Question on speedo gear.....

[MY 85 GT]

Ok i had my gears changed from 3.27's to 3.73's....He did change the speedo gear to 23 tooth ...I have a tomtom and the speedo is off like 6 mph...did he put the wrong speedo gear in?....which one should be in there with a AOD and 3.73's...maybe he put a 5 spd one in by accident?....thanks...

 

[Mustang5L5]

Up to 1990, Purple 21-tooth

1990-later AOD...white 23-tooth

 

[MY 85 GT]

Thanks that whats in there but its off...Before i did the gears the speedo was 100% dead on with the TOMTOM..now its 6 mph off after the gear install....and it has a white 23 tooth gear in there...why is it off?

 

[Mustang5L5]

Is it higher or lower?

I don't beleive the white 23tooth is dead on accurate. I think it's just "as close as you can get"

 

[MY 85 GT]

Its lower...if im going 60 mph my tomtom is saying like 65/66 mph...and it was dead on before i changed the gears...

 

[TRWXXA]

Your speedometer is reading too low, so the speedometer's driven gear has too many teeth. Try a 21-tooth gear. The 23-tooth gear is for 3.73s in later AODs (1989+, IIRC).

The 21-tooth may not get it perfect, but it'll be better if it reads a little high. Don't expect your speedometer to read 100% accurate with 3.73 gears, unless you stumble upon the right speedo gear/tire size combination.

 

[Mustang5L5]

Is this in your '87GT?

You need the purple 21-tooth gear for 3.73's in that year

 

[1991notchbackLX]

Shouldn't we also be asking how tall a tire he's running in the rear??

From everything I've heard, the white 23 tooth will give you an "as close as you can get" approximation of your actual speed. I have a '91 with a 5 speed and 3:73s and I have a white 23 tooth, I don't necessarily care exactly how fast I'm going though. The overwhelming majority of the time I'm driving my eyes are on the tach, oil pressure, and temperature gauge anyway.

 

[TRWXXA]

From everything I've heard, the white 23 tooth will give you an "as close as you can get" approximation of your actual speed.

That is true for 1989 and later AODs, but not for all years. Sometime late in the 1988 MY, the speedometer drive gear (on the tranny's output shaft) was changed from 7 to 8 teeth. The correct speedometer driven gear for the 8-tooth drive gear and 3.73 rear end (with stock size tires) is the white, 23-tooth gear.

With a 7-tooth drive gear, it's either going to be a 21- or 20-tooth driven gear, depending on tire size.

Since MY 85 GT's speedo is reading about 10% too low, he should go from a 23-tooth gear down to a 21-tooth gear (a 9% change). A 20-tooth gear would make his speedo read about 15% faster than the 23-tooth gear he has now.

If anyone is interested in figuring out what speedo gear they need for various rear end/tire combinations, the formulas are:

# of Driven Gear Teeth = 20.168114 / Tire Height x Axle Ratio x # Drive Gear Teeth

Tire Height (in.) = Tread Width x Aspect Ratio / 1270 + Wheel Diameter
Example: 215/65R15 -- 215 (tread width) / 65 (aspect ratio) R 15 (wheel diameter)
215 x 65 / 1270 + 15 = 26.00”