Despite what others may say, size does matter…at least it does when it comes to the size of the tires on your AWD or Active vehicle! Vehicles equipped with all wheel drive or “Active” four wheel drive systems are heavily dependent on correct, matched tire sizes.
Before we get into the specifics of what we mean 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 matched tire sizes. These are vehicles that either send power to both front and rear wheels at all times (i.e. AWD, no 2wd option available) or vehicles with four wheel drive or traction control systems that allow the vehicle to determine when it is necessary to send power to the normally unpowered or underpowered wheels on its own, without driver input (i.e. Active or Torque on Demand, with auto 4wd options). Vehicles with these types of drive systems are extremely dependent on having matched tires to operate properly.
Now that we have determined what type of vehicles require matched tires, let’s define what is meant by “matched” tires. Describing a set of tires as matched or mis-matched is referring 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 or the size on the sidewall all being the same or the age of the tires or the wear of the tires or anything else. All those things will affect the circumference of a tire for sure 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 your 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 types of systems measure how many revolutions each wheel is turning as the vehicle drives down the road. If the vehicle senses that any one wheel is turning more or less revolutions than it should in comparison to the other wheels, it will detect this as loss of traction and will send more power to the other wheels to help regain traction at the wheel that it believes has lost traction.
The circumference of the tire determines how many revolutions it will turn in a given distance. A tire that measures 80” in circumference will turn less 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 and that is why it is so important 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 your AWD or Active drive system to function properly and to not cause premature failure to your 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 your AWD or Active vehicle with a mis-matched set of tires that is more than ¼” different in the related circumference measurement and you will cause your drive system to malfunction and cause premature failure to your transfer case and possibly other driveline components as well.
To determine if your tires are a matched set, it is necessary to measure them. You will hear internet experts claim that you can determine if a set of tires are acceptable or not by drawing chalk lines on the tires and driving forward a specified distance and comparing chalk lines or by measuring the diameter of the tires or by measuring the tread depth, etc. None of these methods are accurate. To do the job correctly you must measure the circumference of each tire, period.
Here’s how; with the vehicle on a lift or by jacking up each corner of the vehicle independently, wrap a tape measure all the way around the round part of the tire. The thinner the tape measure the better this will work. There are “stagger” tape measures made specifically for this that racing teams use to measure their tires that you can easily find online or look at your local home improvement store for a thin tape measure. Normal sized “woodworking” tape measures will work but can be cumbersome to use for this. Once you have wrapped the tape all the way around the round part of the tire, make sure it is pulled taught and record the measurement. Do this for all four tires. Compare the largest measurement to the smallest measurement and if the two are ¼” or less in difference than you have a matched set of tires that will work fine for your AWD or Active vehicle. If the difference between the largest and smallest measurement is more than ¼” you will unfortunately need to replace the tires on the vehicle with a new, matched set of tires to avoid system malfunction and premature transfer case failure. Do not underestimate the importance of this. Most AWD & Active transfer case failures that we see are directly caused by being operated in vehicles with mis-matched tires.
One last piece of advice. If you find yourself in a situation where you need to replace the tires on your AWD or Active vehicle, do not assume that four brand new tires will be a matched set. Most of the time, when you mount and balance four brand new tires of the same brand and size you will find when you measure their circumference that they are a matched set, measuring within the necessary ¼” compared tolerance. However, from time to time, we have had customers run into situations where they have bought four brand new tires of the same brand and size and due to inconsistencies in the manufacturing process those four tires did NOT measure within the necessary ¼” compared tolerance. It is rare but we have seen this happen so if buying a new set of tires for your vehicle we suggest that you do not take for granted that those four tires will be a matched set and that you still have them measured to be certain.
We hope this information is helpful and we are always available to help. Feel free to call us with any questions you may have!