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Speedometer Accuracy

Our friends at the 428 Cobra Jet Registry have an online calculator that will do the work for you. Read their page carefully. The calculator will take into account the adapter/reducer if you check the box. Their page will also give results for driven gears with over 20 teeth. http://www.428cobrajet.com/speedo-calc.html
Read on if you would like to know the history or do the math manually:

SPEEDOMETER GEARS

By Erich Bozzer

Have you changed rear-end gears, tire size or maybe transmission and now your odometer and speedometer are inaccurate? Or maybe you suspect some changes were made since your speedometer and odometer are inaccurate. Before you proceed to fix anything, you must first understand how the speedometer and odometer work. The speedometer cable comes up to and is connected to the back of the instrument cluster. The cable is connected directly to the odometer mechanism. It is then connected through a magnetic coupling to the speedometer pointer. This means that your odometer can be correct but your speedometer may be incorrect or vice versa. If your odometer is correct as measured against the mile markers out on the interstate or highway and your speedometer is incorrect then the problem is in the speedometer itself. If the odometer is incorrect then your problem is in the transmission drive or driven gears on the end of the speedometer cable. A +/- 3.75% error is considered acceptable by design, that is a +/- 0.375 mile error in 10 miles. For proper operation the odometer should be corrected first and the speedometer should be adjusted if required after the odometer is correct.

Fixing the transmission drive/driven gears is done by changing the plastic gear clipped to the end of the speedometer cable that goes into the transmission called the driven gear or by changing the gear attached to the output shaft inside the transmission called the drive gear. By knowing the diameter of your tires and the rear axle final gear ratio, you can now go to the table and pick the right driven and drive gear combination for your car. Note: It is important to know that this table applies only to Toploader 3 and 4 speeds with the cable entering the transmission on the passenger side of the car. In addition, you cannot use the table if you are using the Ford adapter/reducer that attaches between the transmission and speedometer cable.

Theory of Operation:
The way to get the whole system to work right is to make the speed cable rotate 1,000 revolutions for every mile the car is driven. This is done by adjusting the transmission driven to drive gear ratio on the speed cable to match the tire size and rear axle gear ratio of the car.


The first thing to know is how many revolutions the tires make in one mile. To calculate this, measure the diameter of your tires. 26" is about average for F60-15's or 245-60-15's. Multiply this measurement (26") by pi (3.1416). This will give you the circumference of the tire. Next, divide one mile in inches (63360) by the circumference of tire (63360 divided by (26 x pi)=776). This equals the revs per mile of the tire. Now multiply this number by the rear axle ratio to give you the number of drive shaft revs per mile (776 x 3.91= 3033). Next, divide this number by 1000 (3033 divided by 1000 = 3.033). This is the ratio of transmission driven/drive gears on speedometer cable you will need in this case use a driven gear with 18 teeth and a drive gear with 6 teeth (18 divided by 6 = 3.0). This is the best choice which yields a 1.1% high error which means your odometer unit indicates 1.1% greater than actual or 10.11 for an actual 10.0 miles. If you are using the Ford adapter/reducer (reducer was factory equipped on cars with 3.91 or 4.30 rear axle ratios **), multiply the number by 0.75 (3.033 x 0.75=2.274). This is the driven/drive ratio you need. Note: speedometer cables are different when using the adapter.


When picking a driven/drive gear ratio to match the tire and rear axle gear ratio you have a choice of two drive gears and five driven gears. The drive gear is inside the transmission and the tail housing must be removed to get at it. The driven gear clips onto the end of the speed cable. These plastic gears are unique to the old manual Toploader transmission and cannot be interchanged with the gears in an automatic transmission or today's manuals with the internal* shift linkage.

The uniqueness is due to the helix direction of the gear teeth. This is the curvature as seen when looking at the gear teeth. The automatic transmissions and today's manuals (T5* and SROD) are right hand helix. The speedometer cable enters the transmission on the driver's side of the car. The old manual transmissions (Toploaders) with the shift linkage outside of the transmission are left hand helix. The speedometer cable enters the transmission on the passenger side. All driven gears (speedometer cable side) are color coded whether they're used with an automatic or manual transmission The automatic and today's manuals are molded in color. The old manuals driven gears are molded in natural colored plastic (off-white) and the tips are color coded with paint. The drive gear (inside transmission) is also color coded, pink or black. You may be able to see what gear your transmission has by looking in the hole where the speedometer cable goes with a light. Chances are very good that you have the pink gear.

To use these tables: Locate tire diameter row and locate rear axle ratio column in the first table. The number 1-10 indicates the gear combination # needed. Take this combination # to the second table to find the number of teeth each gear will need. The third table has the number of teeth, color code and part #.

Tire Size Diameter Inches 

Revolutions  per mile

Rear Axle Ratios

Gear Combination #1-10, (% error)

3.50 3.91 4.11 4.30
27 747 3 (1.7 HI) 7 (2.2 HI) 8 (2.3 HI) 9 (1.4 HI)
26 1/2 761 4 (0.1LO) 8 (0.8 LO) 9 (1.2 LO) 10 (1.8 LO)
26 776 5 (0.1 HI) 8 (1.1 HI) 9 (0.7 HI) 10 (0.1 HI)
25 1/2 790 5 (1.9 HI) 9 (2.5 LO) 10 (2.6 LO) 10 (1.9 HI)
25 806 6 (0.4 LO) 9 (0.5 LO) 10 (0.6 LO) *  (4.0 HI)
24 1/2 823 7 (0.8 HI) 9 (1.6 HI) 10 (1.5 HI) *  (6.2 HI)
24 840 8 (2.0 LO) 10 (1.5 LO) 10 (3.6 HI) *  (8.4 HI)
  * %, error too high using combination  #10
Gear Combinations
Driven # teeth/Drive # teeth = Ratio
1 - 16/7 = 2.286 5 - 19/7 = 2.712 8 - 18/6 =3.000
2 - 17/7 = 2.429 6 - 17/6 = 2.833 9 - 19/6= 3.167
3 - 18/7 = 2.571 7 - 20/7 = 2.857 10 - 20/6= 3.333
4 - 16/6 = 2.667  
# Teeth
(Left Hand Helix) Color Code Service Part # Engineering Part #
Drive Gears 6 Pink C5ZZ17285 A* C8ZR17285 A
7 Black C4DZ17285 A C8OR17285 A
Driven Gears 16 Orange C2DZ17271 J C2DA17271 E
17 Purple C2DZ17271 G C2DA17271 G
18 Green C2DZ17271 K C2DA17271 F
19 Pink C4DZ17271 A C4DA17271 A
20 Blue C2DZ17271 H C2DA17271 H
* Corrected from original newsletter May, 1987, and 2nd Registry, 1992.

** Added 11/22/2001.

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This page last updated: January 03, 2015