I get crummy mileage with my 18" TI wheels.
I just popped on my 15" steelies for snow tires and when I picked them up I noticed a good difference between the weights. I didn't have a scale to measure but it was noticeable.
The car feels easier to accelerate too. I promptly went and filled up my gas to see what kind of mpg I get next fill up with the reduced weight in wheels.
I was doing about 28mpg before mixed but with more city. about 34-36 on the highway with one 40mpg highway trip.
1965 Mustang Coupe 289 4spd
2003 Land Rover Discovery II SE
2013 Focus Titanium Sedan