A quote from the ecomodder website:
Blue Bomber Man said - To get an idea of the impact of lighter wheels, take your vehicles weight and divide it by your HP. This will tell you how much weight each HP drives. Say you have a 100 hp car that weighs 2000 lbs. Works out to 20 lbs/hp. So cutting 20 pounds from your car would be the equivalent to improving your engine by 1 hp.
Now this isn't really a comparison of apples to apples because the wheels are rotational mass. I think the conversion is roughly 4:1 so if you cut 20 pounds of wheel mass it is the same as cutting 80 pounds off the car.
A debate has been going on about wheel size, weight, and rotational mass. I have a set of 15" rota slips weighing in at 11.9 lbs. I am being told that I need to switch to a 14" Honda Civic HX wheel which weighs in at 11.75 lbs. The weight is negligible so then it goes to wheel mass that a 15" tire weighs more than the 14" tire.
I can see that argument but with the mentioned equation would it really make a difference?
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if you can only race me in a straight line then whats the point 
...91 SMF SCCA
http://www.flickr.com/photos/8341770@N07/5601083432/http://www.flickr.com/photos/8341770@N07/5601083432/ by http://www.flickr.com/people/8341770@N07/, on Flickr
I see people out there spending money on the car to improve it … If you spend half that effort just working on your driving skills, making you and the car work together as a team, then you’ll go faster – period. To make the car go a second faster on the track takes a lot more energy and money than making yourself drive it one second faster.
– the late Tom Thrash SCCA Solo E Production