Im running the same size tires on my Sidekick. 100Kms on the speedo (60 miles) My motors running 3500rpms. Ive got no problems with speed, just a lack of power. I havent owned it very long, and the last owner didnt take as good of care of it as he could. So im not sure if the gears have or havent been changed. All i know is the kick does fairly well with the 30s that are on it. Good gear sets you can get from Calmini, or Rocky road, Im sure there are other places as well. Hope you fix the problem soon.