Truck rear gear recommendations

[Bobacuda]

I will soon be tearing my ‘51 B3B apart for a full rebuild.  Since it needs everything, I am planning on rebuilding the 218, while updating the drivetrain with a T5 (0.72 OD), Jeep Cherokee diff, and at least front disc brakes. 16” tires on original steel wheels.
 

I want it to be able to still be a usable pickup that I can also cruise to swap meets and shows. I want it to be able to cruise around 70 mph and I don’t want to be constantly downshifting on hills. 
 

So, what diff gear ratio would you recommend- 4.11. 3.73, or 3.54?

[bkahler]

If you're going to install front disc brakes I would suggest going with rear disc brakes as well.  I used the Rusty Hope front disc conversion and a Jeep Cherokee diff which had disc brakes and with 3.73 gears.  My original 51 had a 3.73 so no issues with the speedometer.  The truck stops like a modern vehicle with 4-wheel disc brakes and parts are readily available.  You can find details of the conversion in my 51 rebuild thread.

 

The diff I used was a 1997 Jeep Grand Cherokee diff with 3.73 & disc brakes, Dana 44HD-A.  I would suggest staying away from the 44HD because it's an aluminum center section diff.  I would suggest instead look for a Dana 44 from a Jeep.  The jeep diff will require 1.5" wheel spacers if you're using the stock 51 wheels.

[Tony_Urwin]

I recommend an online gear ratio calculator to estimate your final drive ratio in first gear, as well as RPM in OD. With the wrong combination of transmission/rear gears, you could end up with a dog in 1st, or too low of RPMs in OD.

I like http://www.grimmjeeper.com/gears.html

 

Those T5s came in a number of different gear sets, a lot of them had 3.50 or even 4.00 first gear ratios. No problem with 1st gear pep!  Be careful, or you end up mostly starting in 2nd gear.

 

I think your OD might like a 3.73 rear axle, about 2500 RPMs at 75mph.  I'm only spitballing here, and it really depends on how and where you drive. Anyone have some real world experience to share?

 

[PT81PlymouthPickup]

I second the opinion on 3.73! I installed a T5 behind my flathead and used a 3.54 positraction Jeep differential because I had one laying around. With my 16 inch rims and 215/85R16 tires 30.4"dia. It's OK, but too much shifting on hills. The original 4.10 was clearly too low of a ratio to contend with modern traffic. I also had a 3.90 in it for a while which was pretty good but believe the 3.73 is would be the sweetest.

[Robert Harrison]

You cant just pick a ratio without analyzing tire size (over all tire wheel dia.) the final gear ratio of your transmission, the operating requirement of the vehicle such as will you be mostly driving it on flat roads, freeways, or mountainous roads.   What speed do you estimate you will (MOSTLY) be traveling at high speed?  A buzzing engine at 60 miles an hour is a pain in the ass.  So is an under powered vehicle on hills.  One of the mistakes people make is gearing too high for the torque available.. They  think they just have to do 70 mph or need to calculate for that speed.  Drivability is enjoyment.  I would look at say 2200 rpm on the engine at 60 mph for a comfortable drive.  Start by investigating over all tire dia for your rims and the transmission over drive ratio.  You can set your overall high gear (4th which should be 1.1 in the trans. to 60 mph but if you mostly don't drive freeway you could even set fifth gear to about 2200 for higher speeds.  I looked at the Hp and torque ratings for say a 1947 230 cid engine and max power was at 3200 rpm  They list torque at 1200 rpm not really sure what's going on there but there is a big difference between 1200 and say 3200 max power.  I think I would still be thinking of about 2200 but thats a me thing.  The other thing is I assume you are using a stock 230 Dodge engine.  Compression ratio makes a big difference in torque.  Early before about 57 heads have a different combustion chamber the later heads are better, may have better ports and you can mill them some to help with ratio.  Check out Don Bunns books on Dodge trucks.  Another alternative might be a 4.0 liter Jeep engine.  Later engines have a nice intake manifold that should be easy to convert to down draft carb.  In my 03 jeep manual 5 speed I am running 3.73 gears with about 32 inch dia tires and I can get up to 20 miles per gallon in a Jeep Wrangler.  Its quiet and Im pretty happy with the choice.  You would have to deal with motor mounts, fitment dimensions, plug the injector holes, trans swaps but that is probably not a problem, pedal mounts, etc but a possibility.  Also 90% of your stopping power is on the front disk brakes really don't make that much difference and drums are typically, cheaper and more reliable to maintain parking brakes on disks are more problematic so don't shy away from drum rears.   Use the calculators.  Bob Harrison

[Bobacuda]

The T5 I have is a WC from a ‘93 S10, originally coupled to a V6.  The gear ratios are 1st - 3.76 2nd - 2.18 3rd - 1.41 4th - 1.00 5th - 0.72

 

The current diff in my 218, ‘51 B3B is either the stock 4.11 or a 3.72(?). The truck has the 3spd trans - I don’t know what those gears are.  I do know that it would pull trailer loaded with about a ton of calves, or cruise lightly loaded at 55 mph and not bog down on hills.

 

So, if someone knows, how does the gearing of the original 4.11 + 3 spd compare to:

Cherokee 4.11 + my “new” T5

Cherokee 3.73 +my “new” T5

 

Thanks. 
Bob

 

 

[Bobacuda]

Found more info on the gear ratios in the 3 spd trans and the T5 trans. I then used a Tremac calculator to come up with the attached table. Based on memory, it looks about right for the original 3spd and 4.11 diff. 
 

With RPM and tire diameter held constant, the MPH is calculated for each gear, and each diff.
 

Looks like the 3.72 or the 4.11 works with the T5.  Thoughts?

[bkahler]

It'll be quieter in the cab with a 3.73 over the 4.11

[Los_Control]

3:73 is better for highway, 4:11 is better for truck things and slower speeds.

 

I read a post from a guy a few years ago, he had experience with 3:73 and 3:55 with a T-5.

 

He loved the 3:73 and could cruise effortlessly down the freeway .... they worked good.

 

The 3:55 gears, in 5th gear cruising down the highway was good, but if came to a slight or moderate incline ... he was always downshifting into 4th because he did not have enough torque in 5th to pull the incline ..... so with the 3:55 it kept him busy to drive it, enjoyed the 3:73 better.

 

Same time many have ran the 3:55 gears with no problem, they have fresh engines .... some have headers and dual carbs .... they run ok with 3:55.

You have a average motor with moderate miles on it, the 3:73 seem to be the sweet spot.

 

Personally I think I want to stay with 3:73 to limit high speed. With a straight axle front end and factory springs/suspension. I imagine 70mph would feel like 100 mph and plenty fast enough.

[Old CWO]

Depending on the tire size, gearing of 3.73 or 4.10 can both be perfectly acceptable as your posted chart indicates.  Assuming a tire size of 215/85R16, the 4.11 would run just about 200 RPM higher at 70 MPH than the 3.73 (2285 vs 2078).  Given the application, I would personally default to the lower gear set all else being equal.  

 

 

 

 

 

Edited November 26 by Old CWO
added RPM