Quote:
Originally Posted by c0da
Upgrading rims for mpg purposes really won't help much. If you stick with stock size and go with lighter rims and skinnier tires you may see an increase.
I've personally upgraded my rims from 16" 18lb stock rims to 17" 14lb performance rims, but I had to go with a slightly wider tire. It has had a negative impact on my mpg.
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Are you correcting for the difference in size between your new tires and the stock ones? If they're taller (and there's a good chance they are), you are traveling slightly more miles than your odometer says you are.
For example, I actually run a slightly wider (and taller) than stock tire, with steel wheels. This means I have to multiply my miles by 1.02 (since the circumference is 2% greater than stock), which means my actual mpg is a bit higher than strictly dividing miles by gallons.
In addition, a lot depends on the mix of driving you do. Stop-and-go driving benefits most from lightened wheels and tires, whereas interstate driving isn't hindered by- and might actually benefit from- steel wheels.