Calculating Tire Size
 

I often see people inquiring about metric tire sizes and what they are equivalent to in inches. For those who do not know how to convert a metric tire size to an inch equivalent I wanted to post this. Hopefully you can find it helpful.

Let’s use a common stock tire found on the F150:
275x65x18
275 = width in mm
65 = aspect ratio, this is the side wall height as a percentage of the tires width
18” = rim size in inches

To convert this tire to standard units (inches) use the following steps:
(NOTE: I round to the 100th place)
1. Convert the width in mm to inches: 275/25.4 = 10.83" (25.4 mm = 1 inch)
2. Calculate the sidewall height: 10.83 x .65 = 7.04" (don’t forget the decimal as this is a %)
3. Calculate final tire height: (total sidewall = sidewall x 2) 7.04" x 2 = 14.08". Now add your total sidewall height to your rim size, 14.08"+18" = 32.08"

This means a 275x65x18 is ~32.08x10.83x18

Hope this helps.....

Heres a site that will help too!:thumbsup:
http://www.redrock4x4.com/tech/tire_calc.php

Great thread!

Thanks, glad it helps, I know a lot of members are often confused by the metric sizes. Sure you can go to a website and type in the numbers and get a conversion, but its always nice to know how the values are derived. Makes it easy to quickly put sizes into perspective.

Also note that some tire manufacturer's sizes vary somewhat. A BFG may be smaller then say a Toyo in the same shown size. Also a mud terrain can be larger then an all terrain or a highway tread tire due to the large lugs on the mud tires even though they are shown as the same size. This can make a big difference in whether a tire can fit OK or have some rubbing.

Agree 100%, thanks for adding that in. I guess the general purpose of the OP was to educate how to perform the basic conversion. it may never be completely accurate regardless of the tire, but will definitely get you in the ball park. :beers:

Good info for manual ratios. I always use the Wallace Tire height chart to figure the size out, Its automatic.

http://www.wallaceracing.com/tire-aspect-calc.php

I'm "sticking" this, because it seems useful. Nice thread, mdhawkin! :thumbsup:

- Jack

Thanks Jack, its actually been around for a while and I have shared the link when applicable, I think its lack of visibility came from me not including "HOW TO" in the title. Hopefully now it can serve its original purpose. :D

hey can somebody please tell me the (MM) of my 285/70 R17. i mesured and its basically 32 inches tall. im not good with math and i got a edge so i want to add the tire size to adjust my gauges. i wanna say its right about 2xxx. its all throwing me off. if somebody knows what im talking about please email at future_ndn@yahoo.com and let me know the MM. thank you!

2610. i was almost close. lol thanks alot shotgunz! i appreciate it:beers:

Not so fast! - Ya fergot aboot Squish! :lol:

:beers:
MGD

This is my favorite online tire calculator, it allows you to compare two different tire sizes

http://www.miata.net/garage/tirecalc.html

forgot about who/what? lol