Bullnose Enthusiasts Forum

Rear End Swap Question

Classic

List

Threaded

11 messages Options

DH166

Rear End Swap Question

Recently acquired a 1981 F100 Custom SWB 2WD that has an axle ratio of 2.54. I want to beef that up considerably. What's the biggest rear end that this truck will take and still be practical (diff fits and leaves more than 3" of ground clearance, setup doesn't weigh so much that all gains are nullified, etc)?

1981 F100 Custom SWB 2WD 4.9L C4 Auto

Gary Lewis

Re: Rear End Swap Question

Administrator

There's a big difference between "big" and the axle's gear ratio. But there are other issues involved.

First, you've not introduced yourself and don't have a signature, so we don't know enough about your truck to answer the question. I say that because there were two different wheel bolt patterns used on the F100's: 4 1/2" and 5 1/2" bolt circle. So if you have the smaller bolt circle wheels and we recommend a heavier rear axle you'll have to carry two spares.

Having said that, if you just want a different gear ratio that can be done with whatever axle you have. But if you want an axle with a heavier load rating then the first question is why? If you don't upgrade the springs at the same time you still have a weak link and are really wasting your time.

So, as said in the email, please go the New Members Start Here folder, read the guidelines, and then start a new thread there to introduce yourself and your truck. Or, after reading the guidelines, you can introduce yourself here and tell us what you are going to do with the truck, which helps us answer your question.

Speaking of which, a signature that tells us about your truck helps us help you w/o asking lots of questions each time, so is strongly encouraged.

Then we can address the rear axle issues when we understand what you have and what you want to do with the truck.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

DH166

Re: Rear End Swap Question

Thanks for the heads up. I just put a post in the newbies forum. As for the wheels, they're 5.5" x 5 lug. As for the purpose, after the restoration, I want to truck to be fully useful for truck things. I don't have another truck, so this one will be the one. Among other things, I have a small camper that we currently pull with a van and might buy a larger one that would exceed the van's capability. The Ford literature says this model is only rated to tow 2,000 with basic equipment. In the future, I'd like to do some upgrades to the 300 to get it above 200 hp and above 300 ft-lb, and I want the back end to match that output. The current ratio of 2.54 seems unlikely to do the job regardless of what the engine does. Now, if I'm wrong about any of that, I'm more than willing to be educated.

1981 F100 Custom SWB 2WD 4.9L C4 Auto

Gary Lewis

Re: Rear End Swap Question

Administrator

Ahh! Now I see. If you have the 5 x 5 1/2 bolt pattern then you should have a 9" rear axle. There are lots of them out there, and you can either swap one in with the axle ratio you are looking for, or just swap the differential itself, frequently called the "pumpkin".

But before you can settle on the "right" gear ratio you should think a bit about the transmission. The C4 isn't the strongest out of the box, but can be built fairly strong. On the other hand the C6 is quite stout but isn't the most efficient. However, they share the same gear ratios.

Speaking of gear ratios, you might have a play with our calculator (Driveline/Calculators) and see what it would take to get your engine's RPM up a bit. But, with a 3-speed tranny you are in a bind as you need a fairly low ratio to tow, but that will hurt the fuel economy.

A 3.50 ratio is pretty standard for the F150's of the era, but with reasonably-sized tires that will net you ~2800 RPM at 65 MPH in Drive. That's great for towing, but is a killer for MPG. Basically, you need an overdrive gear to be able to tow and to get MPG. So, are you wedded to the auto? If so, you could consider the AOD, which does have an OD, but isn't the strongest tranny out there.

If not you could consider one of the manual transmissions that have an OD.

Anyway, maybe this will start the discussion flowing?

Gary, AKA "Gary fellow": Profile

DH166

Re: Rear End Swap Question

Thanks for the info. Other than wanting to keep the 300, I'm not wedded to any of it. The truck won't be a daily driver, so fuel economy isn't my primary concern. Truckish capability is the priority; tow and haul.

1981 F100 Custom SWB 2WD 4.9L C4 Auto

Gary Lewis

Re: Rear End Swap Question

Administrator

The 300 is said to pull anything, just not all that fast. It is a strong truck engine, and if you put the right tranny and differential behind it it'll serve.

If you don't care too much about MPG then the 3.50 gears might be what you want, and they are very common. You aren't going to be zipping down I35 at 75 MPH, but it'll pull pretty strongly at more reasonable speeds.

But let's see what the others suggest...

Gary, AKA "Gary fellow": Profile

DH166

Re: Rear End Swap Question

I took a look at the calculator as you suggested, and the output numbers look pretty good with the AOD. I might consider that swap, especially since a quick search showed a lot of info on C4 to AOD swaps. If I went with an AOD mated to the 300, what would be the best rear end to go with it?

1981 F100 Custom SWB 2WD 4.9L C4 Auto

ArdWrknTrk

Re: Rear End Swap Question

Administrator

In reply to this post by Gary Lewis

Consider also the braking situation if looking to improve tow capacity.

Don't F-100's come with both power and non-power assisted brakes?

In the rear I don't know if the old 9" axles came with different width brake shoes.
I know the 10.25 in my truck certainly does.

If bigger shoes are available it might be a better proposition to swap the whole axle than to buy a new R&P and have it set up.

Welcome to the forum btw!

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

DH166

Re: Rear End Swap Question

Mine are manual brakes with 11" drums.

1981 F100 Custom SWB 2WD 4.9L C4 Auto

ArdWrknTrk

Re: Rear End Swap Question

Administrator

Gary has all the master manuals to determine what the tow capacity would be with different transmissions, gear ratios, tire load ratings, brake options, etc..

Keep in mind though that that is all tied to your VIN, and while you could upgrade every component you'd still be registered (and insured) according to the door tag.

That might not be a big concern to you unless your DOT has weigh stations set up.
And then that trailer is not going anywhere until you show up with a truck rated for it.
Plus a hefty fine.

I'm not trying to be a killjoy or a hall monitor, I just want you to know the path you are about to venture down.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Gary Lewis

Re: Rear End Swap Question

Administrator

The towing specifications can be found here: Specifications/Towing.

Jim is right. Further, if you were to get in an accident that looked like it was your fault due to an equipment failure they might check how much you were towing for liability. But, I don't know that it has ever happened. It is just something to think about.

Gary, AKA "Gary fellow": Profile