Opinions on gear swap?

[cole71]

I have a stock 71' 9 inch under the truck now with 3.25 gears. I have a wider dent era (78) rear end ready to go. Already narrowed the spring pads and shock mounts. The 78 rear end has 3.00 gears. My question is what do you guys think of going to 3.50 or 3.70 gears in the 78 rear end? Motor is a 302 with flat top pistons, cam, shorty headers, 4bbl edelbrock, and c6 transmission. Not a daily driver but wouldn't mind being able to still drive it say 4 or 5 hours either to car shows.

[cep62]

A 3.50 is a good all around gear many 4x4s came with them.
depending on your tire height a 3.70 might suck a little too much fuel on long road trips.
But depending on the frequency of the long hauls you can change center sections in a couple hours.

[ultraranger]

If you installed an overdrive transmission, you could get the best of both worlds; being able to run lower gears (3.50, 3.70) without the high RPM penalty on the highway or the greater fuel use those gears will induce without on overdrive transmission.

http://www.ultrastang.com/info3.php

http://www.ultrastang.com/info4.php

[cole71]

I hear ya cep62. Will I be able to tell much of a difference with 3.50 from a 3.25?

ultraranger, I would love to have an aod, but I'm gonna have to stick with the c6 for now. I'll be on the lookout for one though.

[papabug71]

I would go with the 3:70's. With the motor work you've done, it would make for a good stoplight screamer/street tuck. You could still make it to the local shos no problem but you would have to take your time getting to the distant shows.

Alternatively, you could leave the 3:00 geared rear end alone, build the current rear with 3:70's & swap them as needed. If your're gonna go to the shows that have a 4-5 hour drive, swap the 3:00 geared rear end in. For local shows, light up the rear tires with the 3:70's !

Best of both worlds!

[F100builder]

3.50 IMO...like it's been said, good all around ratio. As far as having one pig with 3.00's and another with 3.70's, keep in mind that most bump 9's have 28 spline diffs while the dent 9's have 31 spline diffs so they can't be simply swapped without having spare axles as well or installing the gears on another 31 spline diff. BTW...I'm going with a 3.50-geared dent 9 with a Trac-Loc for my '70 which has an open 3.70 now. Of course, this also my too!

[F100builder]

Oh, and one more thing...if you look for an AOD, try to get one from an early 90's Lincoln or F150 ('90-'92 I think???). They've got the better guts. Stay away from the AODE (electronic). With the AOD, you can go for 3.70 to 4.11 too.

[cole71]

Yeah I have the dent rear end so I have 31 splined axles. I think I'll use the 3.00 for a little bit till I figure out exactly what I want. I'm kinda leaning towards the 3.50s though...

[F100builder]

I did a bunch of research when I selected my cam, torque converter, and gears...I have all of them working together in perfect unison.

[ultraranger]

If you are going to flip-flop 3rd members, remember that it's going to mess with your speedometer. You would need to swap speedometer driven gears on the end of the cable, if you want your speedometer to read correctly.

I think you would be much better off saving up your money, as you can, and then install an overdrive transmission. I'm not really a fan of the AOD, but if you were to get one, get one from an '88-'93 model Ford. '80-'87 models had several design flaws that were mostly addressed with the revised '88-up models. The AOD-E was an evolutionary improvement of the AOD. The 4R70W was an evolution of the AOD-E. The 4R70W is the most advanced transmission over the previous two.

You would need a stand-alone shift controller (such as a Baumann shift controller) to operate the 4R70W but it is a much more superior transmission than an AOD, and it will work with either fuel injection or carbureted engines.

[cole71]

I'm not really going to be flip flopping third members, and I'm aware of the speedometer issue when switching. I'll keep the overdrive option in mind, I'm just trying to get the truck back together first.