back to article Want a more fuel efficient car? Then redesign it – here's how

Cars are mass-produced consumer products sold to users who mostly know very little about them. They are optimised to make a profit for the manufacturer, so low build cost is paramount for most manufacturers – which automatically excludes many design and engineering ideas that would raise efficiency. John Watkinson has been busy …

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  1. HKmk23

    A light foot and wheel geometry works wonders

    Excellent article, may I add that making sure the wheels run free (you would be surprised have many cars run around with brakes binding just a little...), Also ensure that the wheels are running in a straight line on the straight and that the geometry is equal on the curves, correct tyre pressures also.

    I get 25mpg from my 5litre V10 Touareg at 130kph and that weighs 2.85 tonnes!

    1. JeffyPoooh
      Pint

      Fig 3 says it all

      One way to improve fuel mileage figures, if that's the ultimate goal, is to move house further from work.

    2. david 12 Silver badge

      Re: A light foot and wheel geometry works wonders

      After working on my own cars and bikes, I was amazed to see a wheel spin, and spin, and spin, and spin, on an upside-down race car. On my cars, you can barely turn the undriven wheels by hand, due to bearing friction, and the bikes were only much better. Evidently race cars have much much much better wheel bearings.

      1. Eddy Ito

        Re: you can barely turn the undriven wheels by hand

        I'm sorry but you've obviously tightened the nut a bit too tight. I've built, and rebuilt, many vehicles starting with a '51 Willy's 4-73 pickup and at no time have I ever had an undriven wheel not spin freely for many revolutions after repacking the bearings and reinstalling the wheel.

        As a rule of thumb, as much as I hate that term, the bearing only need be tight enough to prevent off-axis motion so I'm thinking go a half a turn, depending on where the castle nut lines up with the hole, beyond where it obviously seats but no more or snug it up and back off a half a turn, either way should get you in a reasonable ballpark.

        I'm sure there may be manuals that say tighten to "X" ft-lbs, but that's largely shite if you think about it since there is no way they can know the exact taper of the conical bearing, nominal maybe but exact never. You tell me, what's the derivative of the sine of the bearing angle? Yeah, approximations are ok but certainly not gospel.

        On a similar note, I'll add that the author misses what is to me an obvious point. The wheels, given their contribution to the acceleration of the vehicle - a wheel / tire combination that has a larger portion of the mass on the perimeter is harder to accelerate than an equal mass wheel / tire combination where the mass in concentrated in the center.

        More often than not the latest fad is to have a large diameter, low mass rim paired with a large diameter tire and it doesn't optimize the acceleration since a smaller wheel with a larger sidewall may, actually more often than not, provide a better moment of inertia and therefore less work to accelerate to a given velocity.

        1. DaLo

          Re: you can barely turn the undriven wheels by hand

          It's surprising how many people think a car gets better MPG when going downhill if they go into neutral!

          Leave any modern car in high gear with your foot off the accelerator and it will consume no fuel, in neutral it will need to maintain the engine idling.

          1. passportholder

            Re: you can barely turn the undriven wheels by hand

            I think one exception to that is if you are coating to accelerate down a hill.

          2. Alan Brown Silver badge

            Re: you can barely turn the undriven wheels by hand

            "It's surprising how many people think a car gets better MPG when going downhill if they go into neutral!"

            Depending on the car, slope and engine temp they may be right.

            Using the Torque OBD app, the instant milage changes between overrun and coasting on a few local hills is eye opening (90 vs 180mpg when cold, 180 vs 240mpg when hot). I suspect that modern engines put a small amount of fuel through even on overrun to keep the cat hot (or light it in the first place), even when the manifold pressure is down around -13.5psi.

