New to the M3 game and I have a few questions

So I just got my first M3 the other day, its a 1995 M3 and I absolutely love it. I But I noticed earlier it felt like I was speeding but my speedometer said I was doing the speedlimit, so I cut on my gps and it showed to be about 8mph off. That being said, I decided to take it to redline and see what it does, and the rev limiter hit at 6000 rpms and from what i've read its not until 6,500 on a stock chip in first gear. The car does have a bigger tire size, will that cause my RPM and MPH to be off? If not, what are some things I can do to fix this? Ive calibrated my gauges earlier and it did not help. Any help is appreciated.