Not physics, but engineering. . . Your question is not a physics question, just a units question. You want horsepower per pound, but you neglect to mention the weight of the vehicle. The horsepower per pound of the vehicle is just the total horsepower over the total weight. There is some mathematical error in going a significant percentage of the stock weight in either direction, but I believe it would answer the question at least in concept.

Really this term can be expressed as a ratio multiplied by a constant:

(Stock Weight)
------------------------- * (Stock HP) = HP with new Rims
(Weight with Rims)


The units for the weights cancel (lbs/lbs) and you get a new constant multiplied by your stock horsepower. Some coefficient based on the change in weight of your vehicle.

If you wanted to figure out how many HP per pound you traded away for your new rims:

dHP change in HP (stock HP - New HP)
------ = ------------------------------------------- = HP per lb
dW change in Weight (stock - New)