            There are a bunch of techniques to improve milage. Ripping out weight to the extent shown is a bit radical, but it's probably improved reliability dramatically given my experience of 1960/70s british cars (somewhere along the line, "made in britain" became a warning label)

            Other hacks: lighter/synthetic gearbox/diff oil, ditto on the engine, roller tappets, narrower tyres/higher pressure. The comments on the lights are valid, but they make so little difference that it's not worth bothering with. As for the alternator, dump it entirely - GM proved in the 1950s that an exhaust turbine-driven alternator was a lot more efficient, but had trouble with reliability under heat loads. Belt drives put asymetric pressure on bearings and sap far more energy than could ever be recovered by hacking the regulator circuitry. There are a couple of designs for electrical generation using turbocharger housings - and spinning at 15-25,000 RPM means the entire alternator can be a heck of a lot lighter/simpler.

            1. NullReference Exception

              Re: you can barely turn the undriven wheels by hand

              I don't know about Torque specifically, but a lot of similar tools/apps/devices calculate their mileage estimates based only on speed and the airflow through the engine as registered by the MAF sensor (always assuming an ideal air-fuel mixture.) There will always be some air flow through the engine even if fuel gets cut off, and your app may or may not be taking this into account. If Torque will show you injector pulse width, check that - if it goes to zero, you aren't burning any fuel.

            2. Anonymous Coward
              Anonymous Coward

              Re: you can barely turn the undriven wheels by hand

              "There are a couple of designs for electrical generation using turbocharger housings - and spinning at 15-25,000 RPM means the entire alternator can be a heck of a lot lighter/simpler."

              Nice idea - if your car spends most of its life on the highway. In traffic jams with the engine idling you'll soon find the battery going flat however as the turbo/alternator pack sits there doing virtually nothing.

            3. Vic

              Re: you can barely turn the undriven wheels by hand

              There are a couple of designs for electrical generation using turbocharger housings

              I was playing with a FireStreak missile the other day - that uses a turbo alternator to generate its electrical power, driven from an onboard pressure vessel.

              Of course, a missile doesn't tend to last as long as a car from first ignition to end-of-life...

              Vic.

        2. Peter Simpson 1
          Happy

          Re: you can barely turn the undriven wheels by hand

          You tell me, what's the derivative of the sine of the bearing angle?

          cosine of the bearing angle.

          sin -> cos -> -sin -> -cos

          Clockwise for derivative, CCW for integral. One of the few things in my engineering education that stuck and is very seldom used. But it's there when I need it...like now :-)

          // Have a friend who says the only thing wrong with British cars is the Lucas electrics. Just rip 'em out, he says, and replace them with Delco.

        3. NogginTheNog
          FAIL

          Re: you can barely turn the undriven wheels by hand

          "More often than not the latest fad is to have a large diameter, low mass rim paired with a large diameter tire and it doesn't optimize the acceleration"

          Not only that, they often look SHITE! They look like cart wheels from a distance, and unless you've got decent brakes worth showing off you're just telling the world your car only needs crappy drums to stop!

        4. AJ MacLeod

          @Eddy Ito

          "I'm sorry but you've obviously tightened the nut a bit too tight."

          That's impossible on the vast majority of cars these days as almost all of them use sealed bearing assemblies (or even complete bolt-in hub units) with the preload already fixed and not adjustable in any way. The days of "snugging it down and backing off half a turn" are long gone...

          1. Eddy Ito

            Re: @Eddy Ito

            That may be true but it's still possible on my not quite antique daily driver. Still, a wheel should spin quite freely. Perhaps it's the seals on the newer bearing as I've seen several on capital equipment that were quite heavy initially but the friction usually drops considerably after a bit of running.

    3. werdsmith Silver badge

      Re: A light foot and wheel geometry works wonders

      And brake happy motorists take note, it is very expensive to heat the air behind you by pressing that middle (or left) pedal too much, and too many people do it. There are people that whinge about manufacturer MPG figures being wrong, but never take a look at their own driving style, it is not difficult to get close to, or exceed manufacturer MPG figures in any car.

      As for the author's Jag, I too cannot justify replacing my 7 year old car, because a new one would do no more. It is petrol, but I can drive it to better MPG figures than my diesel colleagues and it just keeps on going through MoTs without needing any work. But, the author's Jag? Something wrong there, I feel that the cost of all the modifications, not to mention time is probably far more than it will ever return of fuel savings. Especially when he could have done a lot better in ££ terms just by getting an LPG conversion.

