So im think of changing from stock 175/65/15 to 205/40/17. The tyre calculators tell me the circumference difference is -2%. what have peoples real world experiences been of changes to fuel comsumption when making the change?

 

I think it boils down more to tire weight, & rolling resistance.

 

In my case, i went wider 205 tire albeit (lighter then stock tire and wheel) and maintained just about the same gas mileage (maybe marginally better). Handles miles better with the wider tire too.

 

wow one of the responders here REALLY answers your question in depth

http://www.wiki.answers.com/Q/Does_r...change_mileage

 

Perceived mpg may change due to-
Difference in tire diameter if speedometer and odometer are affected.
How you drive- if 205/40-17 makes you drive faster and brake more you won't get as good mpg

True mpg may change due to-
Weight of wheel and tire combo if drastically different than stock
Higher rolling resistance of aggressive tire size- wider gives more grip but lower mpg

When I want more street comfort and more mpg I use narrow wheels and all season tires and when I want performance I use 17" wheels with Extreme or Max Summer tires. The difference can be about 2-3 mpg depending on how you drive.

 

Yes I've done what you're thinking.

I went from the oem steel/hubcab rim combo 15x5.5, +55et, 175/65/15 to

17x7, +45et, 205/40/17 on my GE8 Jazz VTi.

Both wheel/tire combo's weigh in @ 14.5kg

The fuel consumption remained the same.

Cornering Grip is better due to wider 205 tire/ 7" rim.

Riding Comfort is similar as well (Kumho KU31 @ 36psi front & 34psi rear)

Slightly quicker acceleration due to the smaller rolling diameter.

If you want better fuel consumption, get some lightweight 15" alloys weighting in @ 5kg & below.

My GE8 uses half a liter less per 100kms rolling on my other set of wheels: Mugen RNR 15x6.5 +45et, 4.7kg

Hope this helps you decide

 

The smaller wheels will likely perform better overall, big wheels are good for style points though.
Good article here:
Effects of Upsized Wheels and Tires Tested - Tech Dept. - Car and Driver