Tyre sidewall marking

The 75 figure is the aspect ratio and dimensionless.

The 75 refers to the profile and is a measurement of the height of the sidewall/tyre width given as a percentage.

Adam

In message , Osprey writes

Surprisingly difficult to find good sites with info on tyre wall markings - KwikFit have a tiny subsection and doesn't address the OPs query. Dunlop.co.uk also had minimal info.

But, (no connection etc.) provides all the info. you need I think.

As others have said, the 75 refers to the aspect ratio as a percentage. So the 'height' of the tyre is 75% of 205 = 153.75

So the overall diameter is (14 x 25.4) + (153.75 x 2) = 663 mm

Not quite the 635 which you measured, but a lot closer than 505!

For tyre markings in general, see

If I might just throw in my own recommendation, take a look at the following:-

It is by far the most comprehensive explanation I have found of all tyre sidewall markings, sizes, wheel configuration, offset, etc.

The whole Car Bible series is well worth a bookmark...

(coupled with

Mathew

The "75" is the aspect ratio. The lower the number, the lower the sidewalls.

Tyre sizes must drive the metrication fascists bonkers; three numbers, one metric, one imperial and one dimensionless, not to mention the speed rating which is an arbitrary index, not in alphabetical order, to a speed in mph (that's the "R").

Huge (Huge ) gurgled happily, sounding much like they were saying:

And is given as a percentage of the nominal tread width.

So, in the OP's example, you've got 355mm + 2x(75% of 205mm) = 355+2x154 = 663mm.

Not exactly "dimensionless".

No, that's R-for-Radial. There's a piece of information we're not given in the original post - the load & speed ratings. A decade or more ago, tyres used to be typically labelled "205/75 HR14" - where the H was the speed rating. Now they're more typically "205 75 R14 87H" - where the 87 is the load rating and the H the speed rating.

So, what are it's dimensions? Acre-feet? Lightyear-barns? Barrels per kiloparsec? It's a ratio, and therefore dimensionless.

Ahh, thank you. My mistake.

Huge (Huge ) gurgled happily, sounding much like they were saying:

It's one of those "it is and it isn't" numbers, I reckons. No, on it's own, it is dimensionless, but taken as a whole at least it's easy to turn into an absolute dimension - unlike the load rating number...

Err, no. It has no dimensions. It is dimensionless. It has rung up the curtain and joined the choir invisibule...

And remains so whoever's company it keeps.

A whole lot of nonsene, maybe.

You can use the ratio to calculate one dimensions from another. The ratio itself remains dimensionless, it does not get turned into anything.

Maths wasn't your strong point at school, then.

MBQ

snipped-for-privacy@hotmail.com (" snipped-for-privacy@hotmail.com" ) gurgled happily, sounding much like they were saying:

I've never lied about having failed A-level Maths, 'tis true.

Nor have I, LOL! OK, I didn't quite fail but got a D due to a bout of complacency and insufficient revision. I got an A the second time round!

MBQ

We forced them to drive the last of those Alfasud Tiis with their 350mm rims, until they realised that rims ought to be sized in inches, if you ever wanted to find tyres to fit. That taught 'em.