Speedo gear help

geostang351 · Mar 12, 2007

I have done some modifications to the rear end this winter and need some help with changing out the rear gear in the tranny to calibrate the speed I'm traveling. I called Mustangs Unlimited and the "know it" guy was out today to discuss this. I'm hoping you guys can help me out.
THis is what we have to work with:

AOD tranny from 89 Grand Marquis
8" rear from 78 Granada 4.11 gears
Tires - BF Goodrich 225 (or 235) 60 15 (25 1/2" tall)

How many teeth am I looking at for the driven gear?

:SNSign:

Ogre · Mar 12, 2007

This might help:

http://www.sccoa.com/faq/speedgr.html

68RCodeConv · Mar 13, 2007

There's a simple way to do this if you are near a freeway with mileage markers. Drive at 60 mph as indicated on your speedo. When you pass a mileage marker start timing. Stop timing at the next mileage marker. If it takes more than 60 seconds to go a mile then your speedometer is "fast" (shows you are going faster than you really are). Take the number of seconds it actually took and divide it by 60. Example: 65 seconds. 65/60 = 1.08 so your speedo is 8% fast. Do this a couple of times to make sure on the timing.

Now pull your speedo gear and count the number of teeth. You need MORE teeth to slow the speedo down. Divide the number of teeth by the next higher number of teeth and note the % difference. Example: you currently have 17 teeth. 17/18 = .94444. 1 - .9444 = .056666 or about 6% different. 17/19 = .89 or 11% different.

You are not going to get it perfect unless you are lucky. In this example you can either be 2% fast (18 tooth = 8% - 6%) or 3% slow (19 tooth = 8% - 11%). It's usually better to be a little fast.

Doing it this way it doesn't matter what your tire size is or how many teeth the drive gear in the tranny has.

dablack · Mar 14, 2007

The above assumes that the mile markers are correct. They aren't always correct. I would do the above over at least a 4 mile stretch so you can average it out.

D.Hearne · Mar 14, 2007

You'll need at least a 23 tooth gear. You cannot change the transmission gear, it's machined into the output shaft. If the 23 doesn't work, you'll need a reducer to make it work with 4.11's

geostang351 · Mar 14, 2007

68RCodeConv said:
There's a simple way to do this if you are near a freeway with mileage markers. Drive at 60 mph as indicated on your speedo. When you pass a mileage marker start timing. Stop timing at the next mileage marker. If it takes more than 60 seconds to go a mile then your speedometer is "fast" (shows you are going faster than you really are). Take the number of seconds it actually took and divide it by 60. Example: 65 seconds. 65/60 = 1.08 so your speedo is 8% fast. Do this a couple of times to make sure on the timing.

Now pull your speedo gear and count the number of teeth. You need MORE teeth to slow the speedo down. Divide the number of teeth by the next higher number of teeth and note the % difference. Example: you currently have 17 teeth. 17/18 = .94444. 1 - .9444 = .056666 or about 6% different. 17/19 = .89 or 11% different.

You are not going to get it perfect unless you are lucky. In this example you can either be 2% fast (18 tooth = 8% - 6%) or 3% slow (19 tooth = 8% - 11%). It's usually better to be a little fast.

Doing it this way it doesn't matter what your tire size is or how many teeth the drive gear in the tranny has.

Say what?

D.Hearne; said:
You'll need at least a 23 tooth gear. You cannot change the transmission gear, it's machined into the output shaft. If the 23 doesn't work, you'll need a reducer to make it work with 4.11's

I could figure this out if I new the gear within the tranny (either 7 ot 8 tooth I think). Thanks D. Looking at some of the catalogs you look correct that 23 is what I'll need.

D.Hearne · Mar 14, 2007

geostang351 said:
Say what?

I could figure this out if I new the gear within the tranny (either 7 ot 8 tooth I think). Thanks D. Looking at some of the catalogs you look correct that 23 is what I'll need.

This is taken from Ford's Official Mustang 5.0 book; the AOD has 7 drive teeth, but doesn't list the gear needed for 4.11's but calls for a speedo driven gear with 20 teeth for 3.73's and the Mustang's normal 25-26" tires. So the 23 should be damned close. The formula for figuring the gear is Drive gear teeth X Axle ratio X Tire revolutions per mile, then divide by 1000.

geostang351 · Mar 15, 2007

D.Hearne said:
This is taken from Ford's Official Mustang 5.0 book; the AOD has 7 drive teeth, but doesn't list the gear needed for 4.11's but calls for a speedo driven gear with 20 teeth for 3.73's and the Mustang's normal 25-26" tires. So the 23 should be damned close. The formula for figuring the gear is Drive gear teeth X Axle ratio X Tire revolutions per mile, then divide by 1000.

Thank You

Speedo gear help

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