The Stovebolt Site Forums The Shop Area Driveline Rear end gears

Can the gear ratio be changed in my 55 2nd series 3100?

depends on what you have, the standard gears are 4.11, 3.90 and 3.38. but i think someone makes a 3.55 can't remember who it is.

It has 4.11's with the SM420 trans. Where do I find gears to make it more highway friendly?

The 3;38 gears are available at most specialty parts stores.

get an OD transmission and forget about it. The 3.38 gears IMHO are a poor compromise. Keep the lower gears for their grunt and acceleration and swap transmissions to get an extra gear for less rpm, more economy and better highway speeds.

Assuming you have a 4.11 as you say (should be a 3.90) the 4.11 would be like a 3.0:1 in OD. The 3.90 would be about 2.85:1 final drive.


The typical OD transmissions will drop rpm by 27-30%

Something to consider, changing to a overdrive tranny will lower the engine rpm,but the rear end will be turning faster and may fail. Lots of 10 bolt rear ends are available and 3.08 gears work well. The engine rpm is lower and the reared is not over stressed. Just something to consider.
That is what I did and it dseems to work well.

Is Patrick's still selling the 3.55 gear set for these?

The rearend will not be "turning faster", it will be turning the same speed as it would with a 1:1 trans gear at the same indicated speed. The engine rpm is the only thing that will change. The only way the rear end will be turned faster is if a higher road speed is run. The stock rearend will hold up fine, I run mine at 70-80 mph regularly, and have had it faster than that.
The 3.55 gears available are not for the '55-'62, they are for the '47-'53 torque tube differential. Only aftermarket gears for the '55-'62 are the 3.38s.

Me too, what Longbox said.

I put a Dana 45 from a 58 GMC in mine. Straight swap and 3.07 gears. Made a big difference.

I have the same rear in my '56 with a 350/400. That is a great ratio for that set-up but if you are running a stock 6, that is too high. That is my opinion based on experience with my 248/SM219/3.90 gears. I think there would be a lot of downshifting to second with a 235 and 3.07s.

I have a ’61 differential in my ’41. It was a 3.90 and I changed it to a 3.38 from Patrick’s. I couldn’t be more happy with it. The truck cruses very nicely at 24 MPH per 1000RPM. I always pull away from a stop using my 3.5 to 1 second gear and have no problems with Michigan hills and roads.

Just in a case a 54 1/2 to owner reads this in the future, the 3.55 conversion ring and pinion work fine in a 1954 enclosed driveshaft but you have to use a 47-53 carrier (for the ring gear).

I'm looking to swap out the 4.56 10 bolt in my 42 3/4 ton. The tech article on this site recommends using the diff, carrier and front case from a 55-62 1/2 ton. Sounds practical except for one thing. Thus far I have been unable to locate a 55-62 1/2 ton rear end.
Anyone have any tips as to who may have one of these available? Naturally, I'd prefer a 3.38 ratio but the 3.90 would still be a big improvement over my 4.56.
Any recommendations for using a different rear that will give me better highway speed in my 42 3/4 ton would also be appreciated. I prefer something that will bolt in without major mods. If I have to mod the driveshaft, so be it.
BTW, my truck is running a 62 235 with dual Rochesters and Fenton headers. The trans is the original 4 speed with granny. I'd like to keep the trans stock.
Thanks in advance.

Tom
1942 Chevy BL 3/4 ton
1942 Dodge WC 1/2 ton
1960 Mercedes-Benz 190SL

I am contemplating both an overdrive tranny and 3.38's for my '59 Apache. Right now it has a TH350 and 3.90 arrangement.

John

I have had OD and 3.38 gears in my 59 gmc. It wasn't an ideal setup, so when the perches were ripped off the 3.38 rear, I went to a 3.70 ratio in an after market rear.

Ghost rider, double check that rear ratio. I suspect it is a 4.11, which will work very well with an OD transmission

4.11 gears with an OD transmission, will feel like 3.00 gears. If you have the 4.56, they will feel like 3.32 gears in OD. You are lucky to have the open rear the 3/4 tons offered, makes things a bit easier.
John

I do not know what rear end I have in my 57, but it seems very low. The rpms are at 3000ish at 65. I would like to do a 3.73 swap I think because it would still be relatively low, but be little easier at highway speeds. Regardless of what ratio I pick (I'm not looking to do it that soon), I am curious as to what the process is to change it. Would I just change this http://www.summitracing.com/parts/SUM-741001/ ?

and if so, is that something I should get a shop to do?

Would it be easier to change the whole differential or the whole rear axle using a junkyard doner?

Just curious

Dalyman,

The stock gears in a 57 1/2 ton are 3.90:1. 3.73:1 gears would hardly be worth the expence of changing rear ends. The only gears available for the stock 55-62 chevy differential are 3.90 and 3.38. Changing the diameter of your tires will drop the rpms further than changing the gears.

My honest opinion is that an od transmission swap is easier and more effective than a rear end swap. If your rear end is excessively worn, that is another story.

What engine/transmission combo are you running now, and how do you drive the truck?

That sounds reasonable since it will need new tires soon. The engine is a 65ish 283 bored .125 over. The tranny is stock I assume, 3 on the tree. I would like to keep it manual, but I have thought about putting it on the floor, and a different transmission would be worth it I feel. I do not drive the truck on the interstates, just cruise the backroads and into town for dinner sometimes. The highest speed limit it would be on is 55, so it is fine for driving as is now I suppose. I just like to feel those fire breathin cylinders every once and a while ya know

I have been looking for a way to get more road speed (6.11 gears, 40 mph). Someone said 5.43 gears were also in that year, but I've given up trying to find them. Do you know what I can use to swap out the whole rear end. Obviously this is a dually. Thanks

It would help if you told us what year and what size truck.

I would take bigbadswingdaddy's advice. The od tranny will give you the best of both worlds, good power down low and hwy friendly top end speeds.
Ron