Tyres

Technical Area Wheels, Tyres & Brakes
2

i bought my 2010 mark 3 2.0 Sport Roadster Powershift last October and it is on 3 Bridgestone Potenza 205/45x17 84W and 1 GT Radial 205/45x17 88W. One of the Bridgestone Potenzas is getting down on tread so I was looking for a pair of good Bridgestone Potenzas to make up a matching set. there are a pair near me in Essex the right size but they are 84V’s. Would they be OK or must they be W rated? Also most of the right size tyres seem to be 88W. As my MX5 is my daily driver I am currently running a set of winter wheels and tyres so need to sort out my summer set before too long. Strangely due to seasonal work I do far more miles in the winter than summer!! Hence not looking for brand new tyres. Many thanks, Robert

3

I wouldn’t mess around with part worn’s trying to save a couple of quid. Just get a pair of new mid range tyres and be done with it. 

4

Thanks for your reply. I did look at new Kumhos but they all seem to be 88W not 84W, would they be ok?

5

There is no problem at all in fitting tyres with a higher load index than recommended by the manufacturer. The recommended tyre (as specified on the plate on the driver’s door) is 84W, so the 88W rated Kumhos will be fine. You shouldn’t really fit tyres with a lower speed rating or load index than recommended by the manufacturer (even though they still appear to cover the actual weight/speed of the vehicle), so strictly speaking those 84V rated tyres you mentioned ought not be fitted to your car.
 

6

As above - any issue will be with underspecified tyres rather than tyres that meet or exceed the requirement.  

7

The original OEM Bridgestone was 84. However Bridgestone stopped making this and the last 6 moths if not the last year of production cars were on 88 load rated tyres. The load rating of a tyre has very much to do with how much air pressure you can put in it and the strength of the fabric belt in the sidewall that resists this load in tension. If you have a tow rope than can pull 1 ton. Two tow ropes should be able to pull 2 tons. But one or two tow ropes, you still can’t push with them and so the side wall collapses just the same with the same air pressure in the tyres. 

Load and speed ratings are a lot to do with what is common for that size. V means the tyre can operate, at normal pressures and loads, continually at a speed of 149mph. (240kmh) and W = 168mph (300kmh) so the speed rating does not necessarily equate to what is required by the vehicle but just what is typical for the size of tyre. 

While I would always recommend fitting a tyre with a speed rating in excess of the maximum speed of the vehicle, a gentleman who used to assess all the warranty claims for a very well known and established tyre manufacture told me that it is far easier defend in law lower speed rated tyres as being capable of operating within the speed limits for the country than it is to have your campervan (or car) on tyres that are not appropriate for the weight of the vehicle. 

In any case, if you can get your MX-5 to operate continually above 149, you are doing well. 

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8

Thank you all for your advice, it’s been a great help.Â