Speedometer Correction Made Easy
Bear with me while I complain like a little girl whose shoes are too tight. I'm sick of answering the same "Why doesn't my speedometer register correctly since I swapped tire sizes?" question over and over every single week. There, I said it. Call me a whiner, call me an elitist, or call me what you will. Just promise you'll read this story and come up with some other question for me to answer hundreds of times. I need a little variety.
Your speedometer reading is all just based on simple arithmetic. The engine spins so many revolutions per minute, which spins the transmission output shaft so many times, and so on all the way down to the T-case, axle ring-and-pinion, and tires. Changing the diameter of the tires or the ring-and-pinion gears not only changes the speed at which the other components in the drivetrain spin but the ratio that they spin in relation to each other. A simple formula that you can manipulate to see how it all works together is:
|axle gear ratio|
|Engine rpm =||x mph x 336 x transmission final drive|
Put larger tires on with no other changes, and your speedometer will register slower because the tires need to make fewer revolutions to cover the same distance. Change to a numerically higher gear ratio, and your speedometer will register faster because it takes more engine and output shaft rpm to cover the same ground. The percentage at which the engine rpm at a given speed is affected by tire or gear changes is the same percentage at which your speedometer is affected. Therefore, when you change one piece of the equation, you've got to adjust the other pieces to match. Here's how to do just that no matter what type of speedometer drive assembly your Jeep has.
If you're going to go the speedometer adapter route, here's a good formula to determine which ratio will get you the closest:
|Speedometer adapter ratio =||1,000 x speedometer drive gear number|
|tire revolutions per mile x rear axle ratio|
Note 1: To determine your speedometer drive gear number, divide the number of teeth on the speedo drive gear and driven gear (for example,10-tooth speedo drive gear and 27-tooth speedo driven gear = 27 divided by 10; speedo drive gear number = 2.7).
Note 2: To find your tire revolutions per mile, check your tire manufacturer's Web site or use:
|tire diameter x 3.14|
If you've got a more modern speedo, just look on this chart for your combo, or come as close to it as you can.
Tire Diameter (measured in inches)Ring-and-Pinon Gear Ratio