Troubleshooter: What happens if I use the wrong tire size?

From computer glitches to void warranties, these are just some of the headaches you’d rather avoid

If you’ve ever done any tire shopping for your vehicle (especially for a different size than what the factory installed) you’ve probably heard this advice more than once; stick with the original size or something very close. If you ever asked why, you probably got some “it’ll mess with the computer” reply with no further explanation. But exactly what might happen if you (or your tire shop) inadvertently mounts the wrong-sized rubbers on your ride?

That “it’ll mess with the computers” is the understatement of the century. Consider that on most modern vehicles there are usually over 12 different computers used to run its various systems. First and foremost, all of these individual modules could easily be replaced by one server, with the ability to recognize virus infection attempts, and which could be located in a much better place to protect sensitive electronics from the environment. But that’s another discussion altogether.

You’d be surprised by the number of computers that require accurate and continual input for vehicle speed. Transmission computers obviously need this to properly shift. Engine units need this info to determine fuel and ignition controls. Anti-lock brake systems absolutely rely on vehicle speed data for proper operation. The list goes on, especially for vehicles equipped collision avoidance systems. The key factor for these calculations is overall tire circumference. Put on a set of tires that are larger in circumference than factory and the vehicle will travel farther for each revolution of the wheel, making the speed data input lower than actual velocity. Go far enough (either smaller or larger) and you’ll likely get more than one instrument panel warning light and if an antilock brake symbol is among them, that system will be disabled until the problem is corrected.

The worst I’ve seen in a shop I worked at was with an early Jeep Patriot. When our techs put the wrong sized tires on, and drove out of the shop, the engine stalled and wouldn’t restart. When the surprised tech plugged in a scanner, they found that the wrong tires created a critical computer fault that shut down the vehicle.

Then there are the physical problems with over-sized tires. If they’re too big they can cause clearance problems inside the wheel well, and when turning, the tires may contact the fender liners creating an ungodly noise and limiting steering movement — not something you want if you ever have to pull an emergency maneuver.

Here’s another that very few will ever anticipate: every carmaker has the same clause in their warranty documents;” warranty will not be applicable to any vehicle where the true distance travelled (odometer reading) cannot be readily calculated”. So, wheel into a dealership with wrong-sized tires and a list of what you think are warrantable items, and a sharp service consultant might ask you how you intend on paying for those repairs, as the warranty is now null and void.

Have a maintenance or ownership question for our experts? Others probably want to know as well. Email us at [email protected],  and we might have the answers you need.