explain tyre sizes??

[StangGT]

Hi folks,

Ok, a question as I'm somewhat confused.

I had always understood (and never had cause to check!) that tyre sizes worked roughly like this:

e.g.: 225/45/17

225= width of tread in mm
45= height of tyre expressed as a percentage of the width of the tread
17= diameter of the wheel in inches.

so anyway, I have just had a measure; tyres in good condition, of the aforementioned size.

I would have expected the height of the tyre to be 101.25mm (45% of 225mm), but no, it's around 82mm.

where am I going wrong?

[kambites]

You're probably measuring the height of the sidewall from the wrong point. I'm not certain, but I think it should be measured from the tyre bead not the edge of the wheel rim (so you can't measure it when the tyre is on the wheel very easily).

Edited by kambites on Thursday 6th November 15:33

[zeppelin101]

Where did you measure from / to?

[StangGT]

well, I measured the tyres whilst they were on the wheels. So from the edge of the alloy outwards to where the tyre stops...

that may explain it I guess!

[kambites]

StangGT said:

well, I measured the tyres whilst they were on the wheels. So from the edge of the alloy outwards to where the tyre stops...

that may explain it I guess!

You can still work it out on the wheel. Just measure from the centre of the wheel and subtract the nominal wheel diameter (which is also to the bead not the rim). Just requires a bit of unit conversion.

[Dannbodge]

Just to add my 2p.

The width of wheels (the J number) is also measured to the inside of the rim where the bead sits.

[brillomaster]

also, 225 is the section width of the tyre, which is usually more than the width of the tyre (tyres normally bulge out a bit on the sidewalls like doughnuts)

racing slicks that have a very square cross section are slightly closer between tread width and tyre width.

[Slidingpillar]

Radials are 82%/80% of the width high as standard. If the labelling was helpful (it's not usually) the VW Beetle standard tyre was 165/82 15".

Cross plys though the standard width is also the height so the 4.00 19" on my vintage car are also four inches high.

This web site has everything:
http://www.carbibles.com/tyre_bible.html