Why Changing Your Tires Can Mess Up Your Speedometer

You just bought a fresh set of larger tires. Your truck or SUV looks amazing, it rides higher, and you're ready to hit the highway. But there’s a hidden problem you might not realize until you see flashing red and blue lights in your rearview mirror: Your speedometer is now lying to you.

It sounds crazy, but it’s true. Changing your tire size directly affects the accuracy of your speedometer and your odometer. Let’s talk about why this happens, how it works, and how you can fix it.

How Your Car Knows How Fast You're Going

Your car isn't actually tracking its speed over the physical ground using GPS (unless you're looking at your phone). Instead, your car’s computer calculates your speed by measuring how many times your tires rotate per minute.

When the car is built in the factory, the computer is programmed with a specific math equation based on the exact size of the factory tires.

For example, a factory tire might have a circumference (the distance around the outside of the tire) of 80 inches. The computer knows that every time the tire makes one full rotation, the car has traveled 80 inches. It uses that math to tell your speedometer dial where to point.

What Happens When You Get Bigger Tires?

Let's say you take off those factory tires and put on a set of big, meaty off-road tires. Now, the circumference of the new tire is 90 inches.

Here is the problem: Your car’s computer doesn't know you changed the tires.

The computer still thinks every rotation equals 80 inches. But in reality, your new tires are covering 90 inches of ground with every single spin. You are physically traveling further and faster per rotation, but the speedometer doesn't know it.

The Result: You are driving faster than your speedometer says you are. If your speedometer reads 60 mph, you might actually be doing 65 or 68 mph. This is a very easy way to get a speeding ticket without even realizing you were breaking the law.

What About Smaller Tires?

The exact opposite happens if you put smaller, lower-profile tires on your car. Because the tires are smaller, they have to spin more times to cover the same amount of ground. Your car will think you are flying down the road because the wheels are spinning so fast.

The Result: You are driving slower than your speedometer says you are. If your speedometer reads 60 mph, you might actually only be going 55 mph. While this won't get you a speeding ticket, it can be incredibly annoying.

How to Check Your Speedometer Error

So, how do you know exactly how far off your speedometer is? You don't have to guess, and you definitely don't have to do the complicated math yourself.

We built a tool exactly for this scenario. If you're planning to upgrade your tires, or if you already have, use our Speedometer Calibration Calculator.

Just enter your original factory tire size and your new tire size. We will instantly tell you exactly what your actual speed will be when your speedometer reads 20, 40, 60, and 80 mph. We’ll also show you the exact percentage difference, so you can decide if it's safe to make the switch!