Outer Limits 4x4 Board

New tyres arrived today, MT 117s.



Since the golden rule is to not spend more on any mod than you did on the actual car, I couldnt buy a 5th tyre for the spare.

My plan is to purchase a 2nd hand spare.

So the MT117's measure 900mm (35.5") in diameter (exactly as silverstone say). Incidently each rim and tyre weighs 43kgs.

So how much variation in diameter is acceptable for the spare tyre vs the main tyres.

Since this tyre has 17mm of tread its conceivable you could have a variation of up to 30mm over the life of the tyres.

So I reckon im after a worn 36" tyre that will be around 35" in diameter. Will a worn 35" tyre (around 34" diam) be acceptable?

it doesnt matter so long as you arent planning to drive far on the spare. in a full time 4wd (which i dont believe yours is) it can be a problem as increased movement in the drive train can lead to more wear. (side gears and such). i suggest if you do get a flat, just arrange the tyres as such that the spare is on the front. -problem solved.

-Dave

or you could let the other tyre down to suit the height of the spare.

joeblow wrote:or you could let the other tyre down to suit the height of the spare.

Whilst letting the tire down will reduce the height of the tire, it will not change the circumference of the tire - which effects how many turns it does at any given speed.

I agree with the first answer, For a short trip you could get away with it as your diff should slip enough to not cause problems. If your driving home in good roads then 2wd and chuck the smaller one on the front.

However if driving off road it should be ok as the reduction of traction should allow a certain amount of slipage anyway.

As long as you dont have welded diffs of anything it should be sweet.

I would go for the 35 as that gives you a half way point between 35.5 and 34.5 which is going to be the rough usage range.

I've had a tire guy tell me, rather emphatically, that you shouldn't do it. But I don't see how running a slightly larger tire, on an open diff, is any different to just taking a corner? The diff is doing whats its designed to do, split the drive between the left and right wheels.

So, it MAY cause increased wear on one side. But thats like saying don't turn left or right all the time cause it will wear one part of the diff more

EDIT: and an for the tire dropping suggestion

djroberts wrote: it can be a problem as increased movement in the drive train can lead to more wear. (side gears and such).

More wear? Vehicles with a front axle disconnect system (such as Pajero and some Hilux models?) have their front diff side gears spinning for thousands upon thousands of kilometres. I've never heard of one of these diffs needing to be replaced due to wear - will a 3% difference in tyre diameter make much difference?

joeblow wrote:or you could let the other tyre down to suit the height of the spare.

Now there's some GREAT advice. Didn't someone at Ford say "just let some air out of the tyres, that will get them riding better" not nlong after there was a spate of tyres exploding attached to Explorers.

DO NOT RUN LOW PRESSURES ON THE ROAD, tyres get hot and let go.

But a cheapo spare that's roughly the same size if you have to.

It is fine with an open diff.
With an LSD, auto locker or welded centre it can cause problems.
Paul.

sierrajim wrote: DO NOT RUN LOW PRESSURES ON THE ROAD, tyres get hot and let go.

Should that be "don't run low pressure at speed"?
Offroad they can heat up and come apart too.

joeblow wrote:or you could let the other tyre down to suit the height of the spare.

This is the funniest thing I've heard for AGES!