[post updated 6/13/2017]
Knowing how fast you’re going is pretty important for those of us who drive our Jeeps off-road and on the streets (unless you plan to just add the cost of your traffic tickets into the upgrade – Woop-Woop).
Whenever you increase tire height (and the tire circumference, or distance around your tire) your speedometer will read slower than you’re actually going. This is because the speedometer actually only reads the number of rotations your tires are making per hour and it calculates your speed based on it’s understanding of how big your tires are and how far they travel in each rotation. Those new larger tires fool the speedometer by traveling further per hour than it thinks is possible,
For each 1” of additionally tire height your speedometer will be off (read slower than you’re going) by approximately 3.35%. So, when you upgrade from stock 30-inch tires:
- With 33” tires, 30MPH is really 33MPH; 60MPH is really 66MPH
- With 35” tires, 30MPH is really 35MPH; 60MPH is really 70MPH
- With 37” tires, 30MPH is really 37MPH; 60MPH is really 74MPH
- With 40” tires, 30MPH is really 40MPH; 60MPH is really 80MPH
If you want to calculate other tire sizes or speeds the formula is:
[Actual Speed] = [New Tire Diameter] / [Old Tire Diameter] * [Speedometer Reading]
If you re-gear your differential (to a higher number gear / lower gear ratio for more power) you can reduce this error.
How to Fix Your Speedometer after changing to Bigger Tires
What if you don’t like all of this “wrong speedometer” and “doing math”. Well, you have a few options:
- You can just remember the 30MPH and 60MPH speeds from above for your tire size. Police don’t normally give you a hard time for being over by a mile-per-hour or two (unless you’ve stolen his girlfriend).
- You can use your GPS. These days most people have one (and they’re a good idea when off-roading), and most will tell you you’re actual MPH based on how quickly you’re changing position.
- You can also buy an electronic tuner that allows you to set the tire size (fixing the speedo issues) and can also reset your transmission shift points for a little better performance.
I use option #3. It costs money to buy the programmer (the price varies depending on the model and features, but they’re generally under $200) but you can solve a lot of other problems when you buy a programmer. The two devices that I can vouch for are the “Spuerchips Flashpaq” and “AEV ProCal”. For example, the AEV PROCAL can adjust the:
- Tire size
- Gear ratio
- One touch lane change
- Daytime running lamps
- Low tire pressure indicator
- Set a temporary extended idle
- Provide a “dead center” steering wheel indication
- Turn off the “check engine” lamp
- Clear diagnostic trouble codes