On FWD cars, the rear wheels spin freely without a care in the world! What an easy life for the rear tires.
What about on the front? Do front-left and front-right tires have to match? What if one of them punctured and you had to replace it with a new tire, so now you have one that is new and thus with more tread and theoretically sits slightly higher, is that a problem?
I don’t know…
Vehicles tend to pivot around the odd tire in emergency situations - just when you wouldn’t want that to happen. Unfortunately, you don’t know this is a problem until it is too late.
So the best sitiation is to have all 4 identical tires - meaning same size, same make, same model, same speed rating, and same state of wear. The further you are away from that, the worse the situation.
Obviously different tires on the front fall into this situation - and since most braking force is generated by the front brakes, this becomes very important.
For the sake of the differential the 2 front tires should match.
Assuming you can put worn tires on the back because they are only coasting along is flawed reasoning. Rear tires are important on FWD cars. The rear tires help stop the car and keep the car stable by not skidding out. Worn rear tires are prone to spin outs and are not safe. Therefore the best set up is good tires on all 4 wheels.
Anyone can state the ‘best’ solution and pat himself on the back for giving great (and costly) advice, but how bad, if at all, is the second-best situation? You have four tires with plenty of life in them but one must be replaced. Must you replace the pair? Or can you get by with the single replacement?
Buy one tire. We all have done it. Nothing bad has happened. This is sound practical advice.
The difference between fwd and awd is the type of differential. On most awd systems, somewhere in the system is some type of limited slip device, and a different tire will chew that up. On most (but not all) fwd cars there is no limited slip device, so two slightly-different sized tires will not result in a quick mechanical failure. The diff will have to operate, but that shouldn’t cause too much wear on it.
However, for handling, I’d want my two front tires to match as closely as possible, since they do most of the braking and turning work. So move the tires around and get matching ones on the front. Also, the newest tires go on the back to reduce the chance of a spin in slippery conditions.
A lot of people live their entire life just replacing tyres when they are almost bald.
Most people who do the minimum, will never be in a situation that it would be a problem. The problem is that small % that will have a problem, possibly a life threatening problem.
It would not be so hard if all cars handled the same and if all tyres handled the same, but they don’t.
If you choose to mismatch tyres, you increase the risk of loosing control in an emergency condition. Many people will never get into an emergency condition so they would never need to worry. However we don’t know when we might get into a situation.
Likely you will never get into that kind of situation. However you may. I have been in that situation twice (both black ice). First was a 17 car accident (I was #17 minimal damage, and the other I managed to do a 360º and gain control. I was lucky and I did have good matched tyres. I doubt if I would have done as well with poorly chosen tyres.
Having said that, I would be less concerned (although I still will make sure my tyres are all in good condition and match) in an area with out snow and ice.
I guess it depends on how much your value your life and the lives of those around you. Yea, most drivers will not need to worry. I just don’t want you or I to be that one that dies need it and does not bother to have proper tyres.
No, you don’t have to replace all four, or even two tires when one goes bad, but if you plan to sell your car soon, it should have four matching tires. An oddball tire is a sign of recent body damage and repair, and many people will see the mismatched tires as a reason not to buy your car.
I used to buy used tires, and didn’t care if they matched, but eventually, I realized that buying new tires, and paying only once for mounting and balancing for the long haul, costs less as a per-mile cost of maintenance. Keeping four matching tires on my car isn’t that difficult, as long as you take care of them and rotate them on schedule. By the time I need to replace one, the other three should be just about as worn or aged, at which point it really doesn’t make sense to replace just one.
The diameter of the front tires DO NEED TO MATCH…on both FWD and RWD vehicles. This is due to the differential on both types of vehicles. Thats the definitive answer if I do say so myself. Everything else is just bunk…Do the right thing and you wont have to second guess yourself.
I’ve found a basic description of what Texases’ has briefly described.
Vehicles with only RWD or FWD have to have some system to allow each wheel to turn at a different speed while both are still under power in order to make turns. The outside wheel has to travel farther in the same amount of time, thus the different speeds. Typically this is done using a gear system. As long as the gears stay properly lubed, they can operate at different speeds indefinitely.
With an AWD system, not only do the right and left wheels on each end have to travel at different speeds, but there needs to be an accomodation to allow the froont and rear to turn at different speeds as well. Typically this us done using some type of coupling such as the one described in the attached link. However, in these type of couplings the difference in the speeds is absorbed by the fluids and dissipated as heat as well as being compensated for by frictional components. It works great for limited differences, but if the differences are substantial and constant, the fluid generates heat and the frictional components wear.
Bottom line: gears will accomodate differences in speed indefinitely. Viscous couplings, used in AWD system, cannot.
The safety issue or having different traction coefficients on different corners of the car, as others have commented on, is obvious.
The FWD differential will slip continuously if the tires are mismatched enough. It simply creates more wear.
If the vehicle has Limited Slip differential on the front keep them matched other you likely will burn it up.
“The FWD differential will slip continuously if the tires are mismatched enough. It simply creates more wear.”
… which is why it is okay to use a doughnut spare on the front, but not for extended periods.
If one of your front tires was worn out and had a diameter of 23" and the other was new and had a diameter of 23.5", then the first would turn 877 revolutions per mile and the other at 858 revolutions per mile. My guess is that driving on a curving road would make your differential gears work harder.
Expressing worry about increased differential wear is fearmongering. If you want to worry about gear wear, worry about your transmission where the gears turn much faster. The differential ring and pinion gears run fast but the other differential gears turn slowly or not at all.
I don’t recall seeing complaints here on CarTalk about worn out differentials and never had that happen to my cars.