speedo gear calculator

[87coupe50]

Not sure if this has already been posted but through my searches of the site I haven't come across anything like it.

I was lookin on EBAY to find a black 20 tooth speedo gear and came across this calculator. http://www.sccoa.com/faq/speedgr.html Not sure on how accurate but it seems to be right on in my case. I have a Tremec 5 speed which I believe has a 7 tooth yellow drive gear and was told the rear end was 3:73. I am running 275/45 17 inch tires. It had a white 23 tooth speedo gear which once the speedo cable was fixed and hooked up showed about 10 Mph low at 60 Mph. I bought a red 21 tooth gear which I had been told would be the right gear but it still came up 3/4 Mph low. I figured dropping another tooth would correct another 3 Mph and get me as close as possible. According to the calculator with my setup it shows that I would need a speedo gear with 19.7 teeth, so a 20 tooth gear would be the closest possible solution.

With all the questions of what gear do I need maybe this could be a sticky or added to the index thread

 

[TRWXXA]

Most of the speedo gear calculators I've tried are pretty good -- at least as good as the accuracy of the numbers the user inputs, anyway.

For your setup, the formulas and calculators say you need a 19.7-tooth gear. The 20-tooth gear should get you within 1.5% -- which is very good. If you're saying your speedo is within 3/4 (three-quarters) of a mph @ 60mph, I wouldn't change a thing. If you're trying to say it's "3 to 4 mph" too low, then you should try one less tooth.

BTW: You still haven't said how you're determining your speed. How do you know it's accurate?

I have found that no matter what a calculator says, be prepared to go up or down a gear size to get your speedo closest to accurate. There are variations in tire heights from manufaturer to manufacturer, and tire height changes with tire pressure, etc..

For those who like to do it themselves, here are formulas for the most common speedometer gear, and rear gear questions:

"What will my engine RPM be if I get these gears?"

Engine RPM = 336.1352 / Tire Height X Axle Ratio X Transmission Gear Ratio X MPH

"Which speedometer gear do I need?"

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

"How fast am I going at this RPM?"

MPH = Tire Height X 0.0000496 X Engine RPM / Transmission Gear Ratio / Axle Ratio X 60

"What rear gears do I have?"

Rear End Ratio = Tire Height X 0.0000496 X Engine RPM / Transmission Gear Ratio X 60 / MPH

"What is my tire height?"

Tire Height (inches) = Tread Width (mm) X Aspect Ratio / 1270 + Wheel Diameter (inches)

 

[Bluemr24]

93 aod question

I have not taken my cable off the tranny to see which driven gear is in mine, but maybe somebody knows which one I should have. My combo is 5.0, aod, 3.55
rear, and stock height tires on pony wheels. My speedo is much too high,
reads probably 20 percent over. Should I increase the number of teeth to read lower?
Help! Thanks, MikeR

 

[1990MustangLX50]

here ya go. straight from 5.0resto.com

Ring & Pinion Speedo Gear Part Number
1983-89 Mustang Manual 5-Speed
3.08 18 Tooth LRS-17271H
3.27 19 Tooth LRS-17271A
3.55 20 Tooth LRS-17271B
3.73 21 Tooth LRS-17271C-K
4.10 23 Tooth LRS-17271D-K
1990-93 Mustang Manual 5-Speed
3.08 20 Tooth LRS-17271B
3.27 21 Tooth LRS-17271C-K
3.55 23 Tooth LRS-17271D-K
1984-89 Mustang Automatic AOD
3.27 19 Tooth LRS-17271E
3.55 20 Tooth LRS-17271F
3.73 21 Tooth LRS-17271G
4.10 23 Tooth LRS-17271J-K
1990-93 Mustang Automatic AOD
2.73 19 Tooth LRS-17271E
3.08 20 Tooth LRS-17271F
3.27 21 Tooth LRS-17271G
3.55 23 Tooth LRS-17271J-K
1996-98 Mustang Cobra Manual 5-Speed
3.27 19 Tooth LRS-17271A
3.55 20 Tooth LRS-17271B
3.73 21 Tooth LRS-17271C-K
4.10 Speed Cal LRS-SPDCAL2
1994-98 Mustang Manual 5-Speed
Except 1996-98 Cobra
3.08 20 Tooth LRS-17271B
3.27 21 Tooth LRS-17271C-K
3.55 23 Tooth LRS-17271D-K
1994-98 Mustang Automatic AOD
2.73 19 Tooth LRS-17271E
3.08 20 Tooth LRS-17271F
3.27 21 Tooth LRS-17271G
3.55 23 Tooth LRS-17271J-K

 

[87coupe50]

I used my GPS to determine speed. I've used it in several other cars and the GPS seems to be dead on at a constant speed.

I used the list of gear sizes from the Useful Technical Thread Index, that shows that I would need the 21 tooth gear. Which was still 5-6.6% off / 3-4 Mph.

The link in the first post takes you to a calculator which if you enter your drive gear # of teeth, tire height and gear ratio then it will tell you how many teeth on the speedo gear you need. If you click the other link you can enter you tire info: 275/45 17: and it will calculate the tire height for you. Then click back to the original screen and enter all your calculations.

Bluemr24, you would need more teeth on the gear to show a slower speed on the speedo. According to the Useful Technical Thread Index at the top of the 5.0 TECH page: "speedo gear part numbers and information" you would have an 8 tooth drive gear.

 

[TRWXXA]

I have not taken my cable off the tranny to see which driven gear is in mine, but maybe somebody knows which one I should have. My combo is 5.0, aod, 3.55
rear, and stock height tires on pony wheels. My speedo is much too high,
reads probably 20 percent over. Should I increase the number of teeth to read lower?
Help! Thanks, MikeR

We need to know what year AOD. AODs before mid-year 1989, had a 7-tooth drive gear. After mid-89, they got an 8-tooth drive gear. "Stock" tires were 225/55-16 -- 25.7" in height. Can I assume you have a '91 or later car since that's when the Pony wheels became available?

A 7-tooth drive gear would require a 19-tooth (tan) or a 20-tooth (orange) driven gear (the calculator says "19.5 teeth").

An 8-tooth drive gear would require a 23-tooth (white) driven gear. The calculation says, "22.3 teeth", but there is no 22-tooth gear. 23 teeth will get you to about a 3% error, while 21 teeth would be almost 6%.