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Discussion Starter #1
Hi guys,

The short version is, how much leeway do I have when mixing tyre sizes on a Classic? (and don't say none, or the thing would surely crap out any time you run on a spare that has a few mm more tread...) :twisted:

The long version is, I'm currently running on 205/80R16 (of different brands, and different ages, and therefore different rolling diameters anyway). I'm looking to buy 4 new tyres (2 this month, 2 next month) and use the current tyres as trailers and spares.

A 235/70R16 would give me a rolling diameter of 734mm, ie about the same as the current tyres, and is probably the safest option. I wouldn't mind gaining a bit of height though. Trouble is that I'm not going to buy new tyres for spares, so my spares will need to mix safely with the new ones.

Anyone care to put some kind of a figure on what would be an acceptable variance?

(FWIW they're the stock 3-spoke alloys, and I'm looking at Goodyear Silent Armors).
 

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If it was an 89 on with viscous i'd say none, as you've got an LT230 that is fully open you should be ok as they are very similar rolling radius.

I'd put the new ones on back first and get second pair as soon as possible.

Martin
 

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Discussion Starter #3
Thanks Martin, but how much bigger do you reckon I could go and still safely mix the old (spares) and the new tyres?
 

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Open diff (non viscous model) run what you like, pretty much. Even a viscous would be quite tolerant i think, especially now. Mine takes a while before locking up, even with wheels spinning.

One odd sized tyre, such as a spare shouldn't matter. Just means that diff will run on the spider gears as if you were turning a corner, but i don't think it would affect that axle ratio.
 

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I think you will find that the tolerance is around 10% for an open diff
Which is quite high in the over diameter (734mm +/- 10% is a pretty big difference in overall rolling radius). Certainly gives you an idea of what you can run front to back.
 

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Which is quite high in the over diameter
Which is quite high in the overall diameter.



Close, but no biscuit :)
 

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Diameter makes no difference, it's rolling radius to be concerned with. Probably what was meant by "rolling diameter" anyway. That's the distance from the center of the rim to the ground, and that's what the drive-line sees as gearing. Just adjust the air until the rolling radii are equal on all four corners. For equalizing pressure, it's easier to measure from rim edge to ground, and just as effective. This can actually make a night and day difference in traction on the slippery stuff, especially the white crap from the sky.

Want to get anal on tire wear? Chalk your tires to measure wear, and adjust the pressure accordingly on the front, then equalize the rolling radius for the rear.
 

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Discussion Starter #9
Thanks guys.

I made a couple of enquiries about equivalent tyre sizes and was quoted the 'rolling diameter' - maybe it's an Aussie thing or maybe I was just speaking to the wrong blokes.

I'm certainly not pedantic about it, I just wanted to make sure that I'm not placing anything under undue stress.

Looks like I can gain a bit of height and still use the existing rubber for spares though :thumb:
 

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fermenter said:
Thanks guys.

I made a couple of enquiries about equivalent tyre sizes and was quoted the 'rolling diameter' - maybe it's an Aussie thing or maybe I was just speaking to the wrong blokes.
You should find this sort of information for most tyres
http://www.bfgoodrich.com.au/config/dow ... onData.pdf
In this group of data you are interested in the "Rolling Circumference"
 
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