Rear Diff Swap
 

Good evening all,

Guys,

I have some questions, please. I recently bought a 37 half ton, and to be honest, I am not real familiar with the banjo differential setup and the torque arms / suspension.

Top speed of this old truck is about 50, and I am not wanting a high speed vehicle but it would be nice to be able to run 55 or 60 with it.

I want to keep this truck stock, but wondering about changing the rear differential and what is entailed in doing something like that.

Can the banjo diff be re-geared, or can the complete rear housing be changed out and if so, what kind of challenges, and what all would need to be changed?

TIA - KT

Re: Rear Diff Swap
 

You should look at an add on overdrive or a T5 or T170 swap

Re: Rear Diff Swap
 

A friend of mine mentioned a T5 but can that be done with an enclosed driveline, or do I need an open driveline?

Re: Rear Diff Swap
 

It looks like Hot Rod Works has a conversion kit for the banjo rear end, to do an open driveline.

Re: Rear Diff Swap
 

A completely different transmission sounds a little extreme if all you're looking for is a little more top speed. Changing rear end gears would be cheaper and easier. From the sounds of it, you probably have 4.44 gears in it now. Perchance, is (or was) your truck equipped with V8-60? Available ratios are 4.11, 3.78. 3.54, or even 3.25, any one of which would increase your top speed. Depending on how hilly it is in your neck of the woods, a 3.78 or a 3.54 would work well. I had a '36 with a 4.11 and 6.00-16 tires and it would do 55-60 easily.

Changing rear end gears is a big job, requiring the removal and disassembly of the rear end and torque tube, but not nearly as big as replacing the transmission. It's still working with all stock components, with no special fabrication required. If you still have the stock brakes, you won't even have to bleed them. Gears are available. You might even get lucky and find some street rodder with a complete rear end assembly with a better ratio available he wants to get rid off.

Re: Rear Diff Swap
 

Moonshine, the advice above is expensive, not needed, and certainly not in keeping with your wanting to keep your Ford stock.
To answer your questions, yes, the gears within your banjo are changeable, with a wide variety of gear ratios available. Doing the job yourself will require a reasonable degree of mechanical ability, but is certainly doable, and at a very reasonable cost. …. I see that Tubman has posted just now before me, si I’ll just say he’s got the full story for you.

Re: Rear Diff Swap
 

Which in itself is another big job.

Re: Rear Diff Swap
 

Taller tires could help too.

Re: Rear Diff Swap
 

yes, The Art Doctor abides!

a tire with a diameter of 25", if increased to 26", will increase the speed at any give RPM by 4% (just a mathematical example), alone, without ANY other changes being made. simple arithmetic is all that's necessary to find the increase in speed one would experience at any give RPM in top gear.

and it's quite possible, considering the modest increase in top speed desired by the OP, that such a simple solution would find success, but i feel it should be noted, that there are practical limits, usually associated with hill climbing performance that are a consideration, whether the increase in speed per RPM be accomplished via transmission, differential, or tires.

when considering such a change myself, i simply bought a set of rear tires (used) with a larger diameter, mounted them, and headed for the nearby hills. for a final test i climbed the Sierra's with them and found that although i got the increased speed per RPM i was looking for on the flats, the hill climbing and increased frequency of downshifting on hills was impracticable for my purposes, and so reverted back to the tires i had.

Re: Rear Diff Swap
 

If the O/P is correct in stating that "Top speed of this old truck is about 50 (mph)", he either has very low (high numerically) rear end gears or a very weak engine. If the latter, none of these proposed solutions will help until the engine problem is corrected. If the former, all of them will work to some extent. As I said earlier, I had a '36 (3-window) with a stock engine, 6.00-16 tires, and 4.11 gears. Given my experience with that car, he must have the 4.44 gears that came with the V8-60 vehicles (yes, they put them in commercial vehicles) if the engine is in decent shape. We really should get to the root of the problem before offering solutions.

O/P, what sayest thou?

Re: Rear Diff Swap
 

I’d say install some 3.78 gears.

Re: Rear Diff Swap
 

Make life easy on yourself and your wallet. Put a 3.54 or 3.25 gear in the rear end. Then change the transmission to a 15 tooth input shaft and a 29 tooth cluster. When you're done go drive your truck 70 mph.


I did this to my '37 Tudor with a stock 221 engine and I can do 70 mph anytime. No problems.

Re: Rear Diff Swap
 

[QUOTE=Seth Swoboda; Then change the transmission to a 15 tooth input shaft and a 29 tooth cluster. When you're done go drive your truck 70 mph.


Hey Seth, Please explain the results and experience of the above swap.

Re: Rear Diff Swap
 

Third gear is still one to one., so changing the transmission gearing doesn't affect top speed.


If your going to higher (lower numerically) final drive ratio, I'd think that you would want to retain the wider ratio transmission gear set to retain drivability.

Re: Rear Diff Swap
 

perhaps you have a engine performance problem— I would drive the 60 hp car with the 4:44 rear at speeds up to 74 mph

Re: Rear Diff Swap
 

I have long wondered what is a "safe" RPM for an extended amount of time for the average 21 stud old Flathead V8 engine with decent oil pressure. With the 4.11 gear in my 36 pickup I rarely exceed 50 MPH. I tend to stay on "pleasure" roads where I can putt along about 40 MPH enjoying the sights and sounds.

Re: Rear Diff Swap
 

Thanks guys, for all this advice, and I just wanted some ideas to ponder on! I will read and digest it and understand.

Thanks!

KT

Re: Rear Diff Swap
 

No, the Flatty has good compression on all 8 cylinders so I am thinking it's the rear being low geared. The engine code starts with 18, so I belive that is an 85 hp.

Re: Rear Diff Swap
 

That is most of my driving, but there are times when we might drive 25 to 40 miles to a car show, and it would be nice to run 60 in a 65 speed zone

Re: Rear Diff Swap
 

I had a 4.44 dif, and could barely make 60 mph with it. I replace it with a 3.78, after which I could get up to 77 mph.
The Ford flathead with stock distributor will not redline. Point float will keep rpms down, safely enabling pedal-to-the-floor for extended periods.