The 3rd member is pretty darn easy to pull if you do not like the gear. Maybe get it in and see what you think, I am with you though, the higher gear in the Mav rear end might not work well.

According to my homebrew engine speed calculator your 250 will be turning 1760 rpm at 70 mph with the 2.79 rear gear.  The AOD gears are the same as the C4 except for the .67 OD.  I would say give it a try and see how it behaves at highway speed.  If you are in hilly country it might want to kick down a bit more than you like but maybe not since that 250 should have pretty good low-end torque. 

I have and AOD with 3.55 rear gears and it seems just about perfect to me.  It's turning 2150 at a GPS indicated 70.  A 3.25 would probably be about perfect for your present setup but I wouldn't go through the trouble and expense of a gear change until I had the rest of the car (5.0 engine) the way I wanted it. 

BB

I believe the Fox Mustangs came from the factory with a 5.0, T5 with .68 OD, and a 2.72 rear gear.

BB

Last edited by Bullet Bob (12/13/2017 7:57 PM)

Lots of factory combos with low rear gears and OD, BUT this wasn't done because it was best for performance; it was done because they needed to meet CAFE standards.  This is why the 5.0 cars typically had 2.73 gears, BUT they used a tall first gear in the stick cars, and more aggressive ECU tuning in the automatics to try and give them off the line performance coupled with decent highway fuel economy.

Tire size also play a role in this conversation.  With factory sized tires I would not go over a 3.50 rear gear.  If you've gone to 17s and tires with a reasonable sidewall that are taller than stock I would go to 3.73s.  That's the combo I run in my '89 GT.  I have 3.50s in my '67 (T5 swap) and that's also a good combo, though if I ever change the rear I'll also change those to 3.73s.  I like that combo.  Great off the line acceleration, reasonable highway RPM. 

Appreciation all around for the replies.  I'm just going to clean it up, clean it out, paint it, lube it, put new bearings on the axles, install it and see how she does.  I'll deal with a gear change later if I need to.  I feel safer already just knowing I have it to go in.  That story MS told about that poor coupe that got destroyed on the freeway after its 6 cyl rear locked up lingers in my mind like some of my father's stories and advice he gave me over the years!
- Not too much hilly terrain around here. Pretty much flatlands, vineyards and orchards although I can reach the Sierra foothills in about 30 minutes.
- My '92 LX has a T-5 and a Z code rear which is a 3.08 traction lock. 
- Driveshaft mod/change is definitely happening since I neglected to shorten it waaaaay back when I installed the AOD.  It's definitely too long now coupled with needing a different u joint.
- As for BB's calculations, if she lags on the freeway I'll just go 75-85 like everyone else does around here...70 mph just gets you blocking traffic. 
 

I'm guessing you had a stock Fox body V8 T5, not one with the Z-spec ratios.  Yeah, for that it is a bit too high, because first gear is 3.35:1.  The Z-spec gear ratios were designed to pair better with a standard performance rear gear ratio.  A lot of guys swapped gears as one of their first mods with the Fox cars when they were new.  Initially it was ignorance about what could be done to get more power out of the engine, since EFI was considered voodoo in the beginning.  But, this meant shifting into 2nd way too soon.  The Z-spec came out claiming increased strength and better ratios.  The strength part of that is BS, and is really only because the first gear ratio is lower than stock (2.95 vs. 3.35), and has to do with how they determine transmission strength (I'll spare you that treatise in stupidity here). 

What you need to look at is overall 1st gear ratio (1st gear in the trans x rear end ratio).  With a stock T5 you end up with a 12.5:1 ratio, which is too much for an engine with only a 6,250 rev limiter.  Go to the Z-spec and that ratio improves to 11:1.  For a smaller engine that's a good overall ratio in my experience.  If you had a 351 or a stroker with more low end grunt a 3.55 would be a better choice, because the goal is acceleration, not turning the right pedal into a tire spin pedal instead of an accelerator. 

But, some of this stuff is subjective too.  Personally, I like close ratios in small block cars that can wind.  In a big block car with absurd power its ridiculous (my uncle had a '67 Vette with an L88 427 and an M22 where you could just start in 2nd and shift into 4th and call it a day). 

Last edited by TKOPerformance (12/24/2017 6:45 AM)