dwb91lx

The 2.73's will be coming out, just not sure between 3.55's & 3.73's. Looking for all around performance AND NOT AFRAID OF BETTER FUEL MILEAGE. Mostly street driving but definitely want to be able to cruise on the highway. I've seen the calculator for 5-speeds but nothing for 4-speed AOD.

Would like to know RPM difference (3.73 & 3.55) @ 70MPH in Gear #4.

Also, anything else to consider @ same time? No need to drive 140MPH & not interested in the strip.

Tony71502

Consider looking into speedometer gears to correct the change in the speedometer reading caused by different rear end gears. I changed my speedometer drive gear(on the cable) and the driven gear(in the transmission) and at 113 mph the GPS matched my speedo perfectly.

I'm using 3.73 with a t-5 trans and it works wonderful. 70 mph on the freeway and im spinning at 2300 rpms, nearly the same as my ranger(t-5 and 3.73's). 3.73 would be great for the AOD in my opinion. I'm willing to bet people will chime in and say go to 4.xx+ gear ratio, I wouldn't.

dwb91lx

Got the gear rpm info from American Muscle site by looking @ the individual Ford Racing sets...It doesn't look too radically different to go ahead with 4.10:1 considering @ 75MPH they are rolling 2529 RPMs vs. the 3.73:1 @ 2301 RPMs! 3.55:1 looks like 2190 RPMs @ 75 MPH in 4th gear (AOD).

dwb91lx

Does it make the most sense to stay with Ford Racing for the new gears? What about the Trac-Lok...replace or re-build (not much cost difference/ $150?)? Looks like Mustangs Unlimited has the best price?

reldla1996

For typical street driving, the 3.55 is an excellent choice, increasing torque 30 percent while keeping rpm's down somewhat. For the drag strip, 3.73 or 4.10 will get you better times (which ratio depending on your power levels and rpm limits). Ford Racing is an excellent source for the gears, and what I used. If your car has a lot of miles, it would be a good time to do the Trak Lok, it will need done at some point. I replaced mine at about 150K miles, and it was still working, but not much was left on the clutch plates. Whether to rebuild or replace is your choice. I found a take off at the time, from Saleen.

dwb91lx

To take this a step further: What's my best bet on speedo caibration?

It looks like my '91 came stock with a 8 tooth drive gear...with the 2.73's

I have 245/50 zr16's...

I'm not a math wiz, but doesn't it calculate needing 22.5 tooth driven gear for 3.55's or 23.5 tooth for 3.73's...

Can the Drive Gear be replaced (how hard)? Anyone been through this this using the black 23 tooth & how far off IS the speedo readings? It may help determining the new gear ratio for me...

mustangGT86_05

It wouldn't make sense to change the drive gear, you would need to remove the tail housing to do so. There are places that can tell you your proper speed gear if you google it. I run a 3.55 gear and autocross, its a great gear for that, but it all depends on your intentions

dwb91lx

I would imagine that you have a 5-speed manual transmission...I'm trying to overcome the "lack of respect" that the AOD has for second gear (skips right thru to 3rd, nothing to lock into 2nd)...The bulk of my driving is around town in the 25-40mph range... I know, I wish I had a 5-speed.

reldla1996

one-half tooth off on a 23 tooth gear equates to a little over 2 percent, or a little over 1 mph off at 60 mph. I like my speedo to read slightly fast, it helps keep me out of trouble.....

Tony71502

I paid a mech around here to change my drive gear about 30 dollars. This was before I decided to explore the inner workings of transmissions.

The drive gear(inside the trans) should be yellow, not black. Go to latemodelresto.com and they sell the combo of driven and drive gear for cheap. Thats what I used. My speedo matches my gps well over 100 mph. It is accurate, as long as your tires are sized properly.