      1. 9Rune5

        Re: A light foot and wheel geometry works wonders

        Not necessarily a good idea, especially if you live in areas that see lots of salt on the roads come winter time.

        My Saab 9-5 has so far covered a little more than 100000 km in five years.

        This spring I had to replace my rear brake discs due to corrosion. My front discs only required a little brushing on the edges, but had corroded a little bit even there.

        Amount of fuel saved: Negligible. Cost of new brake discs: Quite noticable. Of course, having a bit bigger discs than average plays into the equation too, but having proper stopping power can be a money saver in the end (my dad almost plunged into a Skoda that had decided it was a good idea to stop around a bend).

    4. Anonymous Coward
      Anonymous Coward

      Re: A light foot and wheel geometry works wonders

      >Also ensure that the wheels are running in a straight line on the straight and that the geometry is >equal on the curves, correct tyre pressures also.

      >

      >I get 25mpg from my 5litre V10 Touareg at 130kph and that weighs 2.85 tonnes!

      I would suggest a better way to get better mpg is not drive around in a pointless 2.85 ton tank with the aerodynamics of a barn door and buy a normal car instead. An added bonus is people then won't think you're trying to compensate for a problem in the trouser department.

  2. Anonymous Coward
    Anonymous Coward

    Best car based article yet from El Reg.

    This!

    Was very interesting and pretty practical.

  3. Pen-y-gors

    Depends on assumptions

    "Suppose you drive 5,000 miles (8,046km) a year in a car that does 30mpg and costs say, £205.00 to tax. Fuel and tax are costing you about £1,160. Swap it for a new car that does 40mpg and costs, say, £145 to tax. Fuel and tax are now costing you about £860, a saving of £300 per annum."

    But how many cars these days can only do 30mpg? My 1989 Porsche 944 automatic manages 30mpg! And most new cars should manage a lot more than 40mpg.

    With a more realistic set of assumptions: 12000 miles per year, and buy a new car that does 60mpg, with low emissions...

    Costs of old car: petrol 2360 + 205 tax = £2565

    Costs of new car: fuel 1180 + £20 tax = £1200

    Annual saving = £1365.

    Cost of new car £10K? so paid for completely in 7-8 years, less if you manage to get any trade-in on the old one.

    But of course, if you're talking about a classic car, then you don't buy it based on fuel efficiency - you buy it because you love it, and sod the expense!

    Of course, your mileage may vary. Still an interesting article though.

    1. Anonymous Coward
      Anonymous Coward

      Re: Depends on assumptions

      The disadvantages include modern cars being so hideously complex, that even changing a light* often requires a workshop and workshop tools, and I wait to be convinced of the longevity and long term fuel efficiency of the current crop of TSI type micro-sized engines.

      My neighbour has a 1.0L Focus that, despite spending more time at the Ford Dealership than on her drive, STILL only manages 34MPG average, in a rural, small town environment.

      BTW, has the author written a book or thought of setting up a business to help others do this sort of work?? My MPV only averages 30MPG, but there is no newer equivalent, so anything I can do to improve MPG is eagerly looked at. (Official mpg is 20/38/30); I have been thinking of an engine pre-heater, but they arent cheap.

      *Some Skoda Fabias need the front bumper removing to replace the headlights, and dont talk to me about Renaults.

      1. Pen-y-gors

        Re: Depends on assumptions

        Fair point - I think the solution is to have a cheap, boring, modern, economical, comfortable car for day-to-day use (e.g. deisel Skoda) and then the classic Porsche/Jag/whatever for a few thousand miles of expensive annual motoring that's fun!

      2. Dan delaMare-Lyon

        Re: Depends on assumptions

        Owners forums are a wonderful source of all that is good in the world of motoring and have helped me greatly. There is no way, without the forums, I'd have ever done half of what I have on my car - and it being from the VAG stable, you have to faff about inside in "invisible" space with tools to take the headlights out to change the bulbs - usually losing part of said tool into the bottom of the car in the process. Same with the rear lights where the bolt holding them in place is placed such that the likelihood of you losing the bolt/tool tip/magnetic holder into the bodywork is at its maximum. So much so that it's a standing joke for the apprentices at the dealers to be given these jobs then made to find the missing part which involves taking the back of the car apart...

