Reader Help: Replacing tyres on all-wheel drive cars…
A reader has asked about the need to replace all tyres on an all-wheel drive car when just one gets a puncture. Myth-busting time.
Dear Practical Motoring,
I was interested in buying the Santa Fe until I came across Santa Fe problems in online forums. Basically it talks about getting a puncture half way through the tyre’s life. According to the Hyundai manual it says in that case replace all four tyres but according to the Hyundai Service book it says replace only two.
Yesterday I rang Hyundai Australia, the problem still persists, you get a puncture in the outback, the spares useless, you need to replace the whole lot, why bother putting a spare in if you have to replace all 4 or carry 2 spares.
There are many pages on replacing tyres on 4WD/AWD or part-time AWD, what a lot of people don’t understand is replacing a worn tyre with a new spare will cripple the AWD system, because the new tyre is spinning faster than the opposite wheel, thus telling the rear AWD system that the front wheels are slipping thus lock the front wheels, which is a big NO NO on a bitumen surface.
Google, Santa Fe problems in Australia, scroll down to Santa Fe Tyre replacement ends up costing $5k Beware. What this bloke did was replace two tyres which were different to the front and ended up stuffing up his transmission, interesting read.
So glad you wrote in because we love busting scare-mongering myths. There are plenty of things to be worried about with cars but this isn’t one of them. It’s not true you need to immediately replace all four tyres when you get a puncture on all-wheel drive cars, as decades of experience has proven across many, many different types of vehicle. Most of the time the stories you hear of problems are actually caused by something else, such as a faulty transmission, for which the tyres are blamed. But as ever, the answer is not simple so here’s a longer explanation.
An all-wheel drive (AWD) car drives all four wheels; two on the front axle, two on the rear. That means the diameter of all four tyres should be the same so all four tyres rotate in sync; in other words, they have the same rolling radius. Any taller tyre will rotate slightly slower for a given vehicle speed, any shorter tyre will be a bit quicker.
If the tyres have different diameters then the transmission will suffer as it tries to equalise different rolling radiuses in two ways; across the axle, and from front axle to rear axle. The result is that transmission components will suffer. It’s a bit like fitting a horse with three shoes of one size and putting a different sized shoe on the other foot. Incidentally, it’s not legal to run two different types or sizes of tyres on the same axle.
However, not all AWD vehicles run identical tyres front and rear. Most tractors and some AWD ultra high performance cars such as Lamborghinis are designed to run different front/rear diameters, but the principle still holds; the transmission is designed to expect that each wheel, and the front and rear axles, will turn a given amount relative to each other for a given distance the car travels. If a tyre diameter changes, that relative rotation changes and that’s where the problems begin.
So the first and most important point is that AWD cars such as the Satna Fe should run equal sized tyres on all four wheels, and that doesn’t matter who makes the car. It doesn’t even matter much if the tyres aren’t the original size, as long as they are the same diameter. For the same reason the tyres should, ideally, be equally worn. Depending on the type of tyre there may be as much as 20mm difference in diameter between a new and totally worn large offroad tyre, although for cars like the Santa Fe which are smaller and run less offroad-oriented tyres the difference is more likely to be 5mm. That’s only around a 1-3% different diameter from a new tyre to one at the end of its life.
However, the “keep all four tyres the same” is a simple statement and the world isn’t a simple place. Anecdotal evidence indicates you will definitely damage your AWD’s drivetrain if you run say one or two tyres that are significantly different diameter than the others for several hundred kilometers – for example fitting a 245/70/17 size tyre when the rest are 235/65/17. But such a difference would probably be ok for a hundred metres. Similarly, you could probably run one tyre 1% greater diameter for thousands of kilometers and not have a problem, or at least not a significant one. Nobody can provide any exact figures as there’s too many variables, and nobody wants to do the testing to find out exactly what the parameters are, so very rough guesses are all you’re going to get.
Even apparently identical brand new tyres are not precisely the same diameter, and other factors such as inflation pressure as well as load makes a small difference to rolling radius. The terrain makes a difference to tolerance of different diameters; that’s because low-traction surfaces permit tyre slip to equalise different rolling radii whereas high traction surfaces such as bitumen do not. Incidentally, that is why part-time 4WDs run 2WD on high-traction surfaces and only use 4WD on low-traction surfaces.
Aside from wearing out transmission components, mis-matched tyre diameters also affect suspension operation, and throw out electronic driving aids such as stability control which rely on monitoring individual wheel speed.
So here’s the bottom line; yes, it’s true all four tyres on AWDs should be identical in every way, including tread wear. However, minor differences in diameter will not make any significant difference. But the problem for manufacturers is that it is impossible to define ‘minor’ – that’s dependent on so many factors such as the difference in diameter, nature of the car’s transmission, and how long you drive the car for, vehicle loading and the terrain. To protect themselves the manufacturers play safe and stipulate all four tyres must be identical, and with identical wear rates. I would suggest that they should provide a more nuanced answer that explains the situation. Lawyers get paid lots of money so they should be able to produce statements that provide actual, useful real-world guidance while protecting the company’s best interests.
One good indication of how far a vehicle can travel with one tyre a different diameter to the rest is the use of spare-saver spares. These are narrower than normal, but also smaller diameter, usually 3% or so. The manufacturers specify limits for speed and distance, usually 80km/h and 80km. The difference in diameter between a space saver and a normal tyre is much greater than that of four identical tyres when three are worn and one is brand new. The photo below is of the full-sized and spare tyres on a 2015 Suzuki Vitara; the sizes are 195/90/16 and 215/55/17, with diameters of 650 and 668mm – a difference of 18mm or 3%.
The Santa Fe is an interesting case study because it uses an on-demand AWD system, like most of its peers. That means the engine generally drives the front wheels only, sending little if any torque (driving force) to the rear wheels, and brings the rears in as required; computers monitor things like relative wheel speeds to decide how torque should be distributed. Now if the front wheels have marginally larger diameter tyres then they will be rotating slightly slower than the rears, so that implies that there would need to be more wheel slip than normal before the rears come in. If the rears were larger diameter then the reverse would be true, and a big enough difference would actually invoke the AWD system as the computers would be fooled into thinking the front wheels are slipping. However, there are tolerances built in but again – it’s impossible to say at what point any specific tyre diameter change will become a problem.
Now for the real world advice.
- Replace all four tyres at the same time. This applies to two-wheel drive cars but is more important for all-wheel drive cars.
- If you have a full-sized spare, then consider rotating it onto the driven wheels so at any time all of your tyres have the same wear rate.
- If you have an AWD car with four part-worn tyres and a new spare and you have a puncture – fit the spare and drive on. If the damaged tyre can be repaired then it should be refitted so you’ve got even wear across all four tyres; if not, then throw away the damaged tyre, get a new replacement and fit the it to the same axle as the old spare so you’ve got two new tyres on the same axle, and use the newly replaced “old” tyre as the new spare. Then you at least have two identical tyres on an axle. The other axle will have slightly smaller tyres, but you could probably live with that; whenever an AWD vehicle turns a corner the front axle turns quicker than the rear, so all AWD systems are built to accept a certain amount of difference in rotational speed between front and rear axles. The wear rate of new tyres is greater than old ones so things would tend to equalise.
In short, generally attempt to keep all four driven tyres identical; diameter, width, tread wear, but don’t stress if there are minor differences for short periods of time.