i put together a sb d150. tbi 318, 518od. the truck i took the stuff out of had 3.55 gears, but this truck has 2.71 gears. rather than going thru the formulas on the net i found a different formula. my speedo was reading 57 when i was doing 65. if you divide the 65 into 57, it gives a percentage ratio of .876. multiply that times the teeth on current gear should tell you what toothed gear you need. .876x33=28.93. all i had was a 27. i need a 28/29. got them coming.