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TD5 Discovery Tyre Circumferences


boaterboy
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I have recently fitted new tyres to one axle on my 2002 TD5 Discovery leaving the other axle with half worn tyres. All tyres are of the same make and size, although the tread pattern of the new tyres are slightly different to the older ones. I do all my motoring on main roads with no off-roading. My vehicle is not fitted with a differential lock. Will the different circumferences of the front and rear tyres have any adverse effect in any part of the transmission, e.g. increased wear, etc.?

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I assume the new tyres are the same size as the old ones i.e. 235/70R16 or whatever? If so then I don't think the small difference between a new and a part worn tyre would be significant. Fitting tyres that are actually a different size on the vehicle is definitely not recommended apart from perhaps short distance as an emergency spare. The effect is that one or more of the diffs are spinning all the time which they are not designed to do.

How much is the difference when you measure it?

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I assume the new tyres are the same size as the old ones i.e. 235/70R16 or whatever? If so then I don't think the small difference between a new and a part worn tyre would be significant. Fitting tyres that are actually a different size on the vehicle is definitely not recommended apart from perhaps short distance as an emergency spare. The effect is that one or more of the diffs are spinning all the time which they are not designed to do.

How much is the difference when you measure it?

\Thank you for that speedy response.

Yes, all tyres are exactly the same size.

I haven't measured the difference in circumference between the front and back tyres, but I will do and get back.

If both front tyres have the same circumference, likewise both rear tyres but slightly different from the front ones, then presumably the centre differential spins all the time?

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Yes that is correct the centre diff would be spinning all the time. If the difference is small then it will be OK, and it should be if all tyres are the same size though having said that, some "same size" tyres vary considerably between different manufacturers, sit a Michelin XS 7.50R16 sand tyre next to an Avon Rangemaster of the "same" size and the XS is much much taller, for one example! Metric tyres are usually nearer to each other.

I would plan on changing the other two tyres to be honest, as it will make everything else better as well i.e. handling/braking if all the tyres are the same.

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You say the front tyres are half worn, so let's say about 5mm less thread compared to the new ones. This represents about 1.5% difference in circumference, meaning your centre diff is working all the time to cope with this difference.

It should be able to do so without a problem, BUT you are adding extra strain on the system so wear and tear will be up.

I'd say change the front tyres as well and ASAP, if you wait too long too correct the imbalance, you'll end up with another one, because the rear ones will already have worn a bit by the time you put new ones on the front.

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Yes they are but the Freelander transmission is entirely different in operation to a D2

Many thanks for all of your inputs.

I very much appreciated your comments.

I calculated the circumferences of my new Michelin 255/65R16 Synchrone tyres and my 'half' worn Michelin 255/65R16 XPCs and found that the new tyres were 0.8% bigger on the circumference.

I have also calculated the difference in circumference between my retained XPCs and the two XPCs that I replaced and found that to be 1.6% , i.e. unbeknown to me the difference in tyre circumference back and front was greater prior to my fitting new tyres than after - and I am unaware of experiencing any excessive wear in the transmission judging by transmission noise.

On this basis, I think that I will keep the existing situation with regard to buying a further two new tyres.

Interestingly, the Driver's Handbook for the vehicle, whilst stating that different tyres sizes should not be fitted makes no reference to all tyres being required to have the same tread depth / degree of wear.

Yes, I had heard comment about Freelanders being prone to transmission wear in this situation but also that this is because of the central diff. being a type of viscous 'coupling'?

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Yes the Freelander has a viscous coupling and they are sensitive to differently worn tyres (you are only supposed to fit new ones on the rear of a Freelander!) but the amounts you describe will cause no problems at all on a Discovery.

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I had similar thoughts, I've just bought a set of 235/85/16's off another forum member and they have 17mm of tread on 4 of them and the spare has 18mm of tread.

If the tread wears down to 5mm on the 4 tyres through general road use, then I get a puncture and I have to put the spare on, the spare tyre would be 26mm taller than the other tyres - which is a fair difference, surely that would start to cause problems?

Would I be better fitting the spare now with the other three and every so often swap the spare for one of the others to keep them all wearing at a more even rate?

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