A common question from owners of 4WDs who tow a camper trailer is whether a difference in tyre size between towcar and trailer tyres matters…

Hi Practical Motoring,

Would you help me sort out with a 4WD towing problem and, no, it’s not about GVM or gross anything!?

It’s about swapping wheels if needed between the trailer and the tow vehicle.

The wagon has 245/75/R16s and the trailer has 245/70/R16s, so the latter are about 13mm lesser radius. In the event of needing to put a second spare on the wagon, using the trailer spare, will that smaller wheel work?

The wagon is an Isuzu MU-X with independent front suspension and live axle rear. Assuming the smaller wheel goes on the front, will the ESC electronics detect the wheels rolling at different speeds and so kick in?

Hope you can help,

Cheers,
E

Hello E

First, some background to the question. A 4WD towing an offroad trailer has six wheels, and you never know which will be punctured. It is good practice to be able to swap tyres (and preferably wheels) between trailer and towcar, as both towcar and trailer should at least carry one spare each. While you’re at it, ideally you also want the track to be the same between trailer and towcar – the track is the distance between the centre of two wheels on an axle.

E, you are correct to be concerned. Running different tyres on any wheel of a car is a bad idea as it creates handling asymmetry, which is why it is illegal to run different tyres on an axle. That’s simply tyre models, not even different dimensions which is even worse. And yes, space-saver spares seem to be an exception to the rule.

Anyway, back to your question. The different diameter means a different rolling radius, so the smaller wheel will appear to be going faster than the normal sized one. This will throw out the stability control, ABS, EBD and everything else that relies on wheel speed sensors. This could lead to electronic systems kicking in when they don’t need to, or more likely, just giving up in confusion as the computer cannot work out why the yaw sensor is reading nothing, the steering wheel is straight ahead and yet one wheel speed is different from the rest. A similar problem is often also seen when there are wheel alignment problems, for example the steering wheel not on straight.

There is a bit of tolerance built in though, and the difference *may* be small enough not to be concerned, only way is to try it. But there’s another mechanical issue to consider, which is that you’re putting additional wear and tear on your differential. That’s just extra wear it doesn’t need.

While the MU-X is a part-time 4WD, putting the odd tyre on the front won’t help much. This is because the car reads wheel speed off all four wheels, and even though the front wheels aren’t driven on road, the differential still rotates so you’ll still get that uneven wear. Offroad, there’s lots of wheel slip and low speeds so a slight difference in tyre diameter wouldn’t make much difference, although it is still far from ideal.

The best solution is to run 245/75/16 on the car and trailer. Failing that, just be aware of the limitations of your current setup. You would only need to worry about it at higher speeds for long periods, as when offroad in low range one slightly different tyre shouldn’t make that much difference to the sensor systems, and handling wouldn’t be a problem either. However, if you were offroad and found that after a while traction control cut out with an error, leaving you unable to get over an obstacle then switching the car on and off again would most likely reset it and then you could perhaps get over the obstacle.

UPDATE: should have added that not only should you ensure the tyres are the same, but also the rims, specifically, the same diameter, width, offset, PCD and load rating.

The diameter and width are pretty straightforwards. The offset is how far out the mounting point of the rim is to the centre of the rim, and the PCD is the Pitch Circle Diameter – how many studs (bolts) are used to mount the wheel, usually 6 on 4WDs except for the likes of Land Rover who use five larger ones. The PCD also covers the diameter of the circle that the holes for the studs. Even if all that fits, then you may find that the larger brakes on the towcar (especially the front) mean the trailer wheels won’t fit.

Essentially, the ideal is to buy another three wheels and tyres, same as on the towcar, and fit them to the trailer. Most trailer manufacturers will make the necessary changes to the trailer…bit harder to modify the towcar!

Here’s an MU-X, as we didn’t have a stock photo of one towing for the title shot.

Related links

• All you need to know about towing heavy trailers
• Detailed guide to choosing tyres