The truth about AWD reliability & tyre replacement | Auto Expert John Cadogan

Once every two or three weeks I get a desperate e-mail from someone whose SUV’s all-wheel drive driveline has given up the ghost loudly and expensively.
They’ve had the whole nauseating conversation about the repair bill - and it not being covered under warranty. And when you trace the whole thing back to the root cause of the problem, it’s often what they did with the tyres: specifically, replacing them.
As in: They replaced two tyres and not all four. Which doesn’t sound like a big deal, but it is. At least on some all-wheel drive vehicles.
This affects a heap of people, too, potentially, because softer SUV sales are booming, car sales are falling, and that means many people are ditching a conventional car and driving off in an SUV for the very first time.
A lot of those vehicles have all-wheel drive of one flavour or another - not all, but many of them do - and while 2WD is easy and familiar, like the car you’ve always owned, AWD is complex.
One of the operational prerequisites of many AWD systems is to have the same sized tyres on all four corners. Exactly the same size. As in: the same brand, with roughly the same degree of wear.
See, when a vehicle drives here and there, especially in the city, all four tyres follow different paths (except when you’re going dead straight). Trust me on this. On every curve, all four tyres follow different paths.
And that means they’re all turning at different rates - because for any time interval they’re travelling different distances.
And that means all the driveline components: the axles, the front and rear propshafts, limited-slip differentials on the cross axles or the transfer case - whatever other hi-tech malarkey is rotating ‘down there’ - they have to cope with this rotational mis-match.
And these systems do cope with turning at different rates. Up to a point. But if you throw in different rolling diameters on the tyres, by mixing them up - two new tyres of Brand A with two semi-worn tyres of Brand B. That can be enough to tip the driveline over the edge and break something prematurely.
And, I sympathise: if you are the unwitting victim here, it just doesn’t seem very fair.
This process, from the vulnerability to the breakdown to the manufacturer’s denial of a warranty claim leaves you - kinda - out in the cold legally. Because, hey, you failed to comply with the owner’s manual.
It’s unfair, in my view, because who reads every page of the manual? If this is so important, why aren’t you told in a more proactive way? Why isn’t there a training course, if vehicles are that complex, or at least an instructional video online to help you out here?
So it seems to me that many carmakers are doing owners a disservice by not getting these kinds of serious operational imperatives front and centre before purchase.
So, here’s what you do to sidestep this particular mechanical takedown:
It’s dead easy, too. Every 5000 kilometres, have your tyres rotated. And don’t get cute with me here: I know tyres rotate. What I mean is, get your mechanic to change each tyre’s position on the vehicle. In a systematic way.
This is going to depend on the kind of tyres you’re running (because, for example, directional tyres cannot be changed side-to-side without removing them from the rim and turning them the other way around. Trust me on this.)
So, unless you know exactly what you’re doing and you have too much spare time, get the mechanic to do it for you. It’s a small expense. He’s got a hoist, and all the right tools. It only takes a few minutes.
What this does is even out the wear - because different positions on the vehicle wear more severely than others. Fronts generally wear more than rears, because they drive and steer and scrub more, and in right-drive markets like ours, front left tyres wear faster than front rights - especially in cities.
This way, at the very least, all four tyres will be essentially wear evenly, and all be worn out at the same time, and you won’t be tempted to replace only two because the other two seemingly still have some decent life left in them, opening the door to transmission meltdown.
It’s prevention versus cure.