You really need to decide on the rear end before you decide on a tranny...because of the ratio. If you keep your original rear end (not a great idea because it's a 5x5.5 pattern and you have 5x4.5 on the front) the AOD would be great. If you replace the original rear with an Explorer or similar rear you want a ratio of about 3:70 or thereabouts. If you go with a rear end with a ratio of around 3:00 to 3:25 then the C4 or C6 would be a good choice.

 

What is the first gear ratio in a AOD transmission? Reason I ask is that the rear gearing will affect how useful first gear is gonna be, just something to consider.

On your bed options have you considered the simulated wood deck boards for durability? Seems like they would hold up pretty good. My 50 had a steel bed over wood from the factory, so the correct steel stamped bed may be correct for your truck. Personally I like the look of wood better.

 

The first gear ratio is 2:40 in an AOD but that is not as important as the final drive gear. First gears are about the same in the C4 (2.46) and AOD. The C6 has a steeper first gear at 2:71.

 

Good point, I forgot to mention that. lol Camshaft, transmission, rear differential and tire size are a system. You have to decide how you want the entire system to function together. Assuming you're building a mild engine with a 1500 to 3500 rpm operating range, and have a 3.70 or higher rear gear ratio with close to standard diameter tires then the AOD is your best bet. If you're building a performance high reving engine or have a 3.20 rear gear then go the other route. Your builder should be able to help you walk through all this. When most are displeased with their tranny selection, it is usually because they didn't take the rear diff. into account.

 

Thanks, I was just curious as I'm having trouble finding a T5 with a useful 1st gear with my rear gearing.

 

If you went with a Tremec TKO (a little pricey) they have a 3.27 and 2.87 first gear option. 5th gear options of 0.68, 0.64 and 0.82.

 

Hi Guys,
Just got back from meeting the guys helping me with the truck.
Changing out the rear end with an 8.8 seems to be a good idea as you get the same bolt pattern as the front end with the Mustang II IFS. The engine selection appears to be a 351W with a mild cam top end. We were discussing rebuilding the existing 3 speed C4 & putting it back in behind the 351.

Does anything in that plan bother you? What gearing would you go with in the
Rear to get good 65 mph cruising?

I appreciate everyone's comments & thoughts.

Ben D. Austin TX

 

I'd try to get a 3.27 rear gear set with a C-4 tranny.

65 mph + 1:1 (C-4 3rd gear ratio) + 28 inch tire diameter = 2551 rpm

(Tire diameter is just a guess. I have no idea what you're going to use. Bigger diameter, lower rpm obviously.)

This is just my opinion, but with a 3 speed I like those #'s.

 

Hi Corrupt,
Thanks on the gearing - I notice you are driving a 302 - This is my last shot
on engine selection - Any particular reason you went 302 vs 351?

Ben

(I went through Nevada this summer - it was hot - even coming from Texas -
110 degrees is impressive).

 

My reasoning for the 302 is about half way down on page 3 of this thread. The main reason for me though, was that my brother is a Fox Body Mustang guy. I can use his knowledge of the 302, tons of parts and I got my running roller 302 for $125 through a guy he knew. (Great starting point for a build.) If costs were all the same, I probably would have gone with a 351 as well but it didn't make sense to me.

Yeah, we get up to 110-115 degrees every summer and down to -10 or so in the winter. Granted those are the high and low end, but we definitely see all 4 seasons here where I'm at. Make's things a little trickier because we have to plan for all scenarios. I wished I could just build a beach cruiser and never have to worry about snow and sub zero temps.

BTW, here is a good calculator for you to play with.
Engine RPM Calculator

 

That's a good choice but don't pass up a good 3:00 if you run across one and can't find a 3:25...

 

If you are running any direct drive (non overdrive) transmission, and 28" final diameter rear wheel/tire combination, a 3.00 will yield 69.5 MPH at 2500 rpm.

However, using tire diameter adds a lot of error to the calculations because the tires are not round when sitting on the ground. You have to use roll out distance.

I have a 390/C6 with a 2.80 9-3/8 rear. My rear tires are 235/75R15 with an advertised tire diameter of 28.89" and a roll out distance of 88." With this combination I travel 65.8 MPH at 2200 rpm; 71.6 MPH at 2400 rpm; and, 77.4 mph at 2600 rpm.....Yes that .89 inch made 300 rpm difference at the same speed. These are actual dyno speeds/rpm.

Here's an excellent online RPM calculator:

Gear Ratio Calculator

Use 1.0 for third gear and use the 4th gear position for an overdrive gear. And be sure to use roll out distance (tire circumference measured linearly on the ground by rolling the vehicle one tire revolution)

 

I see guys regularly pushing for the same wheel bolts pattern/dimensions. Willard with the Jag IFS (Chxxy pattern) and 9 inch rear will run 2 different bolt pattern/dimensions. It has been a long time since I had to mount a spare. I am not convinced that I will carry a spare. I will decide when I am laying out the exhaust. If I do carry a spare I have a "get me home" adapter that I will carry with it. (I am also running a fatter tire on the rear than the front)
Tires are just a bit more reliable than when our trucks were built.
Just my personal opinion!

 

get the axle flanges re-drilled to your desired bolt pattern, I`ve done this in the past on a project, it was a street strip car with 520 hp.

 

Too late, I purchase and mounted a new set of steelies!
I recognize that I could have redrilled my axles. For the reasons I listed above, I did not see enough value in doing it.