Nismo Rims & 15mm Spacers - MPG Drop?

[Gerg]

So here's a new little wrinkle to putting my new '15 Nismo rims on my '09. I used to get 21+mpg, now with the spacers and the new rims I'm barely breaking 18mpg on the same roads day in and day out (mostly just to work and back).

I thought the Nismo rims were lighter than the Sport '19 Rays. Could the wider offset of the rims themselves and the spacers on top of that really make that big of a difference? I had the 15mm spacers on the Sport '19s and noticed a small drop (from mid 21mpg to high 20mpg), but this seems pretty significant.

Could it be the tires? I've got the stock Bridgestone Potenzas on the Nismo rims. Before that I was running Michelin Pilot Sport AS3s. Do the Potenzas just have that much more grip?

[POS VETT]

Have you put the assemblies on a scale? While the weight data for those wheels are readily available, the weight for the tires is harder to come by. Besides, the wider tread, stickier compound, and probably a few other factors would play a role in the difference in fuel consumption rate.