I have a 1978 F100, I just purchased the vehicle a month ago.
It has a freshly rebuilt 351W I put in last week with a crane 262 cam. It runs very well. However, it has an AOD tranny in it. I'm ok with that, I like the OD feature and flies down the highway but I think I am under-geared.

1st and 2nd are great, shift between 2000 rpm and 3000 rpm.
When it shifts to 3rd around 30-40 mph (guesstimate) rpm drops like a rock. I apply more pedal and engine knock occurs. I can nurse it to a higher rpm in 3rd and into od, without knock, but only if there is no terrain . I wrestled with the timing until I was blue in the face.


The gears in my AOD(out of a 91 T-bird). 1st 3.75
2nd 2.32
3rd 1.0
od .67

tire diameter is 29.1

The metal plate on my rear diff says

WDH-3V4-8HA
2 75 9 351B

from what I've gathered in my quest is that I have an open 9"
rear end with a gear diff of 2.75.
To me that would explain my low rpm in 3rd and OD.

The math tells me with my 2.75 that in
1st gear at 20mph I'm hitting 2380rpm
2nd gear at 40 mph 2950rpm
3rd gear at 40mph 1270rpm
3rd at 50mph - 1580
OD at 70mph is 1500rpm

From what I've gathered, 1270,1580 and 1500 are WAY low and
should be between 2000 and 3000rpm.

My gumption tells me to go with a 3.5 or 3.73 rear diff.
Its my everyday driver but I want it primarily for highway.

Is that plate correct stating that I have a 2.75?
My door sticker just lists the VIN with no axle codes and my metal tab in the engine bay is

52B F10GNCJ1366
1M 2E Z 33 4AD 08H 7
I think the 7 at the end is the axle code, its the only # I could match up with my information that I have on axle codes. If it is- it says its a 3.25.

Will it affect any of the shifting characteristics of the tranny, such as shifting at the required speeds if I put in a 3.5 or 3.73?
I'm in the process of running the numbers on a 3.5 or 3.73.

Will it be better for my vehicle?

I would appeciate any and all input and checking my numbers.
Thanks a bunch.

 

You have a 2.75 Open rear end. I would suggest at least a 3.50 or 3.70 gear and it will work just fine. Most of the 87-96 trucks with OD have 3.55's in them.

The problem with the tranny that you have is that the converter locks as soon as it drops into 3rd gear. This is why it bogs so bad. No converter slippage.
The newer 4R70W's do not lock up like this.
I would go for the 3.70 gears myself, my Dad's 89 has 3.55's in it and it is still a little low on the rpms at 30 mph in 3rd.

Jimmy

 

Thanks for the input lxman1, I think I'm going to go with 3.7, it should still do very well on the highway. I'll let you know how it works out.
Take care

 

The good thing about 9 inch rears is that parts are everywhere. You can find used 4/11 center sections in the paper all the time for 200-400 already set up and with new bearings and seals.

 

2.75 gears and .67 OD! What were they thinking? This is a truck not a car. They must have been trying for maximum mpg. Once you put a hotter cam in an engine it tends to make things worse at low rpms, hence your problem. 3.55 gears are the most common on newer Ford trucks and even that does not work well with oversize tires.

Jim

 

I've got a 351w and AOD in my 71. I have 3.7 gears and would'nt want anything below that. Thinking about stepping on up too 3/89's or 4.11's
70 mph now is right around 2k
Clint

 

Yeah, I was kind of surprised of my tach readings when I hooked it up.
But anyway, I looked around all day to find a diff with 3.7, no luck.

I have had some folks tell me its cheaper to just buy the whole rear end and put it on.

I don't have any experience installing a new ring gear and pinion. Wish I did, I guess I could make this my first time. Its just a bad time right now for a project in my driveway (we are moving).
A guy quoted me $175.00 for labor and that I needed a installation kit also. I would assume its seals and items like that.

I have noticed that my right rear tire has fluid thrown out on the inside of the tire. Could be worn brake drum piston or could it be that the axle is leaking?
I will investigate further.

Can the axle leak from that location?

Will the metal ID tag tell me if I have a 28 count spline or 32??

WDH 3V4 8HA
2 . 75 9 351B
I've tried to look up the code on the net with no joy. And my library at this time really sucks.

Thanks for the input folks.

Take care.

 

Fluid in your tire will either be brake fluid or gear oil. It's easy to tell them apart. If you have the 3600 lb rear axle ( look on the build code at the drivers side door pillar) it will probably be 31 spline. Otherwise it's a crap shoot you just have to take it apart to find out. The install kit has all the bearings, spacers, shims and seals that you need. 175 isn't all that bad if you can't do it yourself. It's kinda complicated if you don't have the right tools, and you will have to have a press to put the bearings on the pinion. If you want 3.50 gears look under 4x4's. Many of them came with those gears or 4.11's.

Good luck.

 

With 29" tires, and a aod, I would go with 4.10's or even 4.56's. I highly doubt it will affect your mileage much. Hell, my stang dropped .5mpg highway going from 2.75's to a 4.10, with 25.6" tires.. I was still only turning about 2700rpm at 70mph