Thread: Advice 3.75 gears

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Hope I am not too late for this thread, but originally, had 3.08 (1976 Chevy 2WD Pickup 350 engine TH350), changed to a 4.11 Rear Auburn Limited Slip Posi, ... how and what criteria do you use to determine what would be a good gear ratio to use for power or speed, or somewhere inbetween? Maybe even burning up the streets, chirping tires in 2nd, 3rd gear etc...???
Any thoughts? Thanks guys.

Rear tire diameter is one factor, but how and where you drive is another. A gear that is nice around town may be horrible on the Interstate at 70 mph. I love the 4:30's in my 27 around town, but on the highway at 60 mph I'm revving to 3000 rpms. By contrast, my 23 had 3:00 gears and just loafed down the highway, slightly off idle, at 60-70 mph.

Personally, I think gears in the 3:50-3:70 range are just about as good of a compromise as you can get.

Don

I have 3.25 gears as well as fairly tall tires - 29.1". These are not the best combination. I went with the 3.25's when I was going to run 26" tall tires - now, I'm considering putting my 3.70's back in for a bit of a kick in the fanny

Originally Posted by SmokeShow01

Hope I am not too late for this thread, but originally, had 3.08 (1976 Chevy 2WD Pickup 350 engine TH350), changed to a 4.11 Rear Auburn Limited Slip Posi, ... how and what criteria do you use to determine what would be a good gear ratio to use for power or speed, or somewhere inbetween? Maybe even burning up the streets, chirping tires in 2nd, 3rd gear etc...???
Any thoughts? Thanks guys.

I built a little excel spreadsheet to plug in my tranny gear ratios including OD, rear tire diameter, rear gears, and left cells open to plug in speed to determine cruise rpm, and rpm in each gear to see where my shift points would land, or you can also run out theoretical top speed with max rpm in OD. Once you have that you can compare to your HP and torque curves to get an idea of your fun factor playing with the gears. I used the formula from TREMEC - rpm = (mphxgear ratiox336)/tire diameter as the basic building block, and I refer back to that spreadsheet all the time to see what changes will do, either rear gears or different tranny ratios. Don is right on wth his advice - with my tall 31" tires and 4.11's still lets me cruise at 1995rpm at 70mph down the highway with the 0.64 OD ratio. With 3.50's I'd be at 1700rpm, which is a little low for me, but others might like the longer legs.