[daveblt]

When it comes to tire sizing there's one thing that never seemed to make sense. The first number is the width and the second is the aspect ratio or height of the tire. However, as aspect ratios get lower the tire also gets wider.A 60 series tire is wider than a 70 series and a 50 series is wider than a 60 series . My van has 215/75/r15 and are about 8"-8 1/4" wide or so. My car has 215/55/r17 and are 9" wide .The same seems to hold true when you look at other tires sizes in general. 9" is more than 215mm so why is it still 215mm? I used to think that the second number was the width but technically it's not, however tires get wider with lower ratios . Would like some feedback on this.

Thanks

 

[Windows on Wash]

The width measurement is supposed to be a standard but you will see variances from tire manufacturer to manufacturer.

The width should be the width indiscriminate of what the aspect ratio is.

 

[ChuckF.]

There are calculators to do this for you, like this one:
https://tiresize.com/calculator/

These are handy if you want to lower or raise a vehicle without messing with the suspension. Also use them to check circumference if you change tire size so you know what will happen to your speedometer.

 

[daveblt]

The main thing I'm wondering about is the width change . In general any tire brand I see ,if the width is rated at say 215, the width of the tire gets wider as the aspect ratio gets lower 215/70 is about 1/4" wider than 215/75. 215 60 is about 3/8' to 1/2"wider than 215/70 and so forth but it's still rated at 215? and yet the second number is supposed to be the height ..... ?

 

[Windows on Wash]

Just means that the wider tire on the smaller width rim, will have more bead roll over is all. The tread should be the same 215mm if that is the size we are comparing.

 

[lenaitch]

The advertised width of a tire is the 'section width' which is widest part of the tire. The 'section height' is the height of the tire from bead to tread (all measured to industry standards incl. wheel width). The 'aspect ratio is the ratio of the section height vs. the section width. So an a/r '70' tire is 70% as tall as it is wide. The tire sizing standards do not measure actual tread width. A 215 tire should be very slightly less than 8 1/2" in section width. The difference between the section width and tread width will depend on many things, including individual tire profiles, the wheels they are mounted on, etc. Also, accurate measurement of tread width can be difficult depending on tread profiles.

 

[ChuckTin]

Probably one of those things that "big" industry tried to put the kybosh to by making it more complex than necessary.
What do we really need to know - will they fit? & will they last?
Fit is limited by wheel (not tire) diameter - can't put a 14" tire on a 15" rim. Then there's whether or not there's enough un-occupied space in the wheel-well, horizontally, vertically, and laterally.
Add whether or not the wheel offset and profile (usually "J" in modern cars) is correct. Finally is the width of the tire correct for the intended install both at the tread (where the rubber meets the road) and above that where the sidewalls bulge. This last sets the ratio, the "70" in my 215/70R-15s. This is the last vestige of the Radial vs Bias-Ply debate (Does anybody use BPs any more?) where the tire carcass in a Radial is inherently more flexible than BPs; they bulge more and that is supposed to keep the tread flatter at the contact patch.
But will they last for you? Well that A/A ratings is supposed (!) to tell that. Good luck.
Tire company M says the letters mean this, Company G says that and B says this other.

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