Tyre Sizes - Want Some Clarity

[NUT515]

Hey guys want to clear up a debate.

Question im asking is as follows. If I had the following 2 tyres 245/18/40 and a 245/20/40 is the actual width of the tyre different or does the 245 stand 24.5cm across the tyre?? I realize the side wall changes in height between the 2 but want to know about the width.

Feel like a dick asking it but figured someone could answer it

[R31Nismoid]

Yes it's different depending on what you're after (and brand of tyre).

The profile is a %, not a set amount, hence it's called profile.

For a visual - use the Miata calc...

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

[XKLABA]

is that how it is written on the tyre, cause that means they are 245mm tread/ 18 and 20% profiles/ and for 40 inch rims ?

if not I'll assume it's 245/40/18 and 245/40/20, 245, is tread width/ 18-20 is rim size in inchs/ 40 is profile which is 40% of the tread width which is 245mm

so yes the width is the same, the profile is the same so the side walls are the same, just the rim size is different one is for 18 inch rims and the other is for 20 inch rims

[R31Nismoid]

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference

245/40-18 3.9in 12.9in 25.7in 80.8in 784 0.0%

245/40-20 3.9in 13.9in 27.7in 87.1in 728 7.8%

[XKLABA]

Hey guys want to clear up a debate.

Question im asking is as follows. If I had the following 2 tyres 245/18/40 and a 245/20/40 is the actual width of the tyre different or does the 245 stand 24.5cm across the tyre?? I realize the side wall changes in height between the 2 but want to know about the width.

Feel like a dick asking it but figured someone could answer it

yes 245 is 245mm of thread width and they are the same width

[XKLABA]

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference

245/40-18 3.9in 12.9in 25.7in 80.8in 784 0.0%

245/40-20 3.9in 13.9in 27.7in 87.1in 728 7.8%

the the width is the same, the profile is the same so the sidewalls are the same, but since they are for different rims there is a 2inch/50.8mm diameter difference which inturn give an circumference difference

[MrStabby]

if not I'll assume it's 245/40/18 and 245/40/20, 245, is tread width

Actually tire width is quite complicated. The 245 number does NOT tell you that what the "tread width" is, that is, how much rubber is on the road. You have to look that up from the specs of the tyre, using the 245/40/18 type number as key. Not all manufacturers publish it. Tread width is probably the best dimension to use if you're worried about clearance at its at the periphery where you're going to get rubbing. For example, when i changed tyres to Falken RT-615s (S spec, more track friendly than regular street tyres) i got rubbing at the rear because the Falkens had a wider tread width than the previous tyres, despite both being 255/40/17s. Google it.