89MustangGT89

I'm about to install 3.73 gears on my automatic 89 mustang gt and I'm guessing my speedo is going to be off. Does anything know what I need to recalibrate it to the right specs. My cars stock gears are 2.73.

AdderMk2

89MustangGT89

Thanks for the feedback. I'm under the impression I need the 21 purple gear but what does the 6t 7t and 8t mean. I find this all confusion lol.

nitrous_bob

first off...you cant make it right period

second your AOD is an 8 tooth drive gear, so you'd need something like a 25 tooth driven gear which is non-existent.

your best bet (or worst bet) is a non ford 23 tooth, it's white and is actually a jeep part. but the pitch of the teeth is incorrect for any ford...worse on an aod and will eventually chew up and need replacing. it will also still have about a 5% error with the 23t

the purple 21 tooth (not the red for a t5) will have you about 10% off but thats dependant on your original gears

almost last....look in your door tag under "ax" m=2.73, z=3.08, e= 3.27

last.... do 4.10's or leave it stock. 3.73 in a aod car will STILL be lacking the ability to launch...and if you hook will still bog a lil bit and still only go 14.40-ish

nitrous_bob

oh and fyi....an aod drive gear is actually part of the output shaft, thats why you cant change it lke a t5

t5 drive gear is just a plastic ring gear thats c-clipped on

89MustangGT89

I already bought the 3.73 and the 21 tooth for an auto 89 and I'm hoping to do a manual conversion in the future which is why I choose these gears. My stock gears are m=2.73.

89MustangGT89

Nitrous bob I also heard that the 8tooth are for 90-93 and that an 89 auto has 7tooth which should make the 21 tooth right in my situation. Does this sound right?

nitrous_bob

NOPE

thats for t5's....all AOD are 8t

and honestly....i have yet to see a 7t output shaft....and even if so...its a complete teardown along with an r&r

you'd be looking at $2k for a speedo change, and still not optimal.....6t is the key

nitrous_bob

and the manual driven is diff from the aod driven

so if u swap later....do it later

89MustangGT89

looked it up and found this "The AOD's up to '89 have a 7 tooth driven gear (inside trans) which is machined into the rear housing thus making changing the driven gear a trans teardown project. Early AOD tail housings also had an oiling issue that was rectified in '89. Using this year housing is suggested when rebuilding the trans.
'90 and up housings had the 8 tooth driven gear which makes drive gear swaps a little more of a PIA. The extra tooth on the driven gear makes drive gear selection somewhat troublesome. The extra tooth makes the drive gear tooth count go up. For example, the 7 tooth '89 gear w/ factory 3.27 gears uses a "red" 19 tooth drive gear, but the '90 and up 8 tooth makes this a "purple" 21 tooth drive gear. This is the largest drive gear that Ford makes. What to do if you have 3.73's or higher with the '90 and up housing? In this case you will need a "23" tooth gear which can be found on certain Jeep vehicles. If you have 4.11's you're SOL. The 23 tooth gears are flimsy at best, (too many teeth for a small diameter) so you'll be changing them frequently. So the conclusion of this may be to change to the '89 tailshaft housing or frequently change your drive gears. This is just an FYI as I went thru this when I had my AOD and did change my gears to 3.73's. I chose to change the tailshaft housing and never looked back."