Tyre diameter and 4WDing

Some advice please from experienced 4WDrivers.
We are leaving this week on a long Top End and Centre trip. We are planning a Simpson crossing during our return to Sydney. Our vehicle is a Landcruiser 79 series, with 265/70R16 tyres on the front and 265/75R16 on the rear. My question is; Will this small difference in diameter be a problem with transmission wind up when 4WDing - mainly in sand. Thanks.
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Reply By: Cravenhaven - Saturday, May 05, 2012 at 10:56

Saturday, May 05, 2012 at 10:56
I wouldnt have thought that you would get any wind-up on a soft surface. The problem is caused when the tyres are on a hard surface and cant slip.
The other problem with different tyre sizes is spares. It is common to carry extra spares on such remote trips, but you will need extra extra spares to allow for the different sizes.
AnswerID: 485033

Reply By: olcoolone - Saturday, May 05, 2012 at 10:58

Saturday, May 05, 2012 at 10:58
Not in sand but any wind up can be fatal to the axles and transfer case..... it's not good practice to run different size tyres front to rear.

How come you run different size tyres?
AnswerID: 485034

Reply By: Bill BD - Saturday, May 05, 2012 at 11:30

Saturday, May 05, 2012 at 11:30
Different tyre sizes from side to side caused me a huge and immediate wind up problem on gravel in 4wd. I went backwards, forwards, round and round, etc and could not get it our of 4WD. Fortunately the jeep had a "full time" 4WD option (full time being drive on any surface). I could drive in that mode until the problem got sorted. Back/front is not as bad I suspect but if you get a flat or two you may end up having to go "side to side". Given my experience I recommend to change them.
AnswerID: 485040

Reply By: Nomadic Navara - Saturday, May 05, 2012 at 15:41

Saturday, May 05, 2012 at 15:41
"Our vehicle is a Landcruiser 79 series, with 265/70R16 tyres on the front and 265/75R16 on the rear."

Was it originally delivered that way? If not then you are driving an un-roadworthy vehicle. Suggest you get it fixed pronto. You must have the same size tyres all round if it originally came that way.
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AnswerID: 485048

Reply By: Ross M - Saturday, May 05, 2012 at 17:59

Saturday, May 05, 2012 at 17:59
G'day Rodge
I am amused at the comment "will this small difference diameter be a problem".

The difference is 27mm bigger at one end and that erquates to 3.5 approx % difference.
In my opinion this is a huge amount of difference.
For every turn of the big ones, the small ones are trying to do a turn + 85mm of tread.
In just over 28 turns of the small wheels, the rears are trying to over take the front ones with one full turn. So you are asking the front to rears to accept a slip rate of 2.5 metres approx in 70 metres or in other words 35 metres per kilometre. An extremely high slip rate in anyone language.

In half a wheel turn you will have transmission wind up.

Normal windup occurs when the front axle is turning outside of the rear axle and is at a very slow rate compared with this and even it becomes a problem and sometimes causes damage.

Because of the restrictive nature of this you will most likely use a massive amount of fuel as the rear tyres make the front ones into slowly rotating bull dozers in sand.

This is a case where a little, not very obvious problem, is really quite different.
You don't want to believe peoples quick mental assessment of this as it can be vastly different to what you think or realize it is.

If you drive like this on concrete you would be able to see the rubber trail it leaves as it just erases the rubber off the tyres both front and back.

A wheely serious and dangerous situation. I doubt if the transmission would last long.

Pi = 3.142 Circumfenerce = 3.142 x 777mm front or 804mm rear

Ross M
AnswerID: 485060

Reply By: Member - nick b - Saturday, May 05, 2012 at 21:13

Saturday, May 05, 2012 at 21:13
Will this small difference in diameter be a problem with transmission wind up when 4WDing - mainly in sand....... 83.26 mm in circumference .......that's huge...
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AnswerID: 485080

Reply By: Member - Phil G (SA) - Sunday, May 06, 2012 at 09:55

Sunday, May 06, 2012 at 09:55
Get 2 more of the 265/75R16 to match the rears. Just dump/trade the 265/70.
Consider yourself lucky that you asked the question - good chance you would have busted the transmission or diff in the middle of nowhere.
AnswerID: 485110

Reply By: Rodge - Sunday, May 06, 2012 at 13:53

Sunday, May 06, 2012 at 13:53
Thanks to all those who took the time to answer my query - especially Ross M. Your explanation was much appreciated. I am taking 2 spares of the larger size and the original plan was to swap these on the front for the Simpson crossing. I now realise that what I presumed was a small difference in circumference is in fact quite significant. Had I needed to use a smaller spare, it could have been expensive. I will be off to the tyre shop tomorrow for two more 265/75R 16's. Thanks again for the advice. Rodge.
AnswerID: 485124

Reply By: GT Campers - Monday, May 07, 2012 at 16:22

Monday, May 07, 2012 at 16:22
Ummm... I don't want to pop your ballon, but is your experience and vehicle prep skill up to the task of a Simpson crossing? Are you travelling in convoy?
AnswerID: 485206

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