tyre sizes

Submitted: Tuesday, Feb 26, 2013 at 17:42
ThreadID: 100778 Views:1797 Replies:4 FollowUps:2
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I have a 105 l/c with constant 4 wheel drive running 265/75/16 cooper st. Fronts have about 60% left on them, rears due to be replaced. Have been given a good price on cooper st maxx which claim to have a O/D of 813mm. Standard st have O/D of 807mm. Will this difference ,( remembering fronts are 40% worn) present me with any problems. Thanks in advance for any imput and sorry if this topic has been covered before
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Reply By: Member - Andrew - Tuesday, Feb 26, 2013 at 21:20

Tuesday, Feb 26, 2013 at 21:20
Hi Riverman

If my maths is correct then there is less than 1% difference in the circumference. That should not be enough to affect anything as your diffs should easily accomodate that discrepancy as it is probably less variation than you would get on a windy road.


AnswerID: 505642

Reply By: Andrew & Jen - Tuesday, Feb 26, 2013 at 23:47

Tuesday, Feb 26, 2013 at 23:47
Hullo riverman

A couple of thoughts for consideration - I think as it is late (for me at least :-)

Given 40% wear, assume 4mm less tread; therefore diam of 803mm on front. Replacement rears diam of 813mm.

This gives a difference of 1.1% in circumference. That is for every 100 rotations of the front wheels, the rears will do 1.1 rotations less - which equals ~1.7m - and hence the diff will be working continously.

Athough I took a quick look at the lcool site, I could not find whether the centre diff is a viscous coupling (as in the 80 Series) or an actual diff. If the former, there may be some heating issues.

However, when the centre diff is locked, you could get continuous windup induced slippage and that could well present a problem (depending on the situation/surface) as the rears will be trying to go ~1m further every 100m of travel.

AnswerID: 505659

Follow Up By: Member - Phil G (SA) - Wednesday, Feb 27, 2013 at 22:34

Wednesday, Feb 27, 2013 at 22:34
Gday Andrew,
New member !!!
Only the Sahara 80series had a viscous centre diff - the rest of the 80series and 105 series had a normal open centre diff, so a minor difference in tyre diameter won't matter.
FollowupID: 782659

Follow Up By: Member - Andrew & Jen - Thursday, Feb 28, 2013 at 15:39

Thursday, Feb 28, 2013 at 15:39
Thanks Phil - when I became a visitor, I did so on the basis that if I was still hanging in here after 2 years, I would join up.
My understanding is that the 1995 GXL with ABS is fitted with a viscous coupliing. But that really is not relevant to the OP.
My point was that when in low range, the centre locked and on, say, a slow rock climb with no slippage, he may get a problem.
But maybe not :-)
What is more interesting is that he found himself with 60% left on the front and having to change the rears.
FollowupID: 782686

Reply By: olcoolone - Wednesday, Feb 27, 2013 at 08:01

Wednesday, Feb 27, 2013 at 08:01
I will make bugger all difference and will not cause any damage or excessive wear.
AnswerID: 505664

Reply By: Ross M - Wednesday, Feb 27, 2013 at 12:07

Wednesday, Feb 27, 2013 at 12:07
Because the centre differential is involved it will have to speed up slightly as it provides it's output of faster to the front shaft and slightly less to the rear shaft.
As mentioned it does make the centre diff operate more than normal but it is only a small amount and possibly within design parameters of the manufacturer.
Your overall engine revs and speedo speed will be a little/small amount higher for the same actual road speed. Just a weenie bit.
AnswerID: 505678

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