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Making Your Speedometer Gear Accurate
By Dan Davis

First off, 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, 4.11 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. A 3.75% error (i.e., a 0.375 mile error in 10 miles) is considered acceptable by design.

When picking a driven/drive gear ratio to match the tire and rear axle gear ratio you have a choice of drive gears and driven gears.

The drive gear is inside the transmission and the tail housing must be removed to get at it. You may be able to see what gear your transmission has by looking in the hole where the speedometer cable goes with a light. Manual transmissions have a plastic gear pressed onto the output shaft. The manual trans drive gear is also color coded: pink (6 teeth) or black (7 teeth). Toploaders use the "type 4a" drive gear while automatics use a gear machined to the output shaft with either 8 or 9 teeth. Automatics have a metal gear machined into the output shaft. To change the latter, the whole output shaft must be changed, a big job.

The driven gear clips onto the end of the speed cable. These plastic gears are unique between the manual Toploader transmission and the gears in an automatic transmission. 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 are right hand helix (known as type 3). The speedometer cable enters the transmission on the driver's side of the car. The Toploader transmissions with the shift linkage outside of the transmission are left hand helix (known as type 3a). The speedometer cable enters the transmission on the passenger side.

All driven gears are color coded whether they're used with an automatic or manual transmission The automatic and today's manuals are molded in color. The Toploader driven gears are molded in off-white colored plastic and the tips are color coded with paint.

Your choices are (tooth count and color code): 16 (blue for AT, orange for MT), 17 (green AT, purple MT), 18 (grey AT, green MT), 19 (tan AT, pink MT), 20 (orange AT, blue MT), 21 (purple AT, red MT) and 23. The 23 tooth gear is from Saleen ( http://www.saleen.com/store2000/detail_new.asp?car=saleen&prodid=047-280 ) and costs $22, but the teeth are thin and the gear will wear out every couple of years. If you cannot make it work with this stuff, you have one more option -- get a reducer box like Ford used with Drag Pak cars. The original is spendy (~$350), but a generic box can be had from a speedo calibration shop for a lot less. I would chose this method over changing the output shaft on an AT unless I had to remove the trans anyway

Here is a handy chart to chose your gear combo after you have done the math shown in the first paragraph. The ranges shown comply with the Ford approved +/- 3.75% allowable error:

Toploader choices:
16/6 = 2.567-2.767
17/6 = 2.728-2.940
18/6 = 2.888-3.113
19/6 = 3.048-3.286
20/6 = 3.209-3.459
21/6 = 3.369-3.631
23*/6 = 3.690-3.978

16/7 = 2.200-2.372
17/7 = 2.338-2.520
18/7 = 2.475-2.667
19/7 = 2.610-2.814
20/7 = 2.750-2.964
21/7 = 2.888-3.113
23*/7 = 3.163-3.409


C4 & C6 choices:
16/7 = 2.200-2.372
17/7 = 2.338-2.520
18/7 = 2.475-2.667
19/7 = 2.610-2.814
20/7 = 2.750-2.964
21/7 = 2.888-3.113
23*/7 = 3.163-3.409

16/8 = 1.925-2.075
17/8 = 2.045-2.205
18/8 = 2.166-2.334
19/8 = 2.286-2.464
20/8 = 2.406-2.594
21/8 = 2.527-2.723
23*/8 = 2.767-2.983

16/9 = 1.711-1.845
17/9 = 1.818-1.960
18/9 = 1.925-2.075
19/9 = 2.032-2.190
20/9 = 2.139-2.305
21/9 = 2.246-2.420
23*/9 = 2.460-2.652
* = Saleen part, not Ford part

Hope this helps!

--
Regards,
Dan Davis

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Another Speedo Gear resource that may be of help is located on the 428 Cobra Jet Mustang Registry website;

Mustang 428 Cobra Jet Speedometer Gear Calculator