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Discussion Starter · #1 ·
I've kept track of the fuel consumption on my 2010 2.2 S type from day one and i've seen very little change from fuel fill to fuel fill over 10000 miles, the average on each tank fill has been around 48.9mpg with the original Bridgestone 225/45/17 and 17'' S Type wheels. Interestingly i've recently switched to Bridgestone 225/40/18 with type R 18'' alloys with similiar remaining tread and correct tyre pressures and now i only get about 46.2mpg. Checked the tyre circumferences and they are within a few mm of each other so pretty sure this is not the cause. Anyone else noticed this?
 

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has any calabration being done done to the size of the wheel change?
the change of size can also throw the speedo out, can it not?
 

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Discussion Starter · #5 ·
wheels

no the mpg will drop as a bigger wheel is heavier so you have more weight to move around, just check a few cars where they give you mpg figures for the same car with different sets of wheels
didn't realise there was that much difference in weight between the 17'' and 18'' surely a passenger would be heavier? I would have noticed that.:(
 

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Dr. Evil
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has any calabration being done done to the size of the wheel change?
the change of size can also throw the speedo out, can it not?
Nope, rolling circumference is identical, doesn't have any effect on the speedo.

no the mpg will drop as a bigger wheel is heavier so you have more weight to move around, just check a few cars where they give you mpg figures for the same car with different sets of wheels
+1

didn't realise there was that much difference in weight between the 17'' and 18'' surely a passenger would be heavier? I would have noticed that.:(
One thing you have to remember, is it's unsprung weight, which can have a more dramatic effect.

You will notice a drop in MPG with the switch from 17" to 18" wheels unfortunately, unless you go for lighter aftermarket 18s. Alternatively you could stick with your type-R rims and look into getting lighter discs/calipers to offset. Just a thought.
 

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Race Taxi Pilot
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As mentioned, 18's weigh more than 17" and as a result, your fuel economy is bound to take a dip.

I think a read somewhere, 1 unit of unsprung weight is equivalent to 5 units sprung - e.g. if 18's weigh 1kg each more than 17" (hypothetically) then:

1kg x 4(wheels) x5 = 20kg extra weight in the car.
 

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Dr. Evil
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