I actually noticed a slight diff. on my '92 civic before and after I got new tires (same size) thanks to almost no tread on the old ones. It's possible that the radar speed signs I was going off of were wrong or calibrated differently from when I drove by them before and after the change, hard to say for sure but w/ relatively new tires my speedo was only reading about 2-3 mph over at 35 and was off by less at lower speeds.