Good Geared Rear End?

[J_Boo]

Will someone explain to me how the gears in rear end work? I don't understand. I physically know how it works but the higher the number lower the number? My axe is a 3.25, is that a positive slip differential?

[BobbyFord]

Most of the rear ends in these era trucks are non-limited slip.
Ford called this era of "posi" a "Trac-loc" rear end.
The lower the ratio number, the lower your RPM's will be at highway speed. 3.00's are good for highway cruising, whereas 4.11's are more suited for 1/4 mile drags.

[ultraranger]

Will someone explain to me how the gears in rear end work? I don't understand. I physically know how it works but the higher the number lower the number? My axe is a 3.25, is that a positive slip differential?

Correct. The larger the number, the lower the gear ratio. Think of it like a transmission. If you shifted to low gear, that would mean you would be putting it into 1st. 1st has a lower (higher numerically) ratio than any of the other gears in the transmission. If you shifted to high gear, that would be the top position of the transmission. High gear has the highest (numerically lower) ratio of the other gears.

2.75:1.is a high gear ratio. 4.11:1 is a low gear ratio.

Ratios do not determine if the differential is 'open' (single track) or if it's a Traction-Lok/Limited-Slip differential. A Traction-lok differential can be added to any rear end gear ratio.

[sargentrs]

As an example, my truck (302/3spd) originally had 3:25's in her. When I first start out in 1st, I'd go about 30' and have to shift because she'd be revving so loud and going no faster. I'd often start out in 2nd. At 70mph she'd be wound out a little, around 3000 rpms but I could still hear the radio. When they went out on me, I put a set of 3:89's in her. A world of difference coming out of the gate. She turned the tires over and I'd go 20 yds before shifting to second. But at 70 mph she'd be turning 3500-4000 rpms and screaming so loud I could hear anything. Higher numbers (lower gear) is great for drag racing or doing burnouts. Lower numbers (higher gear) are better for cruising at highway speeds.

[gregamorton]

I checked my junk truck that I got the other day and it has 3.00 gears. I have never seen a bump with higher than 3.25 gears but have had a few say that they had 3.00 gears. For a good driver I would use 3.00 gears with most engines. I love them 2.47s and 2.50s with the 460 in a daily driver though.

[ultraranger]

If you you were lucky enough to get a truck with a 'live' axle, the '67/'68 F100s would have had a Ford Limited-Slip differential. If the truck was a '69-'72 model, it would have a Ford Traction-Lok differential.

Limited-Slip and Traction-Lok differentials accomplish the same goal of directing power to both axles but they do it in different ways. With the exception of the '69 model Traction-Lok differntials, the Ford Traction-Lok ('70-'85) is a much better differential than the Ford Limited-Slip.

A 3.50:1 rear gear is a good all-around ratio but engine RPMs will be fairly high at highway speeds. This is where an overdrive transmission is very advantageous. You could run a much lower rear end ratio (3.50 3.70, 3.89, 3.91, 4.11) without the higher penalty for gas consumption and wear and tear on the engine, (due to high RPMs of a non-overdriven transmission) at highway speeds.

A non-overdriven transmission is 1:1 in high gear. To the engine/transmission, this means the rear end ratio is whatever it is. An overdrive transmission, in overdrive, is generally around .68:1.

Take your truck's rear end ratio of 3.25:1, for example. If you had an overdrive, the final drive equivelent would be 2.21:1. This is calculated by mulitiplying the rear end ratio by the transmission's overdrive ratio. 3.25 x .68 = 2.21:1. --in overdrive, it would be as if you had 2.21 gears in the rear end.

A 3.50:1 ratio would have a final drive equivelent (with an overdrive) of 2.38:1.

3.70 would be 2.52:1

3.89 would be 2.65:1.

3.91 would be 2.66:1

4.11 would be 2.79:1

http://ultrastang.com/info3.php

http://ultrastang.com/info4.php

Of course there are limitations. The lower the rear end gear ratio, the sooner you will have to shift from 1st to second. If you have an automatic overdrive transmission this may not be a big deal. If you have a manual transmission with a really low rear end ratio, it may be irritating if you are in traffic or have to stop/start frequently.

[crazyhorse]

Mr Ultraranger, thanks for putting the charts up.
That is some good information.
Could they be made permanent in the gear section?

[ultraranger]

Mr Ultraranger, thanks for putting the charts up.
That is some good information.
Could they be made permanent in the gear section?

I guess that's really a matter for the forum owner or moderators to decide.

From my standpoint, I don't mind if my information is placed in the gear section. The only thing I would ask is if it is used, to simply give credit to where the information came from.