The Stovebolt Site Forums The Shop Area Engine Shop 3600 AD rear differential interchange?

Have a friend with a 53 3600 which is apparently very low geared. 3 speed column shifted, all stock. He says he doesn't feel comfortable pushing the motor any harder than 40 miles an hour (sounds like it is revving too high for his liking) and wants to do something to make it more divable and not hold up traffic. He's only looking to be able to cruise at 55 on secondary roads, just wanting to keep the rpm's down on the old gal. I know there are alot of options, however the least expensive would seem to be a ring and pinion change. Question is, what is available or what other 3rd member might bolt right in with the least hassle?
Any help. I tried a search and I know the info has been discussed before but I'm not the most confuser savy. Sorry.

'68-'72 3/4 ton Chevy is a direct swap and was available with a 4.10 gear. That is about as high as you can get with the original diff. It is a direct bolt in.
If you want to swap the entire differential assembly, one from a mid-'70s -'80s Dodge van works very well under the AD truck, and many have a 3.55 gear.

I finally got a chance to get back out to my buddies and check out his truck. We jacked it up to check the ratio and it looks to be in the 4.5 range as close as we can tell by counting wheel revolutions. It has eight lug wheels so I am assuming it is the commercial rear axle. I guess the question now is, will going to a 4.10 gear give him the results he is looking for. That being he would like to be able to get 60 mph out of it without screaming the old 6. I was hoping maybe for a 3.73 or so would be available to avoid having to change out the whole rear and then getting into wheel fitment etc. issues.

Its time for some math.

Calculate the engine speed based on tire height and rear end ratio and then he can decide if he would be comfortable turning the motor that fast.

The different between 4.57 and 4.10 isn't going to make the different between 40 and 60 mph, but he is probably too conservative about how fast he is turning the motor.

We used to put 7.50 17 tires on our GMC 3/4 tons. However, running them 55 all day long with 15" tires won't hurt them. He did it 50 years ago and they will still do it. You just have to get used to the noise. A steel fan, solid lifters and no insulation make for a lot of noise.

Very well could be that he is just a bit too cautious about how hard he drives it. He just doesn't want to push the old gal too hard but at the same time he doesn't want to hold up traffic. I'll check the tire diameter and do the math to see just how hard the old gal is workin and we'll try an decide from there.

Here's a site that will give you some quick answers.

http://www.csgnetwork.com/multirpmcalc.html

I swapped in the 4:11 gears (pumpkin) from a late 60s truck and I love it. Not only will she cruise nicely on the highway but to my surprize it's much better to drive in town. Difficult part is finding the 4:11 diff, I'd place an ad here on the bolt.

Here's where I found one:
http://67-72chevytrucks.com/
The swap from a 4:57 to a 4:10 is well worth the money and trouble in my opinion. The problem isn't just the fact that that motor is really whirlin', it's the wear and tear on the driver. It's draining to drive for any distance with that engine screaming like that. I've lowered my RPM's from close to 3800 to maybe 3200. That is a big diff. My next project is going to be to hang a gearvendors on this thing.

Look up your buddy's tire size on "Tire Rack" and read the specs for the tire. Take the revs/mile figure and multiply by the rearend ratio. The ratio sounds like it is 4.56 which is original. The resulting figure will give you his RPM at 60 mph. 2700 and under should not be a problem.