reading tire sizes

JustGary · Dec 7, 2005

<font color="blue"> "now it has 315 x 75's x 20" </font>

Or maybe written 315/75-20. This means that the tire is 315mm (12.4") wide, the tread is 75% as tall as the width is wide (or 9.3"), and use a 20" rim. So for your case, the mounted tire should stand roughly 9.3" * 2 + 20" = 38.6" tall. Measure your tires to see how close that comes.

For another example, a 315/60-20 is only 60% as tall as the width, so it should stand 7.4" * 2 + 20" = 34.8" tall, which is very similar to your old 35 X 12.5 tires, except for the rim size.

- Just Gary

daTeacha · Dec 8, 2005

So metric designations use aspect ratio, just like car tires. Now what about the English designation with the slash and without the slash? With no slash and 3rd number, is there a way to determine the height of the tire?

rk246 · Dec 8, 2005

Wow they have to make it all very confusing, I am pretty sure my tires on my truck are not 38 inches tall, they actually look a little smaller than my old 35 x 12.50's? I can understand that designation, since it is straightforward, but the whole aspect ration thing throws me off, So is this the same for the tractor tires then...

Shawk · Dec 9, 2005

Here's a shameless plug if you're looking for a new set of tires.... /forums/images/graemlins/wink.gif I have a new set of 4 that were just listed in the classifieds. Any interest... just let me know. 26/12/12 18/850/10 Thanks

daTeacha · Dec 9, 2005

Aspect ratios are simply a comparison of the height of the tire above the rim to the width of the tire.

Picture a tire on a rim sitting in front of you so it could roll directly away. Now cut in half top to bottom so you are looking into two more or less round holes, one at the top and one at the bottom.

Focus on the top one. Measure how high the top of the tire is above the steel rim. Measure how wide it is. Compare the two, height/width That's the aspect ratio. Sporty car tires have aspect ratios between 35 and 60. Let's use 50 as an example. A tire that is marked 235/45 x 17 is 235mm wide at it's widest point. It is half that tall above the rim, or 105.75 mm. Since that 105.75 mm is both above the rim at the top and below the rim at the bottom, you can find the height of the mounted tire by adding 211.5 (105.75 x 2) mm to your rim of 17 inches.

There are 25.4mm in one inch, so a 235/45 tire on a 17 inch rim will result in a standing diameter of 211.5/2.54 or 8.33 inches plus the 17 making for a tire/wheel that is 25.33 inches tall with no load on it. If you mount a tire with a higher aspect ratio, the combination will be taller. One with a lower aspect ratio will be lower.

CT_Tree_Guy · Dec 13, 2005

Let me have a crack at it: aspect ratio is the height of the tire from the ground to the bead expressed as a percentage of the width (as measured at the outmost edge of the sidewalls). A 60 series tire would be 60% as high as it is wide. Higher series (eg. 70, 80) are taller relative to width, lower series (35, 40) are shorter relative to width. (Hmm, I guess you just said all that - oh well).

I think aspect ratios go as low as 25 and as high as 85. Twenty five series would look like rubber bands stretched around a coffee can, handle like the car was on rails, and destroy your rims the first decent pothole you hit. Eighty-five series would have a crazy amount of sidewall flex - definitely not a performance tire. /forums/images/graemlins/tongue.gif

There's a great book that the tire shops use - I got a year-old one that was about to be chucked (car & truck tires only - no tractors). Full of great info - OEM tire sizes, optional tire sizes, lug patterns, wheel nut torques, rims sizes and offsets (5.5J. 5.5JJ, 6J etc.), speed ratings, rotation patterns, approved rim widths for different width tires, who makes what "house" brand of tire, load and inflation tables, how to decode all the little abbreviations stamped on sidewalls, all the good stuff. It's just called (Year) Tire Guide, Bennett Garfield Publication, Boca Raton, FL. 561-997-9229. $10.95/copy (maybe a little more now), or as I say, see if you can mooch an older copy from your tire guy.

Enjoy, it's a real page-turner, I couldn't put it down!

***Spoiler warning - the nail did it.) /forums/images/graemlins/laugh.gif /forums/images/graemlins/shocked.gif

daTeacha · Dec 13, 2005

There's quite a bit on tires that I extracted from a Ford engineer who specs out tires for the different cars they offer posted on the VWenthusiast.com site. Go to the main forum and look in the stickies at the top. This is all car related stuff, but a lot of you guys seem to be car junkies, too. I guess if it has an engine, I'm interested in it.