Tyre sidewall marking

Been trying to understand tyre markings on my trailer tyres. It is marked as 205 75 R14

That should mean (I think) rim diam is 14", height of tyre is 75mm and width is 205mm.

Form this you would assume overall diam of Tyre would be 14" (i.e.

355mm) plus 2 x 75mm so overall width of 505mm Yest if I measure it the overall width is 635mm significantly different.

Anybody know why the discrepancy?

Reply to
Osprey
Loading thread data ...

The 75 figure is the aspect ratio and dimensionless.

Reply to
Dorothy Bradbury

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

Adam

Reply to
ARWadsworth

formatting link

Reply to
Mathew Newton

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.

Reply to
Si

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

formatting link

Reply to
Roger Mills

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.

Reply to
Dave N

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

(coupled with

formatting link
for car battery info!)

Mathew

Reply to
Mathew Newton

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").

Reply to
Huge

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.

formatting link

Reply to
Adrian

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.

Reply to
Huge

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...

Reply to
Adrian

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

formatting link

Reply to
Huge

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

Reply to
manatbandq

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.

Reply to
Adrian

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

Reply to
manatbandq

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.

Reply to
Andy Dingley

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.