How mismatched can tyres be? Can we put a figure on it?

[Ecopix]

I've read the many threads on mismatched tyres on Defender, but does anyone have a feel for how much variation is acceptable before damage is risked?

I was running 4 Bridgestone Dueler M/T 674's on my 300Tdi 130, but the two rear failed on the Plenty Hwy, midsummer and loaded up, as did one spare, so I had to buy bitsa station tyres. All are 235/85's, but different brands/patterns/wear.

Now I have to go home. I'm in Alice with limited but last chance to buy new. No 674's available. So I measure the travel of my tyres, and find that the rear two bitsas are well matched in rolling circumference to each other (better than the front two, despite one being a Wrangler and one a Haulmax!).

But the front 674's travel 160mm further in five wheel rotations (about 10m) than the rear two. Does that mean that in every kilometre of straight road the front axle is trying to travel 16m further than the rear axle, with the centre diff taking up the discrepancy?

And what will that do over 2,500km of straight road home?

None of the tyres I've looked at in Alice (M. Thompson/Maxxis/Toyo) match the Bridgestone M/T's in diametre (e.g. 674's are 818mm dia, Maxxis AT980's are 805mm dia.), and buying four new in Alice Springs, where tyres are $100 a throw more expensive than home, is throwing a lot of money at the problem. Coopers are close, but $405 each!

So my question is, how much discrepancy is acceptable? I've read 10% - 1m in 10m (!), but I've also read sob stories. Many thanks if there's a quantified value to judge this by.

[karlz]

Good question.
I was wondering the same myself recently.

Who can enlighten us?

[Slunnie]

I don't think there is a particular figure. I would think of it more as a continuum where the more it is out the more the wear accelerates. The centre diff doesnt like to do its job, and because of it some of the people in USA recommend to lock the CDL the second you hit dirt.

Interestingly, my Audi uses wheel speed differences to determine if a tyre is deflating! Combine that with LR recommending different pressures from front to rear.

[JDNSW]

The reason for locking the centre diff on dirt is not because it "doesn't like doing its work", but because it does not like the hammering on the thrust washers resulting from "grip/no grip" that is likely on loose surfaces and particularly corrugations.

Continuous low speed differential action due to different tyres is not going to produce undue wear on the centre diff - the front wheels travel less than the rear due to turns on most roads more than the amount suggested due to tyre differences.

The other issue I would point out is that the effective rolling diameter is difficult to establish unless it was determined inflated and loaded normally. I assume the OP did this.

In my view, the difference is acceptable.

[strangy]

The simple answer is drive as far as you would like.
All your tyres are the same placarded size. = close enough. Different batches of the same tyre will have diameter variations.
Straight road with minor variations caused by tyre wear or brand dimensions vs perceived wear as winding roads. = same same/negligible
The centre diff and associated thrust washers are not as fragile as some are concerned about and in this case essentially irrelevant.

Your diffs, whatever their location and associated components are not going to be placed under stressful conditions.
You should be more concerned with the potential handling issues of various tread patterns and compounds.

Just drive it home. If the tyres are stuffed get new ones, regardless of cost, otherwise your overthinking it.

[ramblingboy42]

a problem that could occur is if you lock your diffs with odd tyre sizes....on dirt where the tyres can slip wont be so noticeable but on hard surfaces it will and can pull your rear axle to one side or load your steering to one side

[DeanoH]

Just drive it home. If the tyres are stuffed get new ones, regardless of cost, otherwise your overthinking it.

X2

Deano

[DiscoMick]

Don't lock your centre diff.

Reminds me of a Toyota Surf I saw in Myanmar which had five tyres of different brands on it - no two tyres were the same.
Didn't seem to be causing any problems.

BTW is it right that passenger side tyres on a vehicle tend to travel further than driver's side because most roads are sloping outwards from the centre line?

[Ecopix]

X2

Deano

Thanks all for the really helpful replies. Makes sense to me. I've since worked out, with a lot of looking up manufacturer's specs on the internet, that the differences in diametre are basically the difference between mud tyres and all-terrain tyres. MT O.D.s in my size are all around the 818mm, AT O.D.s all around 805mm*.

I would just drive home, on your advice, but my two spares are no good, so I want to buy two new tyres anyway. My choice seems to be either keep the front the same and buy two new mud terrain tyres of any brand, or keep the rear two and buy 2 new all terrain tyres of any brand. Given I have two new MTs waiting in the container at home, I'll buy two ATs for the front and use the MT's as spares. Then if it rains, I can swap an axle over!

I was wrong about tyres being a hundred dollars dearer in Alice. It was just one shop price-gouging. Other suppliers are maybe $20 or $30 dearer than the coast, which is fair enough considering the freight. The shop I'll deal with is conforming to it's chain's national advertised price, so little pain there.

They have Toyo AT2's and GT Radial AT2's. The locals like Toyo M55's around here. I'm leaning to the Toyo AT2 at $30 dearer, on reputation and a slightly chunkier tread pattern.

Many, many thanks. I'll stick around on this forum and try to contribute. Loved Strangy's line. Hey, I'm a professional photographer - I'm paid to overthink things!
Uncle Wayne

* I drag my big Salisbury diff over termite mounds following Tojos as it is, so 6-7mm less clearance is a bit of a downer. Overthinking again!

[Slunnie]

The reason for locking the centre diff on dirt is not because it "doesn't like doing its work", but because it does not like the hammering on the thrust washers resulting from "grip/no grip" that is likely on loose surfaces and particularly corrugations.

Continuous low speed differential action due to different tyres is not going to produce undue wear on the centre diff - the front wheels travel less than the rear due to turns on most roads more than the amount suggested due to tyre differences.

The other issue I would point out is that the effective rolling diameter is difficult to establish unless it was determined inflated and loaded normally. I assume the OP did this.

In my view, the difference is acceptable.

The reason they gave was what I outlined in relation to wear.