different diameter tyres on same axle

Rob,

Yes.

Wayne

Yes you can not put diff diameter tyres on the same axle with an lsd
You would get away weith it on the front as long as free wheeling hubs are not in the lock position.
Cheers Pete

Absolutely; you will do a lot of damage to your vehicle by running tyres with that much difference. The rolling diameter on tyres fitted to same axle should be very close. The difference in rolling diameter between your tyres will be huge.

Few years ago on a off road rally in the 80 we had 31's blew two tyres and used a 33 on the front. Got us out of trouble to the over night stop. Hense the two extra wheels we carry all over the country side. Also with our camper we ran the same tyres on it as the car.

Hi Rob,
Definitely not a good idea, the diameter difference is too great.

Even the difference between brand new and just legal tread can cause problems in some constant 4wd's. Some people, including me, always fit 4 new tyres each time to avoid this.

The only time I would even consider doing what you suggest would be in a life or death situation where there is absolutely no alternative.

Disco.

No problems on the front of a Patrol (if necessary for a short distance) so long as you are not in 4 x 4 and with hubs preferably UN-locked.

However, different tyres on the same axle is illegal.

Peter

Hi Rob.
If you really needed to use the smaller spare on a LSD, what you would HAVE to do is put the same size wheel/tyre on the other side (Off the trailer) and put the "odd" sizes on the trailer.
Your patrol will look silly with bigger wheel/tyres at front but looks will not stuff your diff and you won't be able to drive in 4WD, as it will wind-up like a bastard potentionly doing damage to driveline.

There are friction plates (like clutchs) inside your LSD and they will burn out VERY quickly if driven with uneven size wheels/tyres

If faced with putting in on the front, I would (make sure hubs are UNLOCKED), strickly to get out of trouble, I wouldn't be cruising down the freeway at 110.
Shane

Thanks everyone, I get the message. Actually, the CT is on order and I've only just found out it comes with 225/75R15's (72cm), which are 6cm smaller in dia that the GQ's (78cm). I'll check out if they (Cub) can fit bigger rubber. Cheers. Rob.

A few years ago I did some research into a very similar problem.

Between standard 265/70R16s on my Prado & 265/75R16s.

Now for one thing not all tyres specified the same size are in fact the same size so the calculations we worked with where purely theorectical. I.E we went to a web site that told us the size of the tyres, rather than actually measure them.

Now te Prao is a constant 4WD vehicle so what we wanted to know was if I fit 4 BFG MT's to the car, but my spare was an original 265/70R16, what effect would this have on the rear diff if we fitted thge spare to the rear & also what would be the effect upon the centre differential.

Now to get 'the effect' into something we could relate to, I worked out what radius a roundabout would be to simulate the same difference in the rotation speed of the two tyres.

So the answers were, the rear diff would have a difference in speed between the two tyres of 1.4%. The centre diff would have a difference between the fron axel & rear half of this, so it is 0.7%.

This worked out to be an equivalent roundabout radius of around 46m.

So if this tyre was fitted, & I was driving in a straight line, the wear & tear would be the equivalent of driving around a 50 odd meter radius roundabout. 50 meters BTW is the length of an olympic swimming pool so it is quite a large circle.

Those who drive in Canberra probably do more curves than straight anyway just in there drive to work!

Also there are many European cars that have an undersized spare since they don't have the space for a full size. They probably suggest to limit your maximum speed & to get the original tyre fixed ASAP, so with consideration of that & the information above re the equivalent curve radius, I wouldn't expect any damage to the car drivetrain if you drove with reasonable caution.

Cheers

If you do , do that Outback gone, pump the smaller tyre up hard and let the taller one down until both are about the same diameter measured from the road surface to rim.

The bigger one usually has to go down to about 15psi , and at this pressure you could only do about 50 kph else in overheats.
Don't go below about 15 , better to leave the bigger one a little higher and let lsd take some wear.

However care and common sense and this will get you to next town - worked in practise when a friend of ours got is 3rd puncture on Birdsville after not getting the original slow down message.

I always ways see this as an advantage of locker diffs actually.

A quick question about the replies???

How does a patrol with a LSD go around corners???
Isn't that what a diff is for?

Surely if you were driving with different size tyres the only effect would be the axles turning at different speeds.
Maybe there is something wrong with my patrol, but it seems to travel in directions other than straight lines with no problems.

Sure it will produce extra wear and tear of the lsd, but so does turning corners. As does tyre wear, different pressures, and 4wd'ing.

I assume outbackogre's question is in relation to using the camper spare if he had allready used the vehicles spare. Ie as a backup spare.
I don't see a problem using it as a spare, short term to get you to the next town.
It would be preferable to use it as a rear than a front so it doesn't affect your steering. As far as using it as a rear, I'm sure the lsd would just think you were turning corners.

Obviously I understand long term usage of different tyres will cause abnormal wear, but as a back-up spare, I can't see why not.

Thanks for the further helpful responses. Yes Brian, my question relates to using the trailer spare on the GQ if I've already holed the GQ spare. I note the comments about making the LSD work hard. The LSD's on the GQ's are notoriously tight and I can hear mine chirp when I turn a tight corner on bitumin. I don't think I'd want to make it work like this for mile afer mile on the flat at high speed. I intend to check my tyre options with Cub. Cheers.

Came across a bloke with Jackaroo about 150km out of Birdsville.
Done in three tyres, only had two spares.
Lent him one of my spares and followed him back to Birdsville.
He had 245/70/16 and I had 235/85/16.
About 2" difference.
Let tyre pressures down to minimise rolling diameter difference, don't know what it did to his diff, seemed to be OK, but it got him out of trouble.

Wouldn't want to go much further without expecting damage to the diff, or serious diff wear. In this case, it could have been really difficult for him, only had one car pass us that day.

Can you find you fit 31/10.5/15 on the CT rims, even if they are 15x6? Or get some cheap 15x7 rims and fit 31/10.5/15s?

I just picked up three cheap 16x6 nissan rims for the paj. i.e. cost nothing, since the tyre place was throwing them out. Needed a sand and a coat of paint. See what you can find at the wreckers.