What rear end gears to use

[67bluegt]

I want to replace the 2.76 gears in my 8 inch rear end with a new posi unit with lower gears. I have a rebuilt stock C4 automatic with a valve body shift kit and better bands installed. My 67 fastback has a new 5.0 @350hp. I think Did Shelby offered a 3.55 as an option? Or maybe that was standard on the GT350. I have no long distance travel plans for this car, mostly around town rides. Is 4.00 to low for this tranny? thanks, Bill

 

[Realmongo]

I want to replace the 2.76 gears in my 8 inch rear end with a new posi unit with lower gears. I have a rebuilt stock C4 automatic with a valve body shift kit and better bands installed. My 67 fastback has a new 5.0 @350hp. I think Did Shelby offered a 3.55 as an option? Or maybe that was standard on the GT350. I have no long distance travel plans for this car, mostly around town rides. Is 4.00 to low for this tranny? thanks, Bill

Your rear gear selection is only limited by how much pain you are willing to endure at highway speeds. 3.55:1 without an OD is probably realistic.

What do you mean by is 4.00 too low?

 

[jb1dsl]

I have 3.55's and a c4 and I think it is a great compromise set of gears. It gives you that good acceleration but they are not to low for the highway. If I were to do it again I would go to a lower gear for more accel.... I never drive over 40 miles one way on it anyway

 

[ultrastang]

No Shelbys came with an 8-inch rear end. They only had 9-inch rears. Typical ratios in the Shelbys ranged from 3.50, 3.70, to 3.89:1.

Some Mustang IIs and Pintos came with 3.40 and 3.55 ratios in their 8-inch rear ends, but most had single track differentials.

The aftermarket offers various ratios for an 8-inch though, as well as a wide variety of Traction-lok-style differentials.

The following link shows two charts with given tire heights and given rear end ratios to give you an idea what your cruising RPMs would be @ 60 MPH, with a non-overdriven transmission or with an overdrive transmission:

--Steve

http://www.ultrastang.com/Rearinfo.asp?Page_ID=3


www.ultrastang.com

 

[gsxrken]

Assuming a 26 inch tire and no overdrive: around town only- 3.90. Any highway plans- 3.50 is the most I would go with (and it might help traction problems that the 3.90s introduce). 3.70s splits the difference, but you might find them annoying over 60MPH
Leave your car in 2nd and drive a few blocks holding it at 3100 RPMs… if engine and exhaust noise are acceptable to you, go 3.70s or 3.90s
Overdrive is awesome, every time I shift to 5th I’m glad I spent the cash.

 

[67bluegt]

Many thanks for all this info. I guess I should go with a 3.55. Makes the most sense if I finally do decide to take a ride out side of town. One thing I did not consider was sound. The car has headers and is fairly loud, so exhaust noise may be really high at 50 mph+
Bill

 

[57fairlane]

I had to compromise the other way.

Going through the traps at 5000 in the 1/8 in second gear was pointless on a 6300 306 with the 3.55s.

Going to 3.80 but cruising around town now with 28" tires as opposed to the 26". Same effective gear on the street, use the advantage at the track with 26x10.5 ET streets.

 

[AdamInChains]

If someone Is running an AOD, what gears would you guys recomend to still be decent on the highway?

 

[thehueypilot]

I run an AOD with 3.73 gears and I believe it turns about 2k at 65 mph and I run 24" tires.

 

[68keyblr]

If someone Is running an AOD, what gears would you guys recomend to still be decent on the highway?

3.73 or 3.89 with AOD.