View Full Version : What Speedo Gear

12-31-2004, 04:39 PM
Well my car is nestled in the garage for the winter months and I'm planning on finishing the suspension changes I plan on making and getting it totally road worthy for spring. In my axle swap, I've gone from a stock 2.73 rear gear to the turbo coupe's 3.73 gearing. My question is, what speedo gear do I need to get to correct my speedometer (yeah, its waaayyy off) and where do I need to go to get it. Thanks guys.

12-31-2004, 08:59 PM
Dealer item... You'll probably need a 21 tooth(the most teeth on a Ford gear), and it may not slow the speedo down enough. There are 23 teeth gears avail from the Mustang vendors, but its a MOPAR gear.. They are said to last about 5-10K mi.

01-02-2005, 12:34 PM
What's with the 23 tooth not lasting very long? I'd like to get it as close as possible. I guess I'll go to a dealer (or ask a Ford mechanic friend of mine) and see what I can find out about the 21 tooth. Thanks.

01-17-2005, 12:10 AM
I have a 21-tooth gear in my '88 w/3.45 gears & 25.6" tires & it reads too fast. It would read even faster w/3.73's. It reads 50mph at a true 46mph.

http://tciauto.com/components/speedometer_gears.htm#ford shows the 23-tooth gear. I heard it's a Jeep part.

01-17-2005, 07:16 AM
pretty sure if you've got 2.73s and an aod then you probably have the stupid 8 tooth drive gear in the trans and it is not changeable unless you tear down the entire trans.
You will need the 23 tooth.
I had one once before i got my mustang aod which has the 7 tooth drive gear and it started losing teeth after about 3 weeks.
That's the reason ford stops at 21,the teeth become to thin and start to wear very quickly.
My trans now requires a 21 and it works mint.

01-17-2005, 07:21 AM
Actually jim your speedo is correct,4mph inacuracy is aceptable for any speedo.
If you goto a 25inch tire it will read perfect.
You also have the 8 tooth drive.3.45s is the max you can go for it to read acurate.
otherwise you will have to use the 23tooth.

Chuck W
01-17-2005, 09:57 AM
I know the older FRPP catalogs used to have the equation listed in them to help choose the correct gear.

The 23T ones don't last very long because the teeth are VERY thin, and they also don't mesh 100% with the Ford drive gear.

01-17-2005, 10:11 AM
You can figure out what you need here:



01-17-2005, 08:11 PM
thanks guys

04-19-2005, 03:34 PM
I'm still having a hard time finding a 23-tooth speedo gear for my '88. I just want to get one in my stock tranny for the summer until my tranny/motor rebuild comes along winter 05/06. Does anyone recommend a certain company/place to get it? I do have the 8 tooth gear drive in my AOD right? Help me please!

04-19-2005, 03:59 PM
Ebay is where i got mine,just search aod speedo,think i paid 10.00us for it.
cj pony parts was the company i think.

04-19-2005, 04:52 PM
Are you sure that I have an 8 tooth speedo drive gear? I've been looking on some of the mustang sites and they say everything earlier than an '89 had the 7 tooth. If so I would need the 21 tooth speedo gear.

04-19-2005, 08:38 PM
I always thought the earlier trannys had the 8 tooth gear, my '90 SC 3.8 AOD has the 7 and a 21 works for me with the 3.73s.

BTW each tooth added to the driven gear slows the reading approx 3mph at 60 mph...

04-20-2005, 09:15 AM
Ok thanks. On a side note, awesome pic in your sig TC50!

05-09-2005, 01:19 AM
http://novaresource.org/speedo.htm has some formulas that might be helpful.

05-09-2005, 09:20 AM
I put the 21 tooth in...seems to be pretty close now