it might throw the speed-o off a few. Nothing major tho. 2-3 MPH. oil pressure isn't affected by wheels/tires. I seriously doubt it anyways. The transmission has a cable that hooks up and basically reads how fast the tranny is turning I think and registers that to the speed-o. Smaller tires are gonna turn a lil farther, thus making the speed-o incorrect. I think you can get it calibrated, but here, we have the digital radar signs up, and I figured that mine shows about 2-3 MPH too much. SO I go between 40 and 45 to go 40. No biggie.