From: Brian Neill Tiedemann (s914440@minyos.xx.rmit.EDU.AU) Date: Wed, 1 Nov 1995 14:23:41 +1100 (EDT)[...] I offer a little info on what the tyre sizes actually mean. The various manufacturers tend to label the real thing somewhat loosely, so for example some 7.50 16 s are larger in rolling diameter than others... Anyhow, one ROUGH idea coming right up:
Sizes of the form 7.50 16, 7.00 16, 8.25x16, 750R16, 10.50 20 etc. First number is the tyre tread width in inches, the second is the rim diameter suited to the tyre. The overall height of the tyre can be simply arrived at, as the `profile' of this type of tyre is normally 100% (sidewall is as high as tread is wide). Overall rolling diameter= 2X(first number) + (second number) eg. A 7.50 16 SAT is approximately (2X7.5) + 16 = 31" tall (actually closer to 32" but you were warned).
Some tyres come with the sizes simply stated in inch measures: 33X12.5X15, 9X34X16LT etc.
Still others come in the dreaded Metric (ish) measures such as 225 75 16 or 235 85 16... The first number is the tread width in millimetres, the second is the profile (percentage), and the third is the rim diameter (inches). Now the profile number is simply an expression of how high a tyre's sidewall is as a percentage of its tread width. This means that a 235 85 16, for example, has a 235mm tread width, and a sidewall which is 85% as high as the tread is wide (0.85X235=199.75mm).
Now cos I speyk um betta sumtimz, inches: 235mm=9.25" and 199.75mm=7.86" (or so), and by the same reasoning as above, overall rolling diameter is: 2X7.86 +16 =31.73" So in reality a 235 85 is near as dammit to a 9X32X16, and a 7.50 is just a bit skinnier and depending who makes it, possibly smaller diameter.
In all of the above, letters are often mixed to and fro: R-Radial ply construction. LT-Light Truck.
7.50 16 type sizes are usually bias ply (rag) tyres unless there appears an R in these somewhere.
Tyres usually have a `Ply Rating' moulded somewhere into them also. This is not necessarily the number of plys used, but an equivalent load rating. This number gives an idea of sidewall stiffness and durability: too stiff may never get spiked, but will ride like a series LR :) and be next to useless to `air down', too weak and sidewalls will develop bubble like lumps after impacts with logs or rocks, and spike all too easily.
I'll try to dig out the recommended rim widths for various common tyres if people are interested.... Trivia: Silverstone make a thing called an MT117 in 235 85 16 !! Might just be the replacement for my Super Swampers...wassat 34X11 ish :) Tread is like an SAT, but one zigzag down the middle, then herring bone. Tread depth at shallowest point is in the order of 20mm (3/4"). Handling is supposed to be MUCH better than SS on road, and as capable as 9X34X16 SS offroad.... This is second hand, I am yet to see or drive a set. I have a faxed picture and the first set is around on a LC I think, others to follow (new import line for the local Silverstone wing). Small drawback: WILL NOT fit in a standard Disco wheel arch.... oh well, I don't own one anyway, and my RR is, well, modified anyway.
Brian.