What axle gear ratio do you have in your truck?

I'm gonna be swapping a AOD into my 71 f100 and will most likely be switching axle gears.

I have 31" tires, so I was looking at either 3.50, 3.70, 3.89 or 4.11 gears.

I don't want to kill fuel economy by going to high or low. But I want it to pull hard too. I'm leaning towards 3.89s or 4.11s.

What do you guys say?

 

A lot is going to depend on what speed you drive......but with 31" tires & 4.11's @ 70mph in OD, you will still only be doing approx 2100 rpm.

So you could even go to 4.56 gearing.

That's assuming you would drive at 70.

 

4.10 would be my suggestion.

 

I do a lot of around town driving, but when I get on the highway to go somewhere I want to get there. So I'd like to drive 80. That's still only 2400rpm.

 

I hope you don't mind paying for gas. These trucks have worse aerodynamics than a barn door given the recesses in front that grab air. Dad's truck turned in 15 MPG at 60, 14 at 65, and it was down to 12.8 when driving 75. It was happy doing it given the ZF5 and 3.50 gears as the engine was still only turning 2000 at 70. But it started getting thirstier and thirstier the faster the truck went.

 

Believe it or not i can get 17+ On the highway doing 65. But without an overdrive that 65 is about 2500 rpm. So once I get my AOD and better matched gears I think I can knock on the door of 20mpg.

 

I can understand that at 65. But not at 80. No matter what you do with RPM the fact remains that aero drag goes up by the square of the speed. IOW, the drag at 80 is 1.5 times what it is at 65.

 

Oh yea I know. I just want to be able to run faster without revving the motor up so gosh dang high. So 4.11 seems to be the recommended ratio in this case huh?

 

Only you can tell because we don't know what you really want. But the AOD's .667 OD ratio is TALL and that would suggest lower final gears. This site lets you calculate vehicle speed based on tire size, gear ratio, and final gearing. I put in 31" tires, 2000 RPM, .667 in 4th, and 4.10 final gearing and got 67 MPG. If you really have the 351C, although you haven't said, then you don't want to turn it any slower.

 

I had an AOD in my (sold) 85 flareside It was always hunted, didn't like OD, then it didn't like 4th,,,OD, 4th, OD ,4th.
Thought it was weaker than other trannnys, bigger tires It may not like it.

 

Yes I do have a 351C. 2v with Edelbrock performer intake and carb and a summit cam. Dual exhaust with glasspacks. Imma guess it's Probably in the 350hp/ 400ft-lb range.

Yea I think 4.11s would be the most fun and powerful. Probably the most economical around town too since it would load the engine less. I doubt the 3.89s would be any better. It's only a 130rpm difference at 80mph. And I might go to 32s someday.

 

AOD's aren't the strongest transmissions out there in stock form, and anything you can do to baby them is good. One thing they don't like is shifting into and out of OD, aka "hunting". Ford put out a warning about that back in the day. And one of the reasons they hunt is that extremely tall OD ratio - so tall that many engines didn't have the power to pull the vehicle when OD came in and dropped the R's to 1500. So the vacuum dropped, throttle input went up, and the tranny dropped out of OD - for a little bit - and then went back in.

Going with rear gears that complement the transmission's gears is necessary and, on paper, it has always looked to me that 4.10/4.11's are the sweet spot as that gives a 2.73:1 overall ratio when in OD. That's high enough to give good MPG but not cause the tranny to hunt. And, the lower gears actually make the tranny's life easier overall as each shift is less of a jump in speed and the clutches have to slip less.

 

Cool. Thanks for the info Gary.

Another question for you guys, what are your general up shift points?

 

Gary,

I find it helpful to "see" numbers when comparing a concept such as drag. These numbers show the difference and back up what you stated.

@ 65mph [65X65=4225]

@ 80mph [80X80=6400]

Just for kicks:

@ 55mph [55X55=3025]

@ 70mph [70X70=4900]

@ 90mph [90X90=8100]

@100mph [100X100=10000]

Using the first two reference points [65 & 80mph], my understanding from what I have read, it requires approximately 51.5% more power to go 80mph as it does to go 65mph.

 

Well. Thinking about it more, I think I'll go to 3.89s. 4.11 would be fine, and a lot if fun. but Im going for maximum fuel economy in the long run. Plus with 4.11 gears I wouldn't be able to get my speedo accurate. 3.89 I can swap drive gears and be set. And there's only a 130rpm difference, so it will be fine. I can downshift if I need to.