Speedo is off, why and how to fix?

opihinalu

Active Member
Feb 10, 2021
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Florida
Hi all. I have a 86 GT with an AOD. Ever since I got the car, the speedo has been off. From around 0-35 mph it is about 5 mph ahead. From around 35-55 mph it is about 10 mph ahead. From around 55-80 mph it is about 15, and so on. Could this be because of a different gear ratio? I don’t know what a regular gear ratio would feel like but I can say I am running around 2.3k rpm at 70 mph. How can I fix this?
 
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It could be gear ratios, tire diameter, or a combination of those factors. The only way to correct the issue is to replace either the speedometer drive or driven gear (maybe both) - OR - if it's your tire diameter find the correct diameter tire.
 
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It could be gear ratios, tire diameter, or a combination of those factors. The only way to correct the issue is to replace either the speedometer drive or driven gear (maybe both) - OR - if it's your tire diameter find the correct diameter tire.
Thank you. What tire diameter was stock on these cars? Also how can I find out my gear ratio?
 
Google search comes up with 225/60-15, which sounds right once I heard it. From there you can find a tire size calculator to do some searching, one like is on the net here.

If your car has not been modified there should be a tag on the rear differential cover. If it's not there, you can get a rough idea by turning the wheel one revolution and counting the revolutions of the driveshaft. If the driveshaft rotates 3 and a half turns, it's most likely a 3.55 ratio. If it only rotates about 2-3/4 times or about 4 times, it's likely a 2.73 or a 4.11 (respectively), and so on. The best way to determine would be to remove the cover, count teeth on both the ring gear and the pinion gear and do the math if it's not stamped on the ring gear anywhere.
 
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Yes, stock tire diameter on 85-93 Mustangs is 25.8" (it can vary a little by tire manufacturer).

The best way to determine would be to remove the cover, count teeth on both the ring gear and the pinion gear and do the math if it's not stamped on the ring gear anywhere.
Some gear sets will have the ratio stamped on the ring gear as well.

20220219_130612.jpg

I have a spreadsheet with various tire sizes and gear ratios. Checking it against your above 2300 rpm @ 70 mph, I get a stock AOD in OD, running stock height tires and a 3.73 rear gear.
 
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We need to know your:

Tire size
What speedo drive gear is in the car
What speedo driven gear
What rear end gears are in the car


I believe all AODs are 8-tooth drive gears, so really need to know what color/tooth count driven gear you have and what rear ended gears and tire size
 
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I think I'm around the same RPM with the 3.73s I just put in, that's probably what you have. Definitely not stock. On another note, changing gears is not the ONLY way to fix it, a Dakota Digital cluster allows you to recalibrate digitally.....and is way less practical and more expensive :jester:
 
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The following info on OEM transmission driven gear was copied from the LMR Speedo Gear Calculator:

  • 6 Tooth
  • Select this if you have a:
    • T5 transmission that has been converted to a 6 tooth gear to correct the speedometer more accurately. The black 6 tooth internal drive gear was originally used in the 4-speed transmission, but it works great to exactly correct for 4.10s when used with a pink 19 tooth driven gear.
  • 7 Tooth
    • Select this if you have a:
      • Factory 1983-1989 Mustang T5 or AOD
      • Ford Racing M-7003-Z World Class T5
      • Tremec 3550, TKO, TKO 500, TKO 600, TKX
      • Tremec T56 Magnum
      • 1996-1998 Cobra T45
    • Or you are converting your 1990-1995 Mustang T5 to a 7 tooth drive gear
  • 8 Tooth
  • Select this if you have a:
    • Factory 1990-1995 Mustang T5, AOD, AODE
    • 1996-1998 GT T45, AODE, 4R70W
    • 1994-1998 V6 T5, AODE, 4R70W

So if the OP's transmission is original, he should have a 7 tooth driven gear (this is a good thing).

My 90 AOD had the 8 tooth gear and with 3.73s I needed the black 23 tooth speedo gear. It would last about a year before taking the teeth off (it is known that the 23 tooth speedo gear teeth are too thin). With my 94 GT T5 I put a 7 tooth driven gear and used the red 21 tooth speedo gear that came in the trans (the car it came out of had 3.27s - so 8 tooth driven gear needed the 21 tooth with the 3.27 rear gears). All good now.

One way I have checked the number of teeth on the driven gear in the trans was to pull the speedo cable and VSS, and mark a tooth on the gear with a sharpie. Then rotate the tailshaft counting the tooth bumps until I got back to the marked tooth. This can be done without disassembling the trans to check the gear.

For those that do not know how to count the number of teeth on a driven gear, you count the tooth ends when viewed from the side of the gear. The pic is a 7 tooth gear.
Driven Gear.jpg
Sorry the text is small font.
 
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Wow guys. This is all very helpful information. So if I have a 3.73 gear ratio like @KRUISR said and I have a 7 tooth driven gear, what speedo gear do I need? What if I switched to 4.10s? How do I calculate what speedo gear I need. Sorry I still don’t really understand much about gear ratios.
 
The drive gears came in a few sizes (colors)
Same with the driven gear roughly 11 to 19 teeth over the years all different colors
If your sppedo is reading fast you need to slow down the cable
That means you need 1 more tooth on the driven gear
The big gear that goes on the output shaft is the drive gear
You take one tooth off that big gear if you cannot get it any slower using the driven gear adjustments
 
AOD's only have an 8-tooth drive gear which is machined into the output shaft. It cannot be swapped out like on the T5 cars.

We need to know the driven gear tooth count or color, and AOD gears are slightly differerent from manual trans gears. I beleive tooth count will vary from 16 teeth (2.73's) up to 23 tooth (3.73/4.10 and beyond)
 
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Yes, stock tire diameter on 85-93 Mustangs is 25.8" (it can vary a little by tire manufacturer).


Some gear sets will have the ratio stamped on the ring gear as well.

20220219_130612.jpg

I have a spreadsheet with various tire sizes and gear ratios. Checking it against your above 2300 rpm @ 70 mph, I get a stock AOD in OD, running stock height tires and a 3.73 rear gear.
Care to share that chart?