The trap of load index

Posted on August 2nd, 2016 in Safety,testing,Tyres by Julian Edgar

This issue we have a story on understanding (and varying) gearing, based primarily on changing tyre diameter. The prompt for the story was the availability of a wide variety of on-line calculators that allow you to very easily correlate road speed with selected gear and engine rpm, and to see how overall gearing changes can be made by changing diff ratio or tyre size.

And there’s nothing at all wrong with those calculators – in fact, it’s easy to spend a few hours trialling all sorts of different combinations of numbers!

However, when looking at making major reductions to tyre rolling diameter, there is a trap that I wasn’t aware of.

And the trap?

Load index!

Load index is the rating given to a tyre that describes the maximum weight that should act through that tyre. The rating is indicated by a number that correlates to a vertical load (in kg or pounds). So for example, a tyre with a load rating of 89 has a maximum load of 580kg per tyre. (And at what tyre pressure does that apply? Again this is an area that most people don’t think about, but that load applies only at a specified inflation pressure – often 36 psi.) Load index tables can easily be found by a web search.

And what governs load index? Most references talk about the strength of the tyre (ie how many layers of steel reinforcement are used, for example) but in fact it also depends to a large extent on the volume of air trapped within the tyre.

And, as you go smaller in rolling diameter, that volume decreases!

Thus, changing gearing by reducing tyre diameter may be difficult if the load index of the smaller diameter tyre has decreased a great amount.

The minimum load index is a legal requirement as stated on the tyre placard. For example, my little Honda Insight, with 165/65 14 tyres, requires a minimum load index of 79 (or 78 in some markets). A load index of 79 means the tyres can cope with 437kg per tyre. That seems really high for this small car – the highest static load the Honda tyres would ever have to deal with is about 330kg – but that’s what the placard says.

If I wished to lower the gearing, changing the wheel size to 13 inch and going with 165/55 tyres (which would give about 10 per cent lower gearing) sounds good – until you realise that the load index of such tyres is only 70, or 335kg. A load index of 335kg is some 23 per cent lower than legal!

In fact, I found it impossible to find a tyre with a legal load index that gave a smaller rolling diameter on the Honda. To go further, I also found it hard to find any cars where these small tyres would be legal, their load indices being so low.

So if you’re thinking of reducing the volume of air inside the tyre (eg a by using a lower profile but keeping the same width, or a combination of smaller wheels and smaller tyres), check the load index of the available tyres first.

It’s honestly not an area I’d ever given much thought to.

3 Responses to 'The trap of load index'

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  1. Greg Griffiths said,

    on August 2nd, 2016 at 11:49 pm

    The same problem can arise when looking for larger diameter tyres. Especially if your car is based on a commercial vehicle. The “XL” load rated tyres in one size may not be available in a slightly larger size. A trap to watch out for when putting offroad-biased tyres on a commercial vehicle.

  2. Gene said,

    on August 3rd, 2016 at 3:18 am

    Load rating is something that has caught out more than a few competitors in Silver State Classic.
    The rules require a load rating equal to, or higher than, original equipment. The car I drive has a stock tire of 255/50-17 with a rating of 101. One of the common choices people make for a larger footprint is 275/40-17; unfortunately, those have a rating of 98. One of the club members had to go out and buy a new set of tires to run. Unfortunately, the closest store with a full set was in Las Vegas, a 4 hour drive from Ely, where he failed the tire tech inspection.

  3. Richard said,

    on August 3rd, 2016 at 11:54 pm

    I am finding a similar problem when trying to find tyres for my Pug 208 GTI. Tyre size is 205/45R17. Not a hard to get size. But the load rating is 88, when most tyres in that size are 84. It severely limits the tyres i can choose from that are half decent. Very annoying