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 |
| tire diameter |
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:
| 5,280 |
| tire diameter x 3.14 |
| 12 |
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
| 35" | 33" | 32" | 31" | 30" |
| 4.56 | 35 | 36 | 37 | 37 | 38 |
| 4.10 | 33 | 34 | 35 | 36 | 37 |
| 3.73 | 31 | 32 | 32 | 33 | 34 |
| 3.55 | 30 | 31 | 31 | 32 | 33 |
| 3.07 | 26 | 26 | 27 | 27 | NA |
Older Jeeps with Spicer 18...
Older Jeeps with Spicer 18 or Dana 20 T-cases are the hardest to re-gear because parts aren't that plentiful and the output retainer requires some disassembly to get to the components. Here you see the speedometer drive gear indexes the speedometer-driven gear, which is captured in the rear ouput housing. The drive gear is a light-press fit on the output shaft.
Due to limited factory axle...
Due to limited factory axle ratios and scarcity of replacement parts, the best bet for speedo corrections on older T-cases is a speedometer adapter of some sort. Shown is a Stewart Warner adapter (called a Drive Joint by the company) that spins at a 1.12:1 ratio to slow down the speedo. Other ratios are available. We found several speedometer adapters on the Web site www.transmissioncenter.net. Check the Math Quest sidebar for more info on calculating the correct adapter ratio.
More modern New Process and...
More modern New Process and even Dana 300 T-cases use a mechanical speedo drive gear assembly to drive either a mechanical speedometer cable or a pickup for an electric speedo. Regardless, both types utilize the same speedometer drive gear that is either machined into the output shaft or is a plastic collar that slips over the shaft with a light-press fit, the speedometer gear, and the speedometer gear housing.
New replacement speedometer...
New replacement speedometer gears with 26 to 38 teeth are available to recalibrate the speedometer. Check out the sidebar to see which one is right for your application. While fully mechanical speedos could use a speedometer adapter like the vintage cases, swapping the drive gears is a much simpler-and cheaper-alternative. Replacement gears are available at the dealership or visit www.4wd.com, www.transmissioncenter.net, or www.quadratrac.com.
The Superlift TruSpeed was...
The Superlift TruSpeed was the unit actually recommended to us by three of the four local Jeep dealerships we contacted regarding TJ speedo recalibration. The TruSpeed features two settings just in case you run two different tire sizes on the same vehicle. Just toggle between setting A and B. The unit wires into the factory electronic speedo plug and taps into the factory fuse box for its power supply. Installation is straightforward, and the instruction booklet is very detailed.
If your output shaft has a...
If your output shaft has a little plug with a wire coming out of it, then you've got the tone ring speedo. We've heard it's possible to have the stock Jeep computer reflashed at the dealership to compensate for tire and gear changes. We called a few dealerships about this and were told the reprogramming is rarely, if ever, effective. They suggest aftermarket solutions for their customers.
There are not provisions for...
There are not provisions for electronically adjusting for tire or gear sizes with the Hypertech Power Programmer for Jeep applications, but the it can make changes to allot for tire and gearing adjustments on Chevy, Dodge, and Ford vehicles. So you're skunked unless you've done a drivetrain and computer swap.
The latest Jeep T-cases are...
The latest Jeep T-cases are fully electronic and use a Hall Effect sensor to pick up magnetic pulses taken off a tone ring on the output shaft. There are no mechanical means to recalibrate these types of speedometers.