Trying to figure speedometer issue out

Topic: Trying to figure speedometer issue out (Read 1571 times) previous topic - next topic

Trying to figure speedometer issue out

July 13, 2014, 06:41:59 PM

Hello to all! This is my first post so please forgive me if I sound like a complete idiot. For the past couple weeks I have been trying to read on the board on the many issues my T-Bird has been having and trying to slowly knock each problem out. First things first. I acquired my 87' T-Bird 5.0L AOD a couple years ago knowing absolutely nothing about them (just needed something that ran to get to college). Now that I have graduated and became quite fond of this car, I would like to get it running as well as I can for (more or less) a daily driver.

First question: My speedometer is about 9-10 mph slow. (Ex. when I get on the interstate I set the cruise at 60-61mph to keep up w/ traffic going 70mph) From what I read on the forum this can be from changes in the rear end, transmissions,differences in speedo gears, etc. The info that I have is that the tranny was changed at one time w/ an AOD from a mustang (dunno the year). The rear axle is still stock as far as I know (2.73).What should I do next? Any help on this would be so appreciated! Also, is there any differences in the AOD transmissions from T-Bird to Mustang and if so, is there anything I should do/alter/modify to make it last? Thanks again everyone and so glad I found this forum!!!

Smirdok - 1987 T-Bird 5.0L w/ AOD (stock as far as I know except transmission)

Trying to figure speedometer issue out

Reply #1 – July 13, 2014, 09:24:58 PM

http://www.latemodelrestoration.com/products/Mustang-Speedometer-Gear-Calculator

find out which one you need and change the gear on the speedo. I just did mine but my setup is different, they have a calculator to help figure out which to get.

Trying to figure speedometer issue out

Reply #2 – July 15, 2014, 09:16:11 AM

There should be a tag on the AOD that will identify what it is but if that is missing then it becomes a bit more of a pain. This is a good write up on the history of the AOD:

http://www.lincolnsonline.com/tech/00080.html

This guy is an AOD genius:

http://www.silverfoxtrans.com/silverfoxtrans.com/Home.html

Another good speedo gear calculator:

http://www.tciauto.com/tc/speedometer-gear-calculator/

You need to reduce the number of the teeth on the driven gear (the plastic gear on the speed sensor - http://www.latemodelrestoration.com/item/LRS-9731B/83-93-Mustang-Speed-Sensor) by about two teeth to get it to speed up 10 mph. General rule of thumb is one tooth on the driven gear is equal to about 5 mph.

My guess is that the original trans that was in your car had an 8 tooth drive gear and the trans you have now has a 7 tooth drive gear. The drive gear on the AOD's is cut into the output shaft and the only way to change them is to replace the output shaft. The T-5 have a plastic drive gear that slides on the output shaft and is clipped into place. A much nicer setup as you can just pull the tail housing and change the plastic gear on the T-5's.

Anyhow, good luck with the correction.

Darren

Trying to figure speedometer issue out

Reply #3 – July 15, 2014, 07:14:03 PM

Quote from: Aerocoupe;435971

There should be a tag on the AOD that will identify what it is but if that is missing then it becomes a bit more of a pain. This is a good write up on the history of the AOD:

http://www.lincolnsonline.com/tech/00080.html

This guy is an AOD genius:

http://www.silverfoxtrans.com/silverfoxtrans.com/Home.html

Another good speedo gear calculator:

http://www.tciauto.com/tc/speedometer-gear-calculator/

You need to reduce the number of the teeth on the driven gear (the plastic gear on the speed sensor - http://www.latemodelrestoration.com/item/LRS-9731B/83-93-Mustang-Speed-Sensor) by about two teeth to get it to speed up 10 mph. General rule of thumb is one tooth on the driven gear is equal to about 5 mph.

My guess is that the original trans that was in your car had an 8 tooth drive gear and the trans you have now has a 7 tooth drive gear. The drive gear on the AOD's is cut into the output shaft and the only way to change them is to replace the output shaft. The T-5 have a plastic drive gear that slides on the output shaft and is clipped into place. A much nicer setup as you can just pull the tail housing and change the plastic gear on the T-5's.

Anyhow, good luck with the correction.

Darren

Thanks everyone for the responses! Aerocoupe, would it be best if I just left things as they lie and deal with the speedometer issue for the time being or could it cause further issues since it could be difficult to switch out the gears?

Trying to figure speedometer issue out

Reply #4 – July 15, 2014, 09:23:25 PM

Just pull the speed sensor out of the tranny and see what color the driven gear is. Look up that color for an AOD which will give you the tooth count. Bump that number by two and order the appropriate color gear. The T-5 and AOD years are different so be sure you get the one for the AOD.

Darren

Trying to figure speedometer issue out

Reply #5 – July 15, 2014, 10:08:55 PM

Another thing that could contribute to speedo issues like that is what is the rim/tire combo on it. My last tc had 235/60/15 tires and that along with an incorrect gear put in with the 5 speed swap made it 10-15 mph off dependent on speed I was actually traveling.

Trying to figure speedometer issue out

Reply #6 – July 28, 2014, 12:05:44 AM

^^^^^I second Tbird-fanatic. I can tell you that my 86tbird had 205/70/R14 Tires. I upgraded to 235/55/R15 :)

This site can help figure out how much different your tires are from stock: http://www.tacomaworld.com/forum/tirecalc.php?tires=205-70r14-255-60r15

Side note, changing the speedo gear is easy, just a c-clip :)