I currently have a 1970 M20 with 3.70 gears in the dif. Is there a possible gear change that would drop my rpm's at highway speeds so I can cruise at 65mph without it racing? Its a mild performance 350, mild cam, long tube headers nothing wild. I should have taken notes of the rpm's at those speeds but never did. As it is I feel it's happy cruising speed is really under 60. It has tons of torque so losing a little doesn't bother me. I can start out in third gear from a dead stop no problem. I have the dif out now and it needs to be freshened up so looking at a possible gear change at the same time. Surprisingly enough zero pieces were in the oil. clips are still on the yoke shafts. It does have excessive end play.
Any suggestions on gear setup that would lower RPM's some but not just make it a slug would be very appreciated.

I appreciate all the help and suggestions you guys offer.
Thanks

 

Many folks who had lower numerical ratio gear sets swapped over to the 3.70 (or 3.73) ratio so you already have a highly desireable unit. If you have a 4 speed, consider putting in a TKO 5 speed. The OD 5th gear will address your concern without any compromise. Very popular choice.

 

Switching from a 3.70 to a 3.36 will only drop your cruising RPM around 500. If you are not happy cruising at 3600 RPM you probably won't like cruising at 3100. I see 3.36 Corvette diffs all the time on Facebook marketplace for a couple hundred dollars. I would buy one and spend an afternoon swapping it out to see if you like the difference in RPM before you spend big money rebuilding yours. I personally wouldn't use a gear lower (numerically) than 3.36 in a small block car.

 

I have 3.55 with a TKX 5 speed and 26.5 tall tier . The car tacs around 2000 at 65

 

I have a 3.73 differential and a TKO 5-speed in both my 68 and 70. I really like the TKO shifting feel. The gearing is great for interstate cruising.

 

I wouldnt go lowerr than 3.55. You may notice it lowering the rpm a little but the truth is the only really good option is th eTKX. You wont like a 3.35 or higher gear like a 3.08. I mild 350 doesnt have a lot of torque..... Are you sure its a 3.70 gear and not a 4.11...

 

10 tooth pinion and 37 tooth ring gear. 3.70. Build sheet said 3.70 and it appears to be. Most of this car is original except an engine swap back in 70's. Transmission and rear end seems to be original never touched. Just freshened up the trans while rebuilding engine over winter. Hopefully this freshening up of the dif will have the running gear in decent shape. After a complete clean up, I found some light pitting on the spider gears and both side yokes were worn. Ring gear and pinion looks good. Nice even wear for both forward and reverse on the teeth. I ended up ordering two new side yokes, complete carrier, bearings, seals and shims. This gave me a reason to finally address the last of what looks like crap under the car. Sand blasting and painting drive shafts and dif today. That will be the last of the grease and oil of 56 years under this thing. After reading many post I do see that the only real fix is the 5 speed. I'll just stay on smaller backroads and off the main highways for now.

 

By complete carrier, I assume you mean the case. The carrier is the housing in GM terms, the posi is the case. A new Eaton case is better than a 1970 Eaton case but the clutch packs are fiber. The net forged spiders are good but I hope you took it apart and checked it. I never use them out of the box and I have 6-7 sitting on the shelf that I went through. Once the spring pack was removed there was a lot of endplay in the spiders. The fiber clutches smudge at every turn, they will be quiet but the oil will be black with fiber in it. I would not use Yukon, Summit, Nitrogear, or other knockoffs of an Eaton and for good reason. Look on YouTube, there is a guy comparing them and it will open your eyes.

New axles sold today are Chinese, the ones I have inspected were case hardened. The dimensions all over the place and they use the cheapest snap rings out there. For a street use, not abusive car they will last about 45k miles, same as the original C3's did. If you plan on launching the car, they will break. Rebuilt axles come from a couple of sources, sold by all. They have to be inspected since I have had a 50% rejection rate with them for years. The good 50% is fine. Some are cored and will snap under load.

 

I meant the posi in the case with the four gears, clutches and springs. I ordered the Yukon YGA-26023. Is this junk? I do zero launching racing or even fast driving. I've gotten that out of my system from a lifetime of going fast. lol My wife and I just want to cruise around in a cool classic ride like this.

 

I have a 3:08

I'll swap with you.

 

Another consideration is the camshaft you have. Your profile name being LT1, an engine with the original LT1 cam profile would not play nicely with a 3.08 rear.