Does anyone have a good link or care to explain tire sizing to me? I understand there is metric and non-metric. Is there a way to know how big a tire is from it's metric sizing? I know there are some Java converters, but I am trying to understand i there is any rhyme or reason to all those numbers.

I am looking for 34 inch tires to put on my 2004 F-150 when it arrives. stock rims are 18" (but I may change them). i would like to understand tire sizing so I can search tire stores or the interet and know if the size will fit my rims. thanks for all the help!

275/65/R18....34x10x16....blah blah blah....

 

Wow, it's been a while since I've gone through this, but let's see if I can do it some justice off the top of my head.....

Non-metric sizes, such as your example, are pretty obvious - 34x10x16 = 34" nominal height (not installed height) x 10" wide x 16" wheel.

Metric sizes are fairly easy, once you know what the numbers actually mean.

The first number is the cross sectional width, in Millimeters (not tread witdth). The second number is called "Aspect Ratio" and is the sidewall height expressed as a percentage of cross section. 'R' for radial, and the last number is wheel size.

So in looking at your example - 265/65 R18, you would know that the cross section is just over 10" wide (265mm / 25.4 = 10.433"). Sidewalls are 65% of 265, which works out to 172.25mm or 6.78", and your rim is 18".

So to figure out the nominal height in inches of your metric tire, you need to add sidewall height, rim diameter, and sidewall height again (got sidewall above and below the rim....)

In this case, 6.78 + 18 + 6.78 = 31.56"

In most cases, the second number will have more effect on height than the first.... Change your example to a 265/75 R18 and your height grows to 33.64" On the other hand, increasing the 265 to a 285/65 R18 would be almost an inch wider at 11.22" but would only be an inch taller, at 32.58".

I guess if I had to come up with an equation, it would look something like:

((first number *(second number / 100) / 25.4) *2) + third number

Example:

((265 * (65 /100) / 25.4) * 2) + 18

(((265 * .65) / 25.4) * 2) +18

((172.25 / 25.4) * 2) + 18

(6.78 * 2) + 18

13.56 + 18

31.56


I guess that makes it look really complicated, but after you have done it a few times it'll make more sense.


I hope I didn't muddy the issue further....

Allen

 

Well, 03 beat me to it, but since I typed all this up, I'm gonna post it!!!

Using your first example, this is the metric size method. The first number is the tread width in millimeters. The second is the aspect ratio (sidewall height as a percentage of the width) and the last number is the rim diameter in inches of all things……...

Your second example is just using standard measurements throughout. The first number is the tire height in inches. The second is the tread width in inches and the last is the rim diameter.

A rough metric conversion is about 25mm to the inch. So that first example would have a tread width of 275 mm. 65% of that is 178.75 mm and the rim dia is 18”. Converting (still pretty rough figures, as I’m just rounding) gives you about an 11” wide tread with a 7” sidewall and an 18” wheel will give you approx. a 32” tall tire by my crude calculations.

For a 34” tall tire, you’ll need a higher number for the aspect ratio. Something like a 70 or even a 75. Or you’ll have to go wider, like 305 or 315. So, a 305/65/18 should get you pretty close to that magic 34” size……

 

Try this link http://www.nyjeepin.8k.com/calc.html It should take to a page with a bunch of handy calculators for tires and gears and a bunch of other cool stuff. If the link does not work google search tire size converter and click on the ny jeepin result.

 

Tirerack's Tech Center has a lot of good information, including the one on Calculating tire sizes.

For your 04 F-150, keep in mind what tires are availible in different sizes. The only size other than stock that I have seen a tire is 285/65R18 which is about 32.6" tire, .6" larger in diameter than stock. You may be waiting a while before more options are availible.

Okay, I looked harder and found a Nitto Terra Grappler 325/60R18 that is 33", but I would not use it on a 7.5" wide rim.

 

Try www.1010tires.com and select "tire tech."

I would bet you'd find all you need to know there. Hope that helps.