What is an acceptable RPM range for a SBC at highway speed? I have a 383 SBC with 700R4 tranny, 4.56 posi, and 28" tall tires...here is what I am looking at with that setup. RPM:2500 Tire Diameter: 28" Axle Ratio 4.56 Transmission Ratio: .7 MPH = 65.27 This is going to be in a 55 chev truck. 2500RPM seems like it would not shorten the life of t he motor and be doable on 2hour trips to the coast. just realized to...when it drops out of OD 65mph becomes 3700 RPM what do you think?