oversize tires on a 87 elite

[buchholz56]

i just installed a set of pirelli SL26 size 90/90-10 tires on my elite 50S, the parts guy at the dealer told me they would be slightly bigger than the stock 2.75"-10 tires. much to my surprise, the tires when mounted are a full 3.75" wide and 1.25" taller in overall diameter, front tire fits fine, rear had to be deflated to clear the engine when installing. rear clears the engine by .25" and only .125" by the kickstand stop on the muffler and the backside of the transmision. the kick stand is now useless, need a 2x4 under it to get tire off the ground. the handling is nothing short of incredible, way more stable and rails through the corners, problem is takeoff is slower and the engine does not seem to wind out all the way, sounds as though its not reaching full rpm potential. it will still hit the same 42-43 mph (gps) that it did before, of course speedo is now 4 mph slow. i realize that the overall gear ratio has been changed, anyone know how to calculate how much? any ideas on how to increase power on the cheap to take advantage of the taller ratio?