Thread: Advice 3.75 gears

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Hello everybody got a question about gears. I had 3.73s to start with they were whining very bad I sent those back they sent 3.75positive back by mistake I don't know much about the 3.75s. I've got a 383/460 hp 700r4 trans.

You will not see the differance

I had no idea that you could get both 3.73 (41Ring, 11Pinion) and 3.75 (45Ring, 12Pinion) for the same carrier sizes. I thought that they picked one or the other for a given rear differential. Guess I learned something today, and I agree 100% with Charlie, you'll never notice any difference between the two. The bigger pinion gear may be a little stronger as it spreads the load over a bigger circle.

Like Roger, I never knew there were lots of different gear ratios available other than the standard 3:70, 4:11, 4:30, etc. But when we ordered the rear ends for my 27 and Dans rpu the guy offered stuff I had never seen, like 4:00, etc.

But as far as your original question, you won't notice a bit of difference between the 3:73 and 3:75.

Don

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.

Hey Roger-these two links have that and more:

Automotive Converters and Calculators

Wallace Racing - Automotive Calculators

Originally Posted by rspears

I had no idea that you could get both 3.73 (41Ring, 11Pinion) and 3.75 (45Ring, 12Pinion) for the same carrier sizes. I thought that they picked one or the other for a given rear differential. Guess I learned something today, and I agree 100% with Charlie, you'll never notice any difference between the two. The bigger pinion gear may be a little stronger as it spreads the load over a bigger circle.

The pinion gear isn't really any bigger with this small a differance in gear ratios--true one more tooth but diameters are pretty well set by the numerical ratio--

The teeth are different size and shape between these ratios and the 12 tooth pinion will have smaller teeth---In some ratios, there is a bigger differance in tooth size as sometimes there will be 2 or more differance and that will be pretty significant

Originally Posted by 1gary

Hey Roger-these two links have that and more:

Automotive Converters and Calculators

Wallace Racing - Automotive Calculators

Gary,
The ones I've seen on the generic calculators don't let you consider overdrive, nor do they give you the flexibility to run all of the tranny gears in one place and see the shift points. I know mine works and it shows everything I want to see, but thanks for the links, anyway. The point I was making is that it pays to run the numbers considering all of your variables before you start buying stuff.

The first link will do that for you if you plug in that final gear per each gear.It does also that for O/D by that final gear reduction.I used it to figure out the RPM at 65 mph with a 700R4 and a 29" tire for Ole Yellar.

Roger I posted that first post and thought about how it came cross.I'm sorry Roger.I didn't mean to do it that way.

Originally Posted by 1gary

The first link will do that for you if you plug in that final gear per each gear.It does also that for O/D by that final gear reduction.I used it to figure out the RPM at 65 mph with a 700R4 and a 29" tire for Ole Yellar.

Roger I posted that first post and thought about how it came cross.I'm sorry Roger.I didn't mean to do it that way.

No offense intended, and none assumed. All's good in my book! We're all just trying to help were we can.