so I've noticed that my speedo reads 2-3 mph faster than what the GPS app says on my phone. doesn't seem to matter if I'm going 45 or 75, always reads 2-3 mph faster. I've checked the GPS app in comparison to a vehicle that i know has an accurate speedo (fire engine), and it reads dead on. So when i think i'm driving 75, actually I'm at like 72-73... Well, my Trip meter/odometer read dead on, according to google maps. Give or take a tenth of a mile over the coarse of a 100 mile trip. Doesn't the trip/odometer measure off the speedo? weird... Also, the rpm that the tach reads is 200-300 rpm faster than what I should be spinning, given my speed. example: at 75mph(GPS) it reads ~2500rpm, when in reality i should be closer to 2200rpm. This is going off every calculator i could find on the internet. I do run 35x12.5x18's and 4.10's with a 4l80e, and I also have a blackbear tune which adjusted for the larger tires. Just kinda wondering if anyone else has discrepancies in their instruments, and what information I should believe.