Thread: gear ratio

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thanks guys, luckily I'm looking to cruise. she seems to drive well and do'nt get much chance to open it up to 70 in the areaa that I live...especially being new to bias ply, totally different car than when I had wide 17" dia radials on it last year. I'll be posting pics of all the body work I did this winter. thanks again

A quick way to guesstimate the RPM change is to swap the numbers. 25" to 30", change to 30 and 25, so where you were turning 30 or 3000 RPM, now you'll turn 25 or 2500 RPM. No rear end ratios or OD ratios needed! It's a simple straight line graph, so 1250 RPM (half of 2500) was 1500 RPM (half of 3000).

Like Don and Tech point out, the rear gear ratio is only one piece of the puzzle. With your TH350 ratios of 2.52, 1.52 and 1.0 you are going to be putting a lot of strain on the tranny every time you take off, especially if you push it. If you use 3000rpm as your "predicted" shift point your speed for 1 to 2 is 39mph, 2 to 3 is 64mph and you will cruise at 97mph in third at 3000rpm. I don't think you will be happy with those tall tires and tall gears if you liked the way it performed last year with 25" tires, but like Don said, bolt it up and try it. Changing to 3.08's yields 2415rpm @ 70, 3.42's gives you 2681rpm, and 3.73's takes you to 2924rpm, just about where they ran back in the day - 3000 @ 70 and gulping gas. An OD gear would make things a lot easier to balance.