Did you read the article I linked?

40 is NOT THE HEIGHT. It is a ratio. It is the height expressed as a percent of the tire width.

Increase the first number (the width), and the height of the tire sidewall will increase if the 40 stays the same.

 

im gona have to go with odog on this. here's another site explaining it: http://www.type2.com/library/tires/tirefaq.htm

and thats why it will mess up the speedo just slightly like i originally said. (not even 1 mph at 60 so its negligible)


EDIT: what would 40 even represent. go out and measure ur tire and if it comes up as 40 anything(i.e.-mm, cm, inch) i'll be surprised. it 40% of the width. so if u have a 225/40/16 the sidewall will be 90mm. 225mm x .6 = 90 mm. go out to ur tire and check it out it works

 

nononononono a 225/40/16 will have a depth of 40mm on the side wall of the tyre no 90mm if it was it would be 225/90/16

 

im no tyre expert though enlighten me

 

on that page:

225/40/17 = 3.5" sidewall
235/40/18 = 3.7" sidewall

So no, the sidewall's are not the same height.

Incidentally, that website is an excellent place to put this to rest. Mess around with the numbers and you will clearly see that I'm right.

 

ahhh right no wonder my car feels like it has no suspension so im going to lose the 17s and go back to 15s for comforts sake, any one want to swap seriously ( UK only )

 

Dave, you do realize that 40mm would be an inch and a half sidewall.

Please just go to any of the links that have been sited in this post. All the links agree.

or . . . just get a ruler and go measure your tires

 

i think me and odog are the only tire experts on this page that know what were talkin bout haha
People are stubborn just look it up, my dad used to work for a tire manufacture so i got the low down when i was younger

 

ahhhh this is what you mean

The actual dimensions of a tyre are dependent on the rim on which it is mounted. The dimension that changes the most is the tyre's section width; a change of about 0.2" for every 0.5" change in rim width.

The ratio between the section width and the rim width is pretty important. If the rim width is too narrow, you pinch the tyre in and cause it to balloon more in cross-section. If the rim width is too wide, you run the risk of the tyre ripping away at high speed.

For 50-series tyres and above, the rim width is 70% of the tyre's section width, rounded off to the nearest 0.5.

For example, a P255/50R16 tyre, has a design section width of 10.04" (255mm = 10.04inces). 70% of 10.04" is 7.028", which rounded to the nearest half inch, is 7". Ideally then, a 255/50R16 tyres should be mounted on a 7x16 rim.

For 45-series tyres and below, the rim width is 85% of the tyre's section width, rounded off to the nearest 0.5.

For example, a P255/45R17 tyre, still has a design section width of 10.04" (255mm = 10.04inces). But 85% of 10.04" is 8.534", which rounded to the nearest half inch, is 8.5". Ideally then, a 255/45R17 tyre should be mounted on an 8½x17 rim.

 

That is true but it is a separate issue.
I am talking strictly about the measurements of the tires as they are manufactured (ie their dimensions when not mounted on rims)

cheers.