Article taken from a saved copy of Liquid’s tech page. All credit goes to him!
• # 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.
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.
What’s P71? HPP?