Replacing front & rear tyres on AWD

Mellermick · Post by **Mellermick** » Wed Jan 27, 2010 11:17 am

I have an 02 Liberty Heritage wagon and have recently replaced the front 2 tyres with new F1 Goodyears. I have the same tyres on the back but quite worn; maybe 5000K left on them. A mechanic has since warned me that the difference in tyre diameter from the rears to the front on an AWD will cause drag on the gearbox and quickly cause damage. I've consulted Subaru Aust who said yes should replace all tyres but wouldn't give any detail. I also spoke to a local Subaru service dept who said there is no problem having replaced only the fronts. Can anyone set me straight on whether there is actually a problem here? Thanks.

AndrewT · Post by **AndrewT** » Wed Jan 27, 2010 12:11 pm

Yeah you should aim to have the same diameter tyres on both ends. You have a viscus limited slip diff in the centre of your gearbox. While it is designed to slip (it needs to when you turn corners etc), it isn't designed to slip 100% of the time to compensate for mismatched tyres so it's life will be reduced quicker than normal. However, with just the slight difference in wear between the front and rear tyres I can't see it causing a big issue but you'd want to sort your tyres out sooner rather than later.
Running huge offroad tyres on one end and stockies on the other, yeah that would be a real problem.

FROG · Post by **FROG** » Wed Jan 27, 2010 3:40 pm

Yeah keep them the same Ive seen too many $x,000 bills for repairing said diff
tyres are cheap in comparison

Venom · Post by **Venom** » Wed Jan 27, 2010 3:52 pm

FROG wrote:Yeah keep them the same Ive seen too many $x,000 bills for repairing said diff
tyres are cheap in comparison

From different diameter tyres (e.g 185/65 to 185/70)? Or from just having the same size and brand tyre but just worn differently?

I can understand different sized tyres been a problem. But surely these things are manufacturered to take into account that tyre ware is never even from front to rear, and as a result the tyres will never be exactly the same diameter.

Happy to be proven wrong, in which case i'm in desperate need of new rear tyres

Cheers,
Rhys

FROG · Post by **FROG** » Wed Jan 27, 2010 4:37 pm

As long as your rolling circumference is the same or as close as possible , you will always have slight variations, I even try to get the same batch so factory differences are kept minimal... And venom 65 and 70 series tyres are different from new...try putting a chalk mark at the bottom of your tyres and rolling the car forward a couple of rotations and see the difference in where the marks are ... You may get away with worn 70's and newer 65's awd user beware is all ,I've seen some awd cars with horribly different sizes boots on with no problems and I've seen a spare fitted which screwed the diff in couple of weeks....

FROG · Post by **FROG** » Wed Jan 27, 2010 4:42 pm

Ps I always rotate all five wheels VERY regularly in order to keep wear even,but that's just me

AlpineRaven · Post by **AlpineRaven** » Wed Jan 27, 2010 10:12 pm

Yeah its not good idea to buy 2 complete different set of tyres because it reduces life of the centre diff, which I am suspicious to my previous liberty, when I got it, it had 2 different sizes tyres and about 2 yrs later (after I fixed it up to correct size) centre diff blew...(ended up swapping gearboxes over) which I believed that the tyres were the blame.. But never know the truth.
Cheers
AP

Mellermick · Post by **Mellermick** » Mon Feb 01, 2010 11:17 am

Thanks for the opinions. As the rear tyres are pretty worn and I'll have to replace them before long anyway, I should probablly bite the bullet and replace them now. It's an interesting subject though and if anyone comes up with any hard evidence one way or the other I'd like to hear it.

YouTube · Post by **El_Freddo** » Tue Feb 02, 2010 6:23 pm

Venom wrote:I can understand different sized tyres been a problem. But surely these things are manufacturered to take into account that tyre ware is never even from front to rear, and as a result the tyres will never be exactly the same diameter.

Happy to be proven wrong, in which case i'm in desperate need of new rear tyres

You've got nothing to worry about Rhys as your AWD box doesn't have a centre LSD - its got a diff lock

Your diff is an open diff until you operate the lock.

So having tyres that are worn down a bit and new tyres on the front shouldn't be a worry for you

Can't wait to get one sorted...

Cheers

Bennie