Hey y'all. I just picked up a used 2000 GMC 1500. It's running on 37/13.5/20 with 4.88 gears. When I got the truck i was given an older model Diablosport Predator along with. The speedometer was reading approx 10mph faster than actual vehicle speed. After advisement from the DS forums I plugged in the tuner to check out the current tune. I found the gearing to be set correctly to 4.88, but the tire size was set to 300/25/18.(my tires equate to 343/63/20 per a conversion cal). I figured this was causing the inaccurate speedo so I changed the tire size to most closely match my actual tires (345/65/20) and took the truck for a test drive while watching a GPS speedo on my phone. Now the speedo is approx 20mph TOO FAST. Replies on the DS forum seem to come very slowly so I figured I would post up here and see if anyone had any insight. I've read tons of threads with different suggestions on how to fix this issue but none seem to have a definitive answer.