        That said though, much of what the author quotes has happened with cars these days anyway. Electric power-steering - check. Revised starting systems - check. More advanced cooling systems - Check. Weight reduction - Check. Changed drive ratios - Check - the list goes on and on.

        To go back and apply it to something designed 40 years ago - hats off!

        Very few people actually check their cars between a light telling them to do so - and most only check tyre pressures when they look low - having driven hundreds of miles, maybe even thousands with them low chewing through extra fuel and causing safety problems....

        Now if only there was a solution....oh wait, attended garages....that could catch on! Check oil, water, tyre pressures.....oh well!

        1. Nigel 11

          Re: Depends on assumptions

          Check oil, water, tyre pressures.....oh well!

          The first two ought to look after themselves. I do check, but I've never had to top up either oil or water (in a Seat Leon now 12 years old). Anyone know, is the oil warning stil just a pressure alert, or does it now alert to a low level in the sump as well? (I discovered recently, my car has a low screenwash reservoir alert. Never knew that until I let it run low).

          You can now buy tyre pressure monitors (TPMS) that replace the valve caps and which are interrogated by a remote box in your car. So now, tyre pressures can be checked continuously, which enhances both safety and fuel consumption. The kit costs about £100.

          I can also vouch for Goodyear Efficient Grip Tyres (er something like that) saving fuel. Low rolling resistance. The cost of of fuel saved over the life of a tyre is in the same ballpark as the cost of the tyre, so it's probably not worth fitting one until your existing tyre is worn out. (I treat 3mm of tread as worn out. Wet grip is very signifcantly compromised from there down to the legal limit).

          1. Solmyr ibn Wali Barad

            Re: Depends on assumptions

            "Anyone know, is the oil warning stil just a pressure alert, or does it now alert to a low level in the sump as well?"

            No good rule here. Quite a lot of cars have an oil level sensor these days. But not all. Would be wise to check the user manual on that.

        2. Alan Brown Silver badge

          Re: Depends on assumptions

          "So much so that it's a standing joke for the apprentices at the dealers to be given these jobs then made to find the missing part which involves taking the back of the car apart..."

          A friend and I made up a gooseneck with electromagnet for exactly those situations back in the 80s..A suitably modified boroscope would work better these days - and bear in mind that a strong enough magnet _will_ move the loose nut/part to where you can get at it, even through sheet steel

          1. Anonymous Coward
            Anonymous Coward

            Re: Depends on assumptions

            I've got a VAG car and can change both front and rear lights in minutes. Perhaps it's model specific.

            Worst I had was a Fiat where I could just about contort my wrist to get to the bulbs but couldn't see what I was doing. Took me an hour to get the job done.

            As someone else said pretty much all of the things in the article have already been done on modern cars.

            One thing which did strike me as strange was the water circulation. I've never owned a Jag but the two or three cars I had from the 70s already have a thermally operated valve which prevented water going through the radiator until the car was up to temperature.

            I wonder if the author has also considered plastic replacement bonnet/boot, lower weight carpeting, more modern lighter weight sound proofing material, lighter exhaust parts, exhaust wrapping, cold air intake etc.

      3. John Brown (no body) Silver badge

        Re: Depends on assumptions

        "Some Skoda Fabias need the front bumper removing to replace the headlights,"

        As does a Kia C'eed. But in my case it's a company car so not my problem.

      4. Voland's right hand Silver badge

        Re: Depends on assumptions

        Engine pre-heaters are cheap. If you use the right pre-heater:

        http://www.wolverineheater.com/

        10 x more economical than a coolant fluid-preheater, you do not need to run it all night to have a usable engine in the morning. An hour or two is enough. Come in both 220V and 120V versions.

