Ok, my stock tires were 245/50/16 and I went to 285/40/17. I used to be at 70mph at 2000 rpm. Now at 2000rpm I'm doing 67mph. Am I going faster than 67 or did my rpm change? Is the only way to change this is w/ to reprogram the comp?

 

The stock tire is 25.65" in diameter. The 285/40-17 is 25.98". That is only a 1.3% difference. So.... at a true 70mph, the new tires will cause the speedo to read 1.3% slower than actual speed, or slightly less than 1mph. Appears you're getting a lot more difference than the tire should have caused.... might want to recheck.

If you decide the difference is enough to require correction, you will need a "prgrammer" like an HPP, Preadtor, etc., or a software programmer like LS1_Edit. You could also send you PCM to a mail order tuner.