A to Z of What Car Owners Should Know About the Size of Their Tires and Vehicles

Despite what others may say, size does matter…at least; it does when it comes to the size of the tires on one’s AWD or Active vehicle! Vehicles with all-wheel drive or “Active” four-wheel drive systems heavily depend on correct, matched tire sizes. Often, these vehicles use transfer case

The Vehicle That Requires

Before getting into the specifics of what it means by “matched” tire sizes, let’s define what type of vehicles this applies to. All-wheel drive and Active four-wheel drive vehicles depend on the transfer case. These are vehicles that either send power to both front and rear wheels at all times or cars with four-wheel drive or traction control systems that allow the vehicle to determine when it is necessary to send power to the ordinarily unpowered or underpowered wheels on its own, without driver input such as Active or Torque on Demand, with auto 4wd options. Vehicles with these drive systems depend on matched tires to operate correctly.

What Is “Matched” Tires?

Describing a set of tires as matched or mismatched refers to one thing and one thing only, the related circumference measurement of the four tires on the vehicle. That is it. Circumference. Not the brand of all four tires being the same, the size on the sidewall being the same, the age of the tires, the wear of the tires, or anything else. All those things will indeed affect the circumference of a tire, but in and of themselves alone, they are not the critical issue as far as your vehicle is concerned.

One of the main parameters that the AWD or Active vehicle drive system uses to determine if it needs to send more or less power to a particular wheel at any given time is how many revolutions each wheel is turning in comparison to each other. These systems measure how many revolutions each wheel turns as the vehicle drives down the road. 

Suppose the vehicle senses that any one wheel is turning more or fewer revolutions than it should in comparison to the other wheels. In that case, it will detect this as a loss of traction and send more power to the other wheels to help regain traction at the wheel that it believes has lost traction. And in this case, transfer case is used increasingly. 

The tire’s circumference determines how many revolutions it will turn in a given distance. A tire that measures 80″ in circumference will turn fewer revolutions in one mile than a tire that measures 79″. The smaller 79″ tire will have to turn more revolutions to go one mile than the 80″ tire, which is why it is essential to AWD and Active drive systems to have all four tires with circumference measurements that match each other. The magic number here is ¼”.

For one’s AWD or Active drive system to function correctly and not cause premature failure to their transfer case, the circumference measurement of all four tires on the vehicle must measure within ¼” of each other. No ifs, ands, or buts. Operate one’s AWD or Active vehicle with a mismatched set of tires that is more than ¼” different in the related circumference measurement, and one will cause their drive system to malfunction and cause premature failure to their transfer case and possibly other driveline components as well.

If one needs to determine if their tires are a matched set, it is necessary to measure them. They will hear internet experts claim that they can decide if a group of tires are acceptable by drawing chalk lines on the tires, driving forward a specified distance, comparing chalk lines, measuring the diameter of the tires, measuring the tread depth, etc. None of these methods are accurate. To do the job correctly, one must measure the circumference of each tire and period.

Last Advice

If one finds themselves needing to replace the tires on their AWD or Active vehicle, they shouldn’t assume that four brand new tires will be a matched set. Most of the time, when mounted and balanced, four brand new tires of the same brand and size will find when one measures their circumference that they are a matched set, measuring within the necessary ¼” compared tolerance.