Speedo Calibration

[retownsendjr]

I've read here before that there are different speedo drive gears available for various set-ups. If I describe my set-up can you tell me what gear I need (and where to buy one)?
Here goes: Donor 88 GT 302, with T5 and 3.55 rear.
I think the original rear was a 3.08, and my speedo reads way too high, like 15% too high.
I can run my GPS to do a comparison of actual MPH, and what my speedo is reading if that is necessary.
Thanks,
Rich

[Richard Oben]

Pull the speedometer cable from the trans and get the correct color gear (driven) which I think is black, but check to be sure. The other part of the equation is the gear in the trans (drive), if it is 7 tooth you are fine but if 8 it is hard to get the right gear as the teeth get very thin. HTH, Richard.

[Big Blocker]

Rich,

As a general rule of thumb, Fords have always been [somewhat] "adjustable" by 7 miles per hour, per tooth on the driven gear. Assuming that you don't want to tear down your tranny to replace the drive gear, pull your driven gear out and check the stamp mark or count the teeth on it . . . then just add as many teeth as needed to "get close". More teeth = slower speedo.

With your setup, I'd be suggesting 21, probably has 19 now. Speedometers are usually calibrated at 1000 rpm (speedo cable speed). And, I'd suggest that you get it to read faster then you are actually going as a safety overhead . . . better to be doing 65 when the speedo reads 70 then the other way around. Don't ask me how I know this . . .

Doc

[J Persons]

This is the 23 tooth speedo gear that's available from most Mustang parts houses. This one lased six months before eating itself. If you want to calibrate the speedo correctly, you will need to change the drive gear to a seven tooth and the speedo driven gear to a 19 for a 3.55 rear ratio. Most Mustang T5's use an eight tooth gear, depending on the year and application. It's fairly easy to change the drive gear but the trans will need to come out of the car and the tail shaft housing needs to be removed to do it.

[retownsendjr]

Thanks for the information.
I don't plaqn on removing the T5 any time soon (Knock Wood!).
So I like the idea of putting in the 21 tooth driven gear, but will it mesh with the supposed 8 tooth drive gear? I'll go back and read more closely.
The driven gear that I have now is orange. I'll have to pull it to count teeth.
Thanks,

[J Persons]

Originally Posted by retownsendjr

Thanks for the information.
I don't plaqn on removing the T5 any time soon (Knock Wood!).
So I like the idea of putting in the 21 tooth driven gear, but will it mesh with the supposed 8 tooth drive gear? I'll go back and read more closely.
The driven gear that I have now is orange. I'll have to pull it to count teeth.
Thanks,

The first thing you need to do is determine which drive gear you have in the transmission. Remove the speedo gear and cable from the trans, look inside and determine the color. If it is a pale green, it's an 8 tooth gear and the the lowest compatible gear is a 21 tooth red driven gear. This combination is just about perfect for a 3.27 rear end ratio. The ideal combination for a 3.55 rear end ratio is a yellow 7 tooth drive gear and a black 20 tooth driven gear. this is assuming a tire that is approximately the same height as a 225/60X15.

[AC Bill]

Found these if it helps?

http://www.fordracingparts.com/download/charts/129.pdf

http://www.ddperformance.com/PDF/SpeedoChart.pdf

Rich, I gather you have a cable drive speedo, rather than electric?

If it's electric, they allow for quite a bit of calibration, with just a push of a button. Might not even need to change gears.

[retownsendjr]

Yeah, it's a cable, from my donor.
My speedo is the AutoMeter Vintage from FFR.
I won't be able to get it up in the air and check it out for days.
Great info!
Rich

[mike-66]

I just swapped my speedo gear and cable last week with the tranny in the car. It only took about 20 minutes to do and I just jacked up the car. I ended up getting a new cable at Napa Auto Parts and ordered my speedo gear from Steeda Motor Sports.

If your tranny is out of an '88 GT, you should have a 7 tooth tail shaft gear (90 and up, 8 tooth gear). And if you have 3:55 gears in the rear you should use the black 20 gear part # 375-C1DZ-17271-A

Here is the link to buy the correct calibration gears.
Speedometer Recalibration Gears for 1979 thru 1998 Ford Mustang

[SPeace-ATL]

I was about to post much the same thing, but Mike beat me to it by a few hours!

Yes, if it is an '88 transmission it will already have the 7-t YELLOW internal speedometer gear. The aluminum tag on the transmission will likely read 1352-169 to confirm that it is of the original type. This is what most all of the 86-89 GT Mustangs came with.

Your local Ford dealer should have the black 20-t gear for less money than buying it and having it shipped to you. C1DZ-17271-A

(The 21t Red one is C40Z-17271-A)

[JoeE]

Mine did the same thing I just went back to the stock one and I am also about 15% off. Why did the white one get eaten and Will th black Ford part last with a yellow drive gear in the trans?

Originally Posted by J Persons

This is the 23 tooth speedo gear that's available from most Mustang parts houses. This one lased six months before eating itself.

[SPeace-ATL]

Ford only makes the gears up through a count of 21t.

The teeth have to be really thin on the 22t and 23t gears to be able to get that many teeth on the gear, but maintain the required diameter.

They just don't hold up.

The internal gear needs to be changed with the 7t or possibly even the 6t version.