[JD6]

I have borrowed the image below from another thread, but I was wondering why there are differing Load Indexes (from 89 to 102) for different recommended wheels/tyres?

Given that the sprung weight of the car doesn't change when changing wheels, this seems illogical, unless I have misunderstood.

[moonshine]

Looking at the relationship, it wouldseem higher profile tyres have a need for stiffer sidewalls/ higher load rating

this might be to reduce sidewall flex.

it is prob a combination of both - a spacesaver needs to be stiff as it is thin and tall (so it has a small air chamber and is high profile)

[JD6]

There does seem to be that relationship between profile and load rating, as you say. I would have expected the need for stiffer sidewalls to have already been taken into account, without the need to raise the load rating for high profile tyres.

[sweels]

Originally Posted by JD6

There does seem to be that relationship between profile and load rating, as you say. I would have expected the need for stiffer sidewalls to have already been taken into account, without the need to raise the load rating for high profile tyres.

This is right. The load rating corresponds with a given load, seemingly regardless of profile e.g as per this link http://www.national.co.uk/informatio...-sidewall.aspx

Some tyres quoted on the stickers on a 3 series can only be fitted on the back (I. E. Any with a 255 width, whereas the 225s of some sizes can be fitted front or back. Whilst the f/r weight distribution when empty is close-ish (depending on engine and whether s or xdrive etc) the max weight of a car changes the weight on the back more than the front. (Minimum of 1 person in front whereas when full, the would be 3 people in the back max 2 in front and a load of stuff in the boot, so you'd think that higher load rating may be needed on the rear. However a 91 rated tyre can be used on the back.

Having said that, a lot of cars have 65% or more of their weight on the front wheels and so most tyres need to take that proportion of a car's weight (divided by 2 as there are 2 tyres on each axle of course!)

It may even just reflect that those load ratings are the minimum required for those sizes that are actually available to buy. Not all size combinations exist on all load ratings and it would possibly cause undue concern to quote a lower load rating (if acceptable) than the lowest available in that size?