[Mike C]

Your tires are approximately 30" tall. (285/25.4=11.2" 11.2*.55=6.17" sidewall height *2+18=30.344) Stock tires were approximately 28 tall. That is why your speedo is off. 2*pi*r for your new tire is 94" while the OEM tire would be 88". The new tire rolls 6" further for every rev or 7% which will be approximately what your speedo is off. Assuming the 3.08 is your original gear.

The 3.08 is a 3 series carrier which would be good up to 3.55 gears. 3.73 and up would be 4 series, in a truck. Different break point on gears in cars.