i recently put my winter wheels on my '07 G35S and have noticed i'm getting better fuel economy.

my winter setup is: 215/55/17 blizzaks

my summer setup is stock.

is it normal to get better mileage with this winter setup or is there something wrong?

(i usually get about 450-500km on a tank with the summers and i'm now getting 500-600km with the winter setup)

Same driving style I am assuming.

But the skinnier wheels/tires will help with rolling resistance, also 1" less diameter wheel is easier to turn yielding better mileage results.

What differences are you seeing?

yup, same driving style.

i'm showing 50-100km longer distance to empty on the winter wheels.

I would think it wouldn't matter much, yes skinner tires help, but the blended fuel we have here makes it a little worse, pluse all the extra warmup time, makes my mpg, go down some.....

Less rolling resistance probably

one inch LESS diameter equals 4% More tire rev per miles and roughly 4 percent less mpg...the trick is usually going to one size larger for better mpg.
Stock rear axles ratio 3:54 with one size larger tire (245 vs 255) the axle ratio now is now theoretically lower . tire is now 10mm larger or 1.5% larger and should yield a .40 better mpg, example on my car w/ 245 rear, 27.1 mpg with change to 255 would yield 27.5...try it ...it works for me. ( one must keep in mind that you will be traveling faster than speedo shows) and for checking mileage one must add 1.5% to speedo mileage traveled.

today here in the states we get 10% ethanol which reduces our mpg by 4%..they have now mandated to15% content so a loss of mpg and power should be 6% which we will be seeing soon

I must be stubborn or not that smart because that makes absolutely no sense to me.

I would think that with a wider tire ( with similar characteristics other than width ), say a 225/45/18 tire vs. a 235/40/18 tire. In theory shouldn't the wider tire yield less mpg because of the added surface contact. Yes miniscule but it shouldn't be adding I wouldn't think.

About the smaller wheel diameter, I could see how if you didn't compensate for the smaller wheel in tire size then you would be turning the wheel more. But if the tire size is equal or close to equal the the rolling diameter would essentially be the same, correct? It just plays into the fact that a 17" wheel is typically lighter than an 18" wheel.

Those are just my thoughts.