        I would definitely advise putting a full front underside shield on the car though to avoid stone chip damage to the filter over time.

        1. MJI Silver badge

          Re: Depends on assumptions

          I bit my oil filter would not get stone chipped.

          It is changed from the top!

      5. werdsmith Silver badge

        Re: Depends on assumptions

        "STILL only manages 34MPG average, in a rural, small town environment."

        Bad driving.

      6. Back to school

        Re: Depends on assumptions

        I have a 1.0T focus, typically get low 40's running around town 2-3 mile journeys and up to 50 on an A road run. It's not supposed to be super economical like the 800kg NA micro cars, it's supposed to be more economical than the 1.6NA it replaces ... and in that it succeeds.

        As for bumpers .. my other car is an elderly but strong diesel passat, it takes about 3 minutes to remove the bumper, hardly the end of the world, when a bulb went a few years back I replaced them all while the bumper was off. Fairly trivial if you can use a screw driver.

      7. Soruk

        Re: Depends on assumptions

        > *Some Skoda Fabias need the front bumper removing to replace the headlights, and dont talk to me about Renaults.

        I can change the bulbs in my Renault Modus - perhaps the fact my hands are relatively small means I can actually do it.

        1. Dabooka

          Re: Depends on assumptions

          "I can change the bulbs in my Renault Modus - perhaps the fact my hands are relatively small means I can actually do it."

          Rear light clusters are a pain though, unless you know the knack and a brave enough to stick a screwdriver where it doesn't belong!

          1. Stacy

            Re: Depends on assumptions

            You mean Volvo are still in the minority of making (front) bulbs easier? I pull a tent peg looking thing out of my headlight cluster and it pops out so you can change the bulbs. Makes it easy to swing the beam when I come to the UK as well.

            Mind you. There isn't even access to the rear cluster! Not without removing the interior lining and soundproofing anyway!

      8. Alan Brown Silver badge

        Re: Depends on assumptions

        "that even changing a light* often requires a workshop and workshop tools,"

        ISTR some EU design rule changes a couple of years back mandating that bulb changes must be doable at the roadside. The result was to hasten the move to leds, as they're not regarded as a user serviceable part.

    2. a pressbutton

      Re: Depends on assumptions

      what modern car does less than 30 mpg

      - alfa 147 2.0 in bristol!

      (changed for a bmw 120d that does 48mpg on the same trip)

      1. Paul Smith

        Re: Depends on assumptions

        bet you wish you still had the Alpha...

    3. Anonymous Coward
      Anonymous Coward

      Re: Depends on assumptions

      Using a car for work, promise of 47mpg by manufacturer, reality 35pmg.

      Average yearly milage 18,000 and 47 tankfuls of fuel a year.

      Bought small diesel van

      Manufacturer claims 67mpg, reality 59mpg and 32 tankfuls of fuel a year.

      Saving 15 tanks of fuel or approximately £825 a year.

      1. Solmyr ibn Wali Barad

        Re: Depends on assumptions

        "Bought small diesel van /.../ Saving 15 tanks of fuel or approximately £825 a year."

        Great. That is, if the diesel engine is a good one, and you don't have to blow the money on maintenance. I don't seem to trust those small highly tuned diesels yet. It's a bit of gamble.

    4. launcap Silver badge
      Stop

      Re: Depends on assumptions

      > But how many cars these days can only do 30mpg?

      Mine - 1.8L Honda FRV (Automatic). It might be my driving style (surely not!) but around town I get 25 MPG. Open motorway gets me to about 33..

      I'd love to still be able to run a manual (likewise still be able to ride a motorbike) but arthritis in various joints cuts that idea out.

  4. AndrueC Silver badge
    Thumb Up

    Best tip to save fuel when driving: DWB (Driving Without Brakes). It's easier if you have a manual but I manage it with an automatic that has a torque converter. Not only will it save a lot of fuel but it makes you a safer driver and adds a lot of interest to driving. To do it well you have to be paying attention and become very good at anticipating what other road users are going to do.

    My instructor (30 years ago) told me "Brakes are for stopping and correcting your mistakes". I've always stuck by that advice. It doesn't mean that you use gear changes instead of braking. It means never needing to slow faster than you can achieve by lifting off.

    1. Pen-y-gors

      Advanced Motoring

      Yep - that's what they teach on the Institute of Advanced Motorists courses - anticipation is very important, looking a long way ahead. Things like adjusting speed gently at roundabouts so that you can just keep going and slot into a gap rather than stopping and starting.

      The courses are well worth doing - they may have changed a bit in the last 20 years, but they really improve your driving, even if (like everyone is) you know you're a "Good Driver". You go out in your car with an 'Observer' (experienced, trained member) who doesn't tell you what to do, but just makes comments and suggestions. When you're ready you do the test (two hours with a trained Police driver - seriously stressful!) - I think they now call it the Skill for Life course and the whole thing costs £149 - which is a real bargain.

      1. Anonymous Coward
        Anonymous Coward

        Re: Advanced Motoring

        Just be aware that when you don't touch your breaks your break lights don't come on!

        If you're not careful you'll have someone to close behind you running into you!

        1. JeffyPoooh
          Pint

          Re: Advanced Motoring

          "breaks" is not the correct word.

        2. Shades

          Re: Advanced Motoring

          "If you're not careful you'll have someone to close behind you running into you!"

          Then the inconsiderate arseholes better have insurance or I'll hunt them down and they'll end up paying for it in other ways!

          1. Nigel 11

            Re: Advanced Motoring

            "If you're not careful you'll have someone to close behind you running into you!"

            Best thing to do with a lunatic drving on your rear bumper is to let him get in front of you as soon as possible. A genuine case of whiplash isn't worth any amount of damages, and that's the least that might happen. But also note, *gentle* pressure on your brake pedal will illuminate your brake lights without actually engaging your brakes. If there's a potential hazard developing ahead you should transfer your foot to the brake pedal and illuminate your brake lights (which is correct, you will be slowing down slightly because your throttle is closed).

            1. Anonymous Coward
              Anonymous Coward

              Re: Advanced Motoring

              I completely agree Nigel.

            2. Anonymous Coward
              Anonymous Coward

              Re: Advanced Motoring

              This is a good point, Nigel 11, and reminds me (reference my own post above) that when as a poor student I was hypermiling my T100 bike, I fitted an extra brake light switch to the front brake lever (on the handlebars) so that when I was using the (phenomenal) engine braking I could move the lever just enough to turn the lights on. In those days most bikes only had a switch on the rear brake, and we were taught to apply front brake first, so rear ends by the dopey weren't that uncommon.

        3. Anonymous Coward
          Anonymous Coward

          @ ChrisZ Re: Advanced Motoring

          "Just be aware that when you don't touch your breaks your break lights don't come on!

          If you're not careful you'll have someone to close behind you running into you!"

          The fact you said this makes me wonder if you are one of those people on the motorway, or dual carriageway, that sits within half a second to a second behind the person in front of you. Whereas *brake*-lights are a vital indicator of what is going on, if you are relying on those brake lights to stop you running into the person in front of you just because they lift off the throttle at those speeds, then i seriously hope you lose your license in other ways before you kill someone because *you* are dangerous.

          1. Anonymous Coward
            Anonymous Coward

            Re: @ ChrisZ Advanced Motoring

            Wow 17 down votes and a lot of assumption about my driving.

            No I'm not one of those idiots. I stay well back!

            I'm just warning others there are others out there that are just potential accidents waiting to happen. Do an advanced course like AIM and they'll give you the same warning! I believe they call it defensive driving to always assume everyone else outside of your own vehicle might an idiot.

            It doesn't matter if it's there insurance that's going to cover it. You'll still have a mark on your insurance for five years, and you'll still be without your own vehicle, and have a whole s**t load of paperwork to work through. After many telephone calls and sitting on hold with a bit of luck you'll get your excess back.

            Well caught on the spelling. I blame tiredness - one of those dev deadline weeks. (As a result my car hasn't moved for over a week!)

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