’71 Duster, 440. I need to change the gear ratio. 3000-3100 pounds, BB auto. It was an eighth-mile car (6.12:1 gears). Will now run ¼-mile (10.50s 10.70s)? 12.5:1, 440 methanol fuel, Comp cam 0.588 lift, 320/320º @ 0.50 283/283º 108º lobe sep rated for 3500-7000 RPM. 31˝ tall slicks. I want to go through traps at 6000-6400 RPM. 4:30s sound about right?
Calculating the ideal gear ratio requires a very complex formula and a lot of data, some of which you didn’t supply, mostly because you probably can’t. Ideally, we’d need the frontal area and CD of the car, engine’s torque and HP curves, tire’s true diameter at speed, converter slippage, and lots more. So we have to take an educated guess mixed with some rough calculations. Based on the limited data you’ve supplied, the car should probably go through the traps at 122-124 MPH. If we take a published revs per mile number for 31˝ slicks (672), and allow a few percent for converter slippage, 4.30 gears should be pretty close—maybe 132 MPH @ 6400, and 125 at 6,000. But with so many variables, no can ever be sure until you run it. It might ET a shade better with 4.56s, depending on track conditions, headwind, and so many other factors. But overall, I think your choice is exactly where I’d start.