When I bought my 1998 GMC Suburban K1500 SLE it had tires that are larger than the truck calls for. Is that the reason my Speedometer is registering my speed as 8 Mph faster than I'm actually moving? The tires on it are P265/75r16 and the truck calls for P245/75r16.

yes, but sometimes you can find a guy to reprogram the computer so it reads right

I'd get the pcm data scanned and see what the PCM reads for speed, as 8 mph off reading HIGH is not right when using a taller tire, it should read lower UNLESS someone has change the gearing on the truck to throw off the speedometer sensor readings OR has reprogrammed the PCM to the incorrect gear ratio.

So, it's possible the speedometer itself is off as well.