The answer is there's no easy formula to figure it out. Also, the effect of just going to lighter wheels is probably fairly small (not saying it's not worth doing).
Weight reduction will have the biggest impact in city driving, obviously. As an example: GM just released "XFE" versions of two of its trucks, and lighter alloy wheels were part of the new mods package. They reduced mass elsewhere and added LRR tires too:
Quote:
Aluminum lower control arms (reduce mass)
Aluminum spare wheel (reduces mass)
Seventeen-inch aluminum wheels (reduce mass)
Automatic locking rear differential
Low rolling resistance tires
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Result: 7% improvement (+1 mpg from 14 mpg) in the EPA city rating.
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