Speedo Gear Replacement

dz01

Member
Mar 31, 2005
274
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Massachusetts
I decided to replace my Vehicle Speed Sensor (VSS) in my 1993 to chase an idle problem (#12 or so on the list). When I pulled the sensor, I noticed the plastic gear on the end of the sensor was pretty beat up i.e. missing some of the teeth and just generally worn. The gear is white and has 23 teeth.

NOTE- When I bought my car I was told 3.55 gears were installed, but the transmission "gear" wasn't changed out right away. It was about 10K miles later. I didn't realize there are two gears, one on the end of the VSS and the mate inside the tranny, so I have no idea what was "replaced", one or both?

I did the wheel spin test and my drive shaft spins about 3.5 turns with one rotation of the wheel so I guess a 3.55 gear ratio is likely.

I need a new speedo gear, but don't know what to buy. Based on a gear calculator on line I see three options since I don't know if my drive gear is a 6, 7 or 8 (stock). Here's are the specs of my car:

5-spd
Tires 235/40/18
3.55 gears

a 6 drive gear is a 17 tooth, a 7 drive gear is a 20 tooth and an 8 drive gear (stock) is a 23 tooth.

I tried to see what color the drive gear was, but it seems white/off white or clearish. Figured I could tell if it were yellow, red or black, but hard to see.

Thoughts? Barring anything else, I guess I can put the same thing back in and pick up a 23 tooth gear.
 
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mikestang63

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Aug 27, 2012
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Your T5 if it is from a 1993 is actually an 8 gear internal and the white 23 tooth speedo gear is correct,. 23 tooth speedo gears are long known for getting chewed up quickly, as the driven gear in tne trans is limited to 21 teeth. There are simply too many teeth on the gear that are thin and do not sync with the driven gear. Take a flashlight and look inside the trans at the driven gear. If it is green- which yours will be, then it is 8 tooth and the closest you can get is teh 23 tooth speedo gear. If you want to correct this, you need to replace the green driven gear inside the trans to a yellow 7 tooth, and then you can use a 21 tooth speedo gear. If not, live with your speedo being 10mph off and stock up on 23 tooth gears.

Another great idea from Ford,

Be glad you don't have an AOD. THey dont even list a speedo gear for 3.73 or 4.10's. My speedo about 12-15 mph off at cruising speed.
 

jrichker

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Here's some resources for you...

Available speedo gears:
1979-1993 Mustang Speedometer Gear & Speed Correction

Speedo gear selection chart
Mustang Speedometer Gear Application Guide - LatemodelRestoration.com®

speedo-gear-selection-from-ford-racing-gif.72909
 

dz01

Member
Mar 31, 2005
274
10
19
Massachusetts
Thanks for the info. The drive gear was hard to see, but looked more off-white than green. I'll have to check again. How off would my speedo be if I used a 21 tooth gear with my stock drive gear?

What are the symptoms of a worn gear? I did notice my speedo bounced a little.
 

mikestang63

SN Certified Technician
Aug 27, 2012
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If you put a 21 tooth gear in , the speedometer is going to be pretty much a weathervane. It will be off probably 15-20mph. Your best option would be taking the speedo to a shop and have it calibrated or if still available, places like Dallas Mustang used to sell a device that would adjust for different gears. i think it was called a "speed cal"?

A bouncing speedo could be anything from bad speedo or driven gears, a binding or bad speedo cable, or a bad speedo.
 

dz01

Member
Mar 31, 2005
274
10
19
Massachusetts
Thanks guys. I ordered a 23 tooth for now. I'm hoping the new VSS fixed my idle while stopping problem. RPMs bounce up and down when I come to a stop. This didn't happen until I put in a new throttle body, but base idle should be spot on.
 

jrichker

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The 23 tooth gear may last 15,000 -25,000 miles... not long if the car is a daily driver.

I did some searching and came across this post...

Mustang Speedo Gear chart.JPG



More help - this enables you to change tire size, driven gear teeth or drive gear teeth. All of these need to be taken into consideration when selecting the speedo gear combination.
Speedometer Calibration Program - Method 1
 
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jrichker

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Speedo works good. Car still stumbles a bit sporadically at stop lights which is why I went after the VSS in the first place. Back to chasing my ghost.

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 159,000 hits, which indicates it does help fix idle problems quickly and inexpensively.