Tyre Size Codes - Mark

I get the feeling that you're going to laugh.....

My question: on a tyre it has its size, for example 325/50R18, or something like that.

What does the "50" indicate ? I am assuming that "325" is the width of the tyre and "18" the size of the wheel itself.
Re: Tyre Size Codes - Neil
50 is the tyre wall profile, a percentage of the tyre's width.

Therefore, a 325/50R18 tyre would be 325mm wide (more than a foot), and the tyre wall would be 162.5mm (almost 6.5"), fitting on an 18" wheel.
Re: Tyre Size Codes - Michael
its the height of the tyre wall as a percentage of the width. ie in this case the height of the wall is 50% of 325mm = 162.5mm
Re: Tyre Size Codes - Mark
Thank you gentlemen.

Is there any reason why a single description uses a percentage, metric and imperial measurements ?

Are tyre described the same way on the continent or is there a "metric" equivalent ?
Re: Tyre Size Codes - Neil
Good point, perhaps Sunderland Council will take Kwick Fit to court for selling in imperial measures.
Re: Tyre Size Codes - John Kenyon
Neil wrote:
> Good point, perhaps Sunderland Council will take Kwick Fit to
> court for selling in imperial measures.

You don't sell tyres by the inch or millimetre.

(Who wasn't taught ANYTHING about inches/feet/yards/chains/furlongs
or miles at school - good job my dad was an engineer...)
Re: Tyre Size Codes -- useful websites - Roger Jones


for technical good info on tyres.
Metric tyres - David Lacey
A small selection of metric sized tyres were made by Michelin as standard equipment for various Ford, BMW & BL fitments, notably the Granada, 5 series & Metro (84-90)
Quite why they used metric tyres is beyond me.....
It may have had something to do with runflat technology - you could run them flat and they wouldn't come off the rim - they were known as TDXE tyres they even used special valves.
But all I know is that they were stupidly expensive
That's why you can see many post '84 metros running about on skinny 135/80TR13 wheels from the early models which could use cheap tyres...
Re: Metric tyres - John Slaughter

Yes, I remember them. It was associated with the run-flat tyres and used a particular rim section to prevent the tyres coming of the rims. I can only assume it was marketing ploy to make sure that you couldn't subsequently fit 'standard', reasonably priced, tyres as replacements. Whether than would have been a safety issue on those rims I don't know - I suppose they wouldn't want people assuming they had run-flat tyres when they actually didn't.



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