You do understand that you lose a bit of performance when you're VSS isn't calibrated, right? Also, the % of error should be progressive, so the faster you go the more it's off. I suspect it's accurate enough at lower speeds then probably around 60-70, you start noticing it. Becase of the larger tires, the PCM thinks you're going slower then you actually are, which throws off the timing and air/fuel mixture a bit.