Up-sizing or Down-sizing
Up-sizing, or installing a taller tire, will lead to a speedometer reading that is slower than your actual speed. This is because a taller tire will have a larger overall circumference, which will cause it to travel more distance per revolution than the original equipment tire. Down-sizing, or installing a shorter tire, will cause the speedometer to read a faster speed than you are actually driving. This is because the smaller tire will have a smaller circumference, causing the tire to travel less distance per rotation than the original equipment tire.
Changing the tire height can alter the speedometer reading because most vehicles measure speed within the transmission based off of tire revolutions.
Plus sizing means replacing your original equipment tires with a tire that is a different size, but it has the same overall height. This can also refer to replacing your original equipment wheels/rims with a larger or smaller size, but also maintaining the same tire height. Plus sizing should not interfere with the speedometer accuracy.
Learn more about Plus Sizing
Tire Size and Speedometer Calculation
There are ways to manually calculate your speed. For instance, you can use road mile markers and time the exact number of seconds it takes you to travel the distance of a single mile and calculate your speed based off of that time. However, these methods often prove less than accurate. You can get a more accurate estimate of your new speed with our tire size and speedometer calculator. Most modern vehicles can have the tire diameter changed in the vehicle computer, which will recalibrate the speedometer properly.
If you have any questions about how tire size effects the speedometer accuracy on your vehicle, reach out to the experts at your local store.