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just a quick discussion between me and a few mates. For the mathematitions out there....would there be much of an increase in MPG on the same car with different wheels. For example 14" wheels v 16" wheels on the same 406 hdi?
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Not really. Very high tyre pressures will improve fuel consumption measurably though, at the cost of poor ride and reduced grip especially in the wet.
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If you have bigger wheels (i.e. bigger circumference) they will do less rotations per mile. But the linear amount of rubber contacting the tarmac will be the same, and therefore so will the amount of work the engine has to do. The only discussion is whether the engine and drive train are geared to perform the same amount of work done, with the same efficiency.
Or .. ?
Oz (as was)
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I would imagine this question will have been investigated scientifically by somebody, but my guess would be that unless huge or tiny wheels are used there would be little difference. The energy used in driving the car forward goes on overcoming inertia (accelerating), air resistance (speed), rolling resistance and friction in the drive train. Only the rolling resistance will be affected by larger wheels, and that will depend mostly on the tyre pressures.
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it will improve the mpg but only slightly as the bigger the wheel the less the rolling resistance ,we used to do things like this as teenagers with a mini on 12 inch wheels instead of the standard 10 inch
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In theory they would, and they would also increase the top speed as you are in effect raising the gearing. As has been pointed out, however, they would also need more power to drive them so the effects would cancel each other out.
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>>In theory they would, and they would also increase the top speed as you are in effect raising the gearing...
Popular beliefs, but not correct ones I'm afraid.
The engine power required to propel a car at a given speed, on the level, is dominated by drivetrain efficieny, and aerodynamic and rolling resistance. Upping the rolling radius can slightly reduce the rolling resistance, and this may have an effect on the mpg - but this effect isn't due to gearing.
The top speed of the car is where the engine power matches the sum of drivetrain, rolling and aerodynamic losses - the gearing isn't really relevant.
As a demonstration of this, most cars will do their top speed not in top gear, but in the next lowest. Top gear is usually overgeared, and the engine will never make it to its maximum power in that gear, unless you are pointing down a steep hill!
Another way to consider this is that the top speed I can manage on a bicycle has nothing to do with the gear ratios at my disposal or the size of the wheels; it has far more to do with my meagre power output, and un-streamlined profile!
Number_Cruncher
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If it's on the same car and they are standard fit wheels, chances are the 16" wheels will have lower profile tyres on and so both wheel sizes will have the same rolling radius and theoretically there would be no difference. Having said that there must be some real world difference because in my car's handbook it quotes slightly less mpg on the same model with larger diameter wheels, even though the rolling radius is the same.
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When I chose my Co. Car, the CO2 output rose with bigger wheels - ie it used more fuel.
You could try one of the car config systems (eg. the one at Ford.co.uk) to get some figures.
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A few years ago I used to run a Fiesta 1.1 van. I had it for about 13yrs so I did replace a few tyres. About half way through my ownership I decided to replace the standard 135 x 13 tyres with 145's. the difference in tyre wear was amazing - the 145's seemed to last twice as long. When changing them I noticed that the 145's had a noticeably larger OD than the 135's. A quick check showed that the 145's travelled about 5" further per revolution. I can't remember any difference in fuel consumption, but I do remember thinking that the van felt more 'relaxed'.
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Problem is, if you put on wheels with a bigger rolling diameter (and it's already been pointed out that they tend to come with lower profile tyres so the rolling diameter is the same), the speedometer/odometer won't know this and will misread. If the fuel consumption is then the same for the odometer's estimate of miles covered, then the car is being more economical.
It's also been pointed out that bigger driving wheels mean higher speed for same revs, but need more power to drive the car through the air.
I put some bigger-than-standard Michelin Xs on my Citroen Bijou. Nothing could possibly have made its performance more languid, and it just wouldn't break adhesion in the dry at any speed although it would perform extraordinary wriggling lurches at its normal very high roll angles. Wet cobbles were another matter however, and I well remember a long and terrifying understeer slide at the top end of Seven Sisters Road, the junction with Green Lanes. Can't remember the name of the tube station. Got away with it though thank goodness.
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Fitted 17 inch wheels to my 2001 model V70 t5 estate ,which replaced the standard 16 inch ones ,the thing used so much more fuel i sold it last week and bought a diesel !
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Perhaps the fact that larger wheels, although having lower profle tyres, are generally wider (ie 205/55 vs 195/65) has an impact? I thought one of the benefits of wider tyres was more grip (subject to the laws of physics!), but more grip must mean more friction... Thinking aloud here though, so I bow to those with knowledge in this area!
Peter
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The reason why the lowest spec is the most economical: narrower harder tyres. But I wd guess the sympathy of the driver's foot wd count for more.
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