ok, this is what I've noticed about my NHBP Fit. I now have over 4k miles on it and my
fuel economy has changed due to the heavier/wider 17" wheels. 
with stock wheels, I was getting as much as 45-47 mpg on highway, 32-35 mpg city/highway combined.
with the 17" wheels, I was getting a lower figure: 30-32 mpg highway, and as low as 23-25 mpg city.
I drive conservatively, no more than 60-65 mph, but there are times where aggresive accel. is a must for passing (maybe up to 5,500 rpm; normal accel. anywhere between 3,000-4,000 rpm).
in any case, I learned to deal with the fact that fuel economy was lost with the aftermarket wheels. it really is a lot heavier than stock (even by feel, without putting it on the scale). it looks good though
. I'd post a pic, but no camera at the moment.