ok enough of this speedo BS

riceslayer302 · Nov 14, 2008

ive been trying to solve this POS problem for over a year now... i have a 1990 T5, 8 tooth drive gear. I have (i am 90% sure) 3.55's in the rear... wtf driven gear do i need!?!?!? mustangs unlimited has a chart in their catalogue but its a pos, ive gotten the wrong gear twice from them.

i know a million people out there run 3.55's w/ a 1990 T5, what gear are you using? red 23 tooth?

Beasty306GT · Nov 14, 2008

Ring & Pinion Speedo Gears
1983-89 Mustang Manual 5-Speed
3.08 18 Tooth
3.27 19 Tooth
3.55 20 Tooth
3.73 21 Tooth
4.10 23 Tooth
1990-93 Mustang Manual 5-Speed
3.08 20 Tooth
3.27 21 Tooth
3.55 23 Tooth
1984-89 Mustang Automatic AOD
3.27 19 Tooth
3.55 20 Tooth
3.73 21 Tooth
4.10 23 Tooth
1990-93 Mustang Automatic AOD
2.73 19 Tooth
3.08 20 Tooth
3.27 21 Tooth
3.55 23 Tooth
1996-98 Mustang Cobra Manual 5-Speed
3.27 19 Tooth
3.55 20 Tooth
3.73 21 Tooth
4.10 Speed Cal
1994-98 Mustang Manual 5-Speed
Except 1996-98 Cobra
3.08 20 Tooth
3.27 21 Tooth
3.55 23 Tooth
1994-98 Mustang Automatic AOD
2.73 19 Tooth
3.08 20 Tooth
3.27 21 Tooth
3.55 23 Tooth

TRWXXA · Nov 15, 2008

-sigh-

Are you running stock size tires, or something else?

Do you KNOW you have 3.55s, or just "think" you have 3.55s?

Are you positive you have a 8-tooth (green) drive gear in the tranny?

You need to KNOW what gear ratio you have. A bunch of folks on here thought they had one gear ratio, then ultimately discovered they had something else.

Put the rear of your car up on jack stands, with the tranny in neutral and parking brake off. Mark one rear wheel with a piece of tape, and put a mark on the driveshaft with tape or Sharpie. Have a helper slowly rotate the wheel exactly 10 times, while you count the number of times the driveshaft turns. 35 times = 3.55ss. 33 times = 3.27s. 37 times = 3.73s. You might be lazy and tempted to just rotate the wheel only once, but it is very hard to tell the difference between 3 1/2 turns from 3 1/4 or 3 3/4 turns -- trust me.

Here's a couple of formulas you need to know:

# Driven Gear Teeth = 20.168114 / Tire Height (inches) x Axle Ratio x # Drive Gear Teeth

Tire Height (in.) = Tread Width (mm) x Aspect Ratio / 1270 + Wheel Diameter

It is unlikely that even the correct speedo gear will get your speedo perfectly accurate. Just a change of 1 tooth either way changes the speedometer by about 5%, so it's possible that your tranny, axle, and tire combination might fall between two gears. A manufacturer would use the gear that makes it read too high, since speedos are not allowed to read too low from the factory.

Mustang5L5 · Nov 15, 2008

riceslayer302 said:
i know a million people out there run 3.55's w/ a 1990 T5, what gear are you using? red 23 tooth?

The red gear is a 21-tooth.

The 23-tooth gear is white.

If you are POSITIVE you have 3.55's and a GREEEN 8-tooth drive gear inside your t-5...then you need the white 23-tooth drive gear. Order a few...these like to strip out

riceslayer302 · Nov 15, 2008

TRWXXA said:
-sigh-

Are you running stock size tires, or something else?

Do you KNOW you have 3.55s, or just "think" you have 3.55s?

Are you positive you have a 8-tooth (green) drive gear in the tranny?

You need to KNOW what gear ratio you have. A bunch of folks on here thought they had one gear ratio, then ultimately discovered they had something else.

Put the rear of your car up on jack stands, with the tranny in neutral and parking brake off. Mark one rear wheel with a piece of tape, and put a mark on the driveshaft with tape or Sharpie. Have a helper slowly rotate the wheel exactly 10 times, while you count the number of times the driveshaft turns. 35 times = 3.55ss. 33 times = 3.27s. 37 times = 3.73s. You might be lazy and tempted to just rotate the wheel only once, but it is very hard to tell the difference between 3 1/2 turns from 3 1/4 or 3 3/4 turns -- trust me.

Here's a couple of formulas you need to know:

# Driven Gear Teeth = 20.168114 / Tire Height (inches) x Axle Ratio x # Drive Gear Teeth

Tire Height (in.) = Tread Width (mm) x Aspect Ratio / 1270 + Wheel Diameter

It is unlikely that even the correct speedo gear will get your speedo perfectly accurate. Just a change of 1 tooth either way changes the speedometer by about 5%, so it's possible that your tranny, axle, and tire combination might fall between two gears. A manufacturer would use the gear that makes it read too high, since speedos are not allowed to read too low from the factory.

i know its a dumb question that a million people have asked... im not retarded or anything i know what im doing, i dont exactly know what gears i have in the car. i am pretty sure they are 3.55's, last year i matched speeds and rpm with my friends car on the highway and he has 3.55's. i matched up the rpm and speed on a gear ratio calculator and it came up as 3.55's. the car is torn apart right now i matched the trans codes and it is a 1990 T5. I WAS using 16" wheels with 225's, this year ill have 15" with 275/60s.

ill figure it out, i appreciate the responses guys.

TRWXXA · Nov 16, 2008

For 3.55s, with an 8-tooth drive gear, and 275/60R15s:

Tire Height (in.) = 275 x 60 / 1270 +15 = 27.99"

# Driven Gear Teeth = 20.188114 / 27.99 x 3.55 x 8 = 20.46 ( use a 20-tooth, black gear)

But be prepared to still need a 21-tooth or 19-tooth gear for better accuracy, since no chart or calculation can account for every variable than can affect your speedo.

MustangLX-5.0 · Nov 17, 2008

No listing for a 92 AOD with 3.73's.I'm installing a set of FRPP 3.73's and finding that gear for that year in an AOD is nearly impossible.

riceslayer302 · Nov 17, 2008

i have a black 20 tooth in it right now, ill let you know how it works

TRWXXA · Nov 18, 2008

MustangLX-5.0 said:
No listing for a 92 AOD with 3.73's.I'm installing a set of FRPP 3.73's and finding that gear for that year in an AOD is nearly impossible.

23-tooth is as close as you can get without installing a special gearbox on the back of your speedometer. If you have stock size tires you'll only be about 2% to 5% off.

ok enough of this speedo BS

riceslayer302

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