390 Fe c6 automatic with 3.25 gears

[Charlie67f100]

Hey everyone I finally got my 67 up and driving here around my question for everyone is it normal for my truck with a 390 and c6 auto with no overdrive and 3.25 gear to be cruising at 3000 grand on the highway at 70mph she's got only slight mods to the motor aluminium intake 4 barrel carb headers and and stock tires size as well? Any input I fairly new to these truck and the fe.

[colnago]

This is all back-of-the-envelope math, and I assumed that your tires were 265/70R16 BFG A/Ts (I had to start somewhere). Using the tire calculator at Tire Rack, it said that those tires rotate 679 times per mile. In 3rd gear, your C6 is 1:1. So, basic math says:

(679 revs/mi) * (70 MPH) / (60 min/hr) * (3.25 rear gearing) = 2575 RPM

If you have smaller tires, your RPMs will be higher, since you'll have more revolutions per mile. The engine itself doesn't matter; this just tells you the RPMs the engine must turn, given the above assumptions.

Joseph