Oh & if yer rilly wondering 'where': Aspect Ratio and Rim / Pan Width.
aspect ratio Aspect ratio is, as you know if you read the bit above, the ratio of the tyre's section height to its section width. The aspect ratio is sometimes referred to as the tyre 'series'. So a 50-series tyre means one with an aspect ratio of 50%. The maths is pretty simple and the resulting figure is stamped on all tyres as part of the sizing information:
Aspect ratio = Section height
Section width
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.
An ideal rim-width calculator
Blimey I'm good to you. Can't figure that maths out either? Click away my friend and Chris's Rimwidthulatortm will tell you what you need to know. Obvious disclaimer : the results should be verified with the tyre dealership/manufacturer.
Your tyre size: / R x up to x
Too wide or too narrow - does it make a difference?
Given all the information above, you ought to know one last thing.
A rim that is too narrow in relation to the tyre width will allow the tyre to distort excessively sideways under fast cornering. On the other hand, unduly wide rims on an ordinary car tend to give rather a harsh ride because the sidewalls have not got enough curvature to make them flex over bumps and potholes. That's why there is a range of rim sizes for each tyre size in my Rimwidthulator above. Put a 185/65R14 tyre on a rim narrower than 5inches or wider than 6.5inches and suffer the consequences.
