Since going from OEM 17" Pegasus to my 18" BBS CH rims the mpg has significantly dropped to an average of 46mpg max. I do have a remap but I've had it 3yrs or more and before fitting the BBS' I was getting 50/51mpg easily. Am I missing something here, can my mpg be re-calibrated or do I just have to live with it?

I'm aware that some Vrs Mk2's have 17" and others 18" but on paper the mpg is the same, how come?

Rolling resistance does affect fuel economy but I wouldn't have thought that much. When I drop from 18 to my 16 inch winters I get 2-3 mpg more on my petrol vrs. Someone else mmay have a better idea if the loss is whats expected with a diesel.

If the overall diameter has changed.... It will be miscalculating distance/speed by a small %..therefore the figures could be out a little too.

There's a bit more unsprung weight with the larger wheels, but the effect on MPG would probably be negligible. But team it with wider wheels, stickier tyres and lower tyre pressures and you could see a considerable drop

Edited 19 December, 201312 yr by dobbey

The OEM rims are 17x7.5 and BBS 18x8 so only .5 of an inch difference in tyre width and an inch in diameter so not a great deal to justify a loss of 6-7mpg?! I changed the Oil and fuel filter and serviced her in the summer so she should be economical!

yep, as above wider rubber, more grip = reduced mpg. that's why eco cars have special low rolling resistance tyres.

weight of the rims is also a significant factor if you consider the energy needed to spin up a heavier rim everytime you accellerate.

What are your tyres sizes on the 17" and 18" wheels?  Should be 225/45-17 and 225/40-18.

yep, as above wider rubber, more grip = reduced mpg. that's why eco cars have special low rolling resistance tyres.

weight of the rims is also a significant factor if you consider the energy needed to spin up a heavier rim everytime you accellerate.

I didn't think I'd lose that much though, I have Falken Ziex 914's fitted and they are meant to be econ tyres, fantastic grip in this weather but maybe that's why.

What are your tyres sizes on the 17" and 18" wheels? Should be 225/45-17 and 2245/40-18.

Spot on.

Changing from 225/45-17 to 225/40-18 tyres raises the overall gearing by less than  ½% so I can't see that affecting your mpg.  The 40-section tyres on the 18" wheels will probably have slightly stiffer side walls than the 45-section tyres on 17" wheels, but can't see that alone making so much difference to mpg.

 

You are reporting a 10% drop in mpg.  I do the same 60 mile journey at least every 10 days or so and my mpg for that journey varies by more than 10% - more like 15%.

I used to get 500-520 miles to a tank but now I'm lucky to get 460, more like 440-450 at the minute without spirited driving!

Edited 19 December, 201312 yr by billywhiz040480

Could it correspond with the cooler weather we've had? MPG drops significantly in the winter in my experience.

Phil

Not the same car etc. But when I went from 225/45/17 to the OE 205/50/17 I gained about 4mpg.

Never noticed the drop down mind you when I first put the 225'S on

My mpg has dropped considerably since the temperature has dropped like 10 mpg (maxidot indication only).

So it may just be the lower ambient temperature effecting it which may coincide with changing your wheels?

But if you have had the car 3 years you would probably have noticed a seasonal mpg change before now.

No idea really :(

My mpg has dropped considerably since the temperature has dropped like 10 mpg (maxidot indication only).

So it may just be the lower ambient temperature effecting it which may coincide with changing your wheels?

But if you have had the car 3 years you would probably have noticed a seasonal mpg change before now.

No idea really :(

The wheels were fitted in the summer and the mpg has not been the same since. I've always had good mpg out of her, not quite as good after the remap because I clearly use more acceleration but significantly lower with the wheels fitted.

I haven't weighed the wheels mind so there could be a significant difference, yet having held both they don't feel much different, well not enough to justify 6+ mpg on an average tank.

I changed from standard 18" Neptune wheels weighing 13½kg fitted with 225/40-18 Conti2s to 17" Dezent RE wheels weighing 9½kg fitted with 225/45-17 Nokian WR D3 winter tyres.  No idea of the weight difference between the tyres.  My overall mpg over 1000 miles has not changed at all.

I changed from standard 18" Neptune wheels weighing 13½kg fitted with 225/40-18 Conti2s to 17" Dezent RE wheels weighing 9½kg fitted with 225/45-17 Nokian WR D3 winter tyres.  No idea of the weight difference between the tyres.  My overall mpg over 1000 miles has not changed at all.

 

You probably have two competing factors at play there though. The winter tyres will likely have higher rolling resistance than the summer ones, the effect of which is being offset by the reduction in the weight of the tyres, so it's hard to conclude anything from this comparison.