I had a 89' Mustang GT with an AOD a few years ago that I had 4.10's put in. I'm almost positive I had put the speedo gear for 3.73's in it because that was the biggest one that was available at the time. It wasn't dead on but it was close. I think it was 5 miles an hour off on the highway.
I have 4.10's with my AOD and just used the white 23 tooth gear. Its not perfect, but a lot closer than the stock tan gear was. If all else fails, you could contact a local speedometer shop and have them make a gear reduction box for you. They will want you to mark your milage and drive exactly 5 miles then mark it again. From there they will make you a box that splices into your speedometer cable and corrects the speedo. I priced one and they wanted 100.00.
Unless im mistaken the highest gear they make for the 90-93 is the 23 tooth and it only takes 3.55's but that is the same gear that is used in the 84-89 trans for the 4.10's so it might work
I got the same problem any AOD 1990 and higher you are screwed unless U change the drive gear wich is part of the tail shaft which means the whole trans has to come out. We have an 8 tooth drive gear and need a 7 tooth. I checked it with the GPS at 60mph it was reading 18mph too fast so I was going 42!!
+1 for the white gear. It's the best I could do for my 89' AOD 4.10's set-up. I drove for 2 years with the stock gear and speedo reading 80 @ 60 actual MPH. I wish I didn't because I want those miles off my odometer!
my 90 vert had aod until a week ago and i can tell you for a dead on fact because i am so pickey about my car that with the 23 tooth white gear it is exactly 10 miles per hour fast.60 will actually be 50.also holcomb motorsports sells what they call a 4.10 black gear.dont be fooled its the same as the 23 tooth white only its black and about 17.00 dollars more.alsomy local ford dealership was out of the white gears so they sent me to the local chrysler dealership and there exact same gear was only $3.60 vs fords$12.00.go figure.one last tidbit of knowledge. the white and black 23 tooth gear is the closest you can get for aod /4.10s,but its actually for 3.73 gears so after about a year it will start to strip and you will have to replace it.i have a feeling thats why the other fella said his was 17 mph off.if he pulls it out i bet he will be surprised to see it stripping.consequently when my aod took its final nose dive i blame the plastic shavings from these gears due to the fact they were all in my trans.just thought you would like to know.
by the way check out my 90 vert triple white on cardomain.com under 90306vert
i just swapped in the 5 spd and am currently installing all black interior.check it out.
thanks jeremy for all the knowledge...i will def check ur car out......triple white is always good....i dont know what to do paint wise with my white steed