The Stovebolt Site Forums The Shop Area Driveline Rear Diff Ratio for my 1955.2 1/2 ton

I have a 55.2 1/2 ton. It probably has a 3.90-1 crown and pinion.
Gearbox is SM420.

Driveshaft has some excess play, and I may look to replace the crown and pinion gears.

About all I can find is as a replacement is 3.38-1 on ebay.

Whilst this would be great on highway and around town, can I expect a drop in power on moderate hills.

Currently I can pretty much push up most in 4th gear. Is the higher ratio going to change that enough that it will be a nuisance.?

Busta1 - Without getting into too much technical stuff, the 3.38 gear is going to affect your up-hill driving. Guessing you are still running the 235 as mentioned in your DITY story??? If so, trying to climb some hills with this gearing is going to cause your engine to lug down and require some down shifting to keep it in the best operating power band to handle the hills.

I tried to look for some 3.90 gear sets, but like you could only find 3.38 sets available.

That 3.38 set of gears will make your current granny first gear of that SM 420 usable from a standing start.
That gear set will also allow you to you reach highway speeds without the engine screaming.
If it starts lugging in forth gear on hills, just downshift into 3rd.
I had a rear axle with 3.08 gears and a Saginaw 3 speed with a 235 in a '50 3100, and it had no torque in first gear, but highway speeds were very pleasant.

Thankyou for the info. 👍👍

Just a suggestion- - - -the Chevy 6 bolt wheel lug pattern is shared with numerous Japanese pickups- - - -Toyota, Mazda, Nissan, Mitsubishi, etc. You might be able to find a suitable ratio by swapping the entire rear axle assembly instead of just the gears. The early Land Cruiser rear end just might be worth a look- - - -just about the right width as well as possibly a suitable gear ratio.
Jerry

Rear end swap is a good idea. Depending on how much you can tolerate a 94 ford explorer differential is an option. On any differential swap you will definitely need to measure wms to wms, (wheel mounting surface).
Spring perches will probably need to be relocated.
You will need to re-drill axle flanges for lug nut bolt pattern for chevy bolt pattern.

Advantages are Rear disc brakes, positraction, and a very strong ring and pinion. A D4 code in the door jamb code plate. Gear ratio is 3.73:1
Just another idea. I'm considering this on my 52.

Thanks for the reply.

No problem with the idea of a rear end swap, but would be a bit more costly than I want to do at the moment.

The stock Eaton differential in those trucks is a relatively sturdy unit. Some movement is normal.
You didn't say if it was noisy. Check the universal joints and the rear flange. Pull the rear cover and take
a look at the gears. If there is no obvious damage or wear, change the oil and move on.
It's a lot of work and expense to change out something that could be serviceable.
Fred

Thanks for the advice Fred

I checked the diff a couple of years ago when I replaced the axles. All looked good except for a bit of excess play in the driveshaft.
Some minor knocking when changing gear etc. its not confirmed it’s the diff but sounds like it.
UJ’s and bearing seem ok.
Had my mechanic check it all a few weeks ago.

I have just ordered 3.38crown and pinion off eBay as I want to increase the gearing a bit to make it nicer around town.
(Hope I’m on the right track.)