Speedo Gear Selection

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Article taken from a saved copy of Liquid’s tech page.  All credit goes to him!

Measurements needed:
• # of output shaft drive teeth (See bottom of article for numbers)
• Axle ratio (i.e. 3.55)
• Tire size (i.e. 225/60R16)

So you’ve got a pre 2003 Crown Vic / Grand Marquis / Town Car or Mustang and you’re dramatically changing your tire size? Swapping in a different gear set? This is more than likely going to throw off your speedometer. Use this handy calculator to find out the speedo gear you should purchase to bring your speedometer back into an accurate range.

Step 1: Determine your tire diameter.

You probably won’t know this off the top of your head, as it is less used than the common tire size designation which is Width/Sidewall Ratio/Wheel Diameter.

Here is a calculator that will help you figure out the tire diameter. Tire Diameter Calculator (and comparison)


Please note this calculator is modified to include an approximate tire deflection/squish in its calculation. The most accurate way to get your tire diameter is to contact the tire manufacturer and ask for Static Loaded Radius–you will have to supply a PSI value and vehicle weight at the wheel. Overall Diameter will give you a false reading!

Step 2: Determine the number of teeth your speedo gear should have.

Enter the value from Step 1 and other info into the calculator below to produce the # of teeth your speedo gear needs to send the correct signal to the speedometer. Applications where the value is less than 15.5 or greater than 23.5 should consider an electronic ratio adapter available on the aftermarket to maintain speedo accuracy (check with your local speedometer shop).

Check out the calculator on TCI’s webpage here: Speedometer Gear Calculator


Step 3: Obtain the proper speedo gear.

Round off the Step 2 result, and select your speedo gear from the chart below.

# of
Color Part Number Stock RAR
(ref. only)
16 Blue D0AZ-17271-A 2.73
17 Green C7SZ-17271-A 2.73, 3.08
18 Gray C7SZ-17271-C  
19 Tan C7VY-17271-A 3.08
20 Orange C8SZ-17271-B 3.27, 3.55
21 Purple D0OZ-17271-B 3.27
23* Black See below  
*The black 23-tooth is available from ADTR.


Installing your new speedo gear:
Full write-up at TCCOA

Determining Output Shaft Drive Teeth

The number of output shaft drive teeth is generally dependent on the vehicle’s axle ratio. The following table lists drive teeth per year, verified by Crown Vic owners. ALL Panther owners are encouraged to crawl under their car, pull out the VSS, and write down the color of their speedo gear.

Year & Model Stock

2002 P74 3.27 7 19

2001 P71 3.27 7 18

2000 M74 2.73 7 16
2000 M74

(Jan ’00 build)
2.73 8 17

1999 P74 HPP 3.27 8 21
1999 P71 3.55 7 20

1998 M74 2.73 8 17
1998 *stock tire size changed*

1997 P73 2.73 8 17

1996 P71 3.27 8 20

1995 P74 2.73 8 17

1994 P74 3.27 8 21

1993 P73 3.08 8 19

1992 M74 AOD 3.08 7 17

1991 P72 AOD 2.73 8 17

What’s P71? HPP?
Additional notes:
•  The 2003MY and newer Panthers do NOT use a VSS or colored plastic speedo gear to derive vehicle speed. If you alter the tire size or axle ratio in a 2003+ Panther, the speedometer reading can only be corrected by modifying the PCM (easily done with a chip). For tranny swaps, keep in mind that the output shafts still vary–see statements above.


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