how do you measure tires?

[tabahr]

example : 285/40 R17

is the 285 measurement taken directly from sidewall to sidewall or diagonally from the bead on one side to the tread edge on the other?

I'd swear I've seen it both ways in the past, but tirerack's website only shows the strait accross method. Am I going nuts?

[Injuneer]

the "285" number represents the "section width".... .the width of the tire at its widest point, on the sidewalls. No other meaning.

[Jeff_in_Atl]

I love pictures. http://www.tirerack.com/tires/tiretech/general/size.htm

[raul.garcia]

285/R40/17
285=thread width / R=radial, 40=percent of over-all width/ 17=wheel size
285mm is the over-all width of the tire.
40 (40% of 285) is called the Profile, which is the width of the sidewall (285 x .4 = 114 mm)
convert mm to inches 114/25.4 = 4.49 inches
add wheel size and sidwalls for total height of tire.
4.49+4.49+17 = 25.98 inches

edited by raul.garcia at 18:20 to reflect posted errors:

[Black97]

Originally posted by Jeff 95 Z28
I love pictures.

I always say pictures are worth more than a thousand words.

Before taking a machine apart I take a lot of pictures. When it comes time puting is back together people ask me where what goes. I can show them the pictures and the aha effect sets in.

[SteveH37Formula]

You almost got it. R stands for radial tire. usually you will have a Z or H if its speed rated by the rim size like
P275/40 ZR17. Thats whould mean

P for passenger tire.
275 tire width.
40 series aspect ratio.
Z for speed rating.
R for radial wich all pretty much are anyway.
17 the rim diameter.

[Jeff_in_Atl]

According to the Tire Rack and Fred, the number is the tire width not the tread width. I didn't know that either.

[tabahr]

Originally posted by Jeff 95 Z28
According to the Tire Rack and Fred, the number is the tire width not the tread width. I didn't know that either.

So... there is no diagonal measurement. I should stop smoking so much crack.

[wolfdogs]

Originally posted by tabahr
So... there is no diagonal measurement. I should stop smoking so much crack.

no. no diagonal measurement.

yes.... the 285 is the section width, as stated... to get the tread width..you have to measure the tread itself. i.e.: a firestone firehawk sv50 ep's tread is slightly narrower than a goodrich gforce kd....yet they are both classified 285's....thats because..the section width..on the sidewall at the widest point...is the same.

the "p" stands for "passenger".....or "p metric".
P type tires come in only one load range rating, which is affected by air pressure to max inflation. LT (light truck) tires however, come in load range C, D, and E. at max inflation...a C load range may only fall 2/3rd's of the way to the max load range and inflation of an E.

speed ratings start a S and go all the way to Z. Z is an "open ended" speed range... the manuf. states that in that size..it will perform at the top speed of the vehicle its placed on.... W goes to 168mph and Y goes to 186..... most commonly..at the higher speed fitment of cars... the manuf. just goes with Z. you will also notice a "load index" sometime.....that refers to what load the tire will carry at its max speed rating....for instance.. P285/40R217 94Z........... that means...the tire will perform at the top speed of the vehicle with 94% of the load on it. the higher the speed...the lower the index drops... on some exotics...its common to see 90Z...... load causes flex, which causes heat.

alot is taken for granted with tires...... but they are much more of a complex animal than folks think. ive been a tire engineer and tire testing engineer now since 1983.....ive seen it all....and then some..... these puppies need to be respected...and kept properly inflated . check your tire pressures once a week at least. it only takes a second........ or two. and when it gets alot colder..checkem right away.. you will loose air as the temp drops.