|
Howdy,
Tires with only the tread width and rim size (165R15), have an aspect ratio (sidewall size) equal to 82% of the tread width. The 'R' is simply 'radial'. This convention is known as Eurometric and was the way Michelin and others sized tires in the 1970s and into the '80s. Some tires carried an "SR" designation. I am not sure but I think the SR is Speed Radial. They are still available if you know where to find them.
Tires sized as original to our cars (5.90-15 on the early cars progressing to 6.40-15) were bias belted tires following the inch based convention for tire sizing in effect from the end of WWII until about 1969. Before the mid-sixties the aspect ratio was 90% of the tread width. As cars got heavier and more powerful the aspect ratio was reduced to 80% and tires got wider. Toward the end of production for the 122S, Volvo began to specify tires with the radial sizing convention. The widest tire from the factory was 165mm.
For a short period during the late sixties there was a radial sizing system that listed the tread width as an inch size without the decimal. 640SR15 would be a Speed Radial 6.4 inches wide.
As tires grew ever wider and a variety of aspect ratios were introduced it became necessary to add another designation. A letter equivalent to the width was prefixed before the aspect ratio (F60-15, C70-14, etc.). Sized A through ?? to describe the tread width; sorry, I don't know how high the letter designations reached or the widths they describe. Aspect ratios ranged from 50% to 80%.
Which brings us to the current crop of readily available tires and the answer to your question. The three numbers on modern tires are: tread width in mm/aspect ratio 'R' rim size in inches. The 'R' still stands for radial.
For your