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Discussion Starter · #1 ·
I have just had my car serviced and they pointed out a cut in the tread of one of my front tyres which showes the wires if you lift the loose bit of rubber. I have replaced the tyre but the fitters said that you should ideally replace both tyres on the same axle on 4 wheel drive vehicles. The new tyre has 8mm tread and the other front tyre still has about 6mm left after about 20K miles. I am hoping that a difference of only 2mm wont cause me any problems. Any feedback appreciated.
 

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You've been given the correct advice for a Kodiaq, but I'm not certain that applies to all 4x4. If it's permenant 4x4 then ideally all four tyres should be changed but occasional 4x4 cars like the Kodiaq are treated like 2wd - ie. best practice is to change both tyres on same axle.

It's really up to you, personally I can't see what difference 2mm will make. That said you're doing pretty damn well if you've 6mm tread after 20k miles on the front tyres of a Kodiaq ! You defo sure it's 6mm? It's worth double checking because it doesn't sound right to me. How much tread was remaining on the rears after 20k miles?
 

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Also surprised by having as much as 6mm left on the front after 20k miles, but that said, my car is still on it's original Pirelli Scorpion Verde tyres at nearly 29k miles and the fronts are somewhere around 3mm left and the rears are at 6mm, so suppose it's possible.

2mm difference shouldn't make a difference. The issue with having a difference in tread depth across an axle lies within the differential, particularly on the rear. Whilst the difference between a new tyre at around 9mm tread and a barely legal one at 1.6mm sounds pretty slight, it's a theoretical difference of double that in terms of diameter, then multiplied by pi for the difference in circumference. So for a 7mm difference in tread depth, that's a theoretical difference of 4.3cm in circumference.

As the two tyres will have differing circumferences, the effect is that when driving in a straight line, you are in effect cornering as the number of rotations required for the same straight distance travelled on each wheel is different. This leads to more wear through the differential. Whilst it's designed to cope with different rates of rotation (otherwise you couldn't turn a corner), it's not designed to be doing it constantly, even when going straight.

Of course in reality the differential will actually cope fine with this (up to a point) and of course the tyres are not perfect circles - they 'squish' at the bottom with the weight of the car and also heat up with use, so there are far more variables than the rather simplistic view I've outlined above.

And 'proper' 4x4s have some form of differential in the middle as well, hence the advice to change all 4 tyres at the same time. The Kodiaq however doesn't actually drive the rear wheels most of the time, only when the fronts lose grip, and the rear axle is then coupled to the engine by a Haldex clutch, so don't believe it has a centre differential as such, so it would be less of an issue to have a difference in tread depth front to rear. That said, the previous advice about always having the better tyres on the rear axle still stands as loss of traction during aquaplaning will happen first on the tyres with the least tread, and if that's the rear axle then this leads to oversteer, often uncontrollably so.
 
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Discussion Starter · #4 ·
Thanks for your replies. The tyre that was damaged had 5.5mm in the centre which is about what the remaining one has. The rear ones still have 7mm. I don’t drive it like a sports car but I don’t drive particularly slowly and I also tow an 18ft caravan, so I’m quite happy with these tyres.
 
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