Sidewall height...I didn't realise that....

A

Angelus666

Senior member
 Essex
So, I didn't realise the 35 in the 255/35/19 means the side wall is 35% of the 255mm width. I just presumed it was a MM measurement to a certain point of the wall.

Learn something new most days....
 
Yes that why the staggered wheels on the e89 take 255/30 rear and 225/35 front
If it was mm they would both be the same
 
Something I picked up from here was how to check the age of a tyre, the series of letters and numbers after 'DOT' will have a four digit sequence relating to the week number and year of manufacture. I had a bit of a shock when I checked a tyre on my Forester runabout last year, it must have been the spare originally and was a 2004 !! :o

tyre manufacture.jpg
 
Angelus666 said:
So, I didn't realise the 35 in the 255/35/19 means the side wall is 35% of the 255mm width. I just presumed it was a MM measurement to a certain point of the wall.

Learn something new most days....
Click to expand...

This ratio between the tread width and sidewall height is called the Aspect Ratio. I still think it's weird that in this age of standardisation etc we have a descriptive series of numbers that contains millimetres, percentage and inches ! :cry:
 
patriot66 said:
This ratio between the tread width and sidewall height is called the Aspect Ratio. I still think it's weird that in this age of standardisation etc we have a descriptive series of numbers that contains millimetres, percentage and inches ! :cry:
Click to expand...

I suppose it is a bit of a mish-mash when you think about it!

But when my Dad had cross-ply tyres in the 60s the width was also in inches! :)

Maybe because Michelin (IIRC) were the first to make radial tyres, all of a sudden width changed to millimetres and has stayed that way ever since.

But diameter has always been in inches - maybe because by then every car on the road already had wheels in inch sizes.

A couple of times in the 80s a couple of manufacturers did try metric wheel sizes (MK 2 Granada 2.8is, and Metro 1.3s) but thankfully this didn't take off - only Michelin made tyres for them, they were hard to find and seriously expensive!

But at least percentages fit with either Imperial or Metric.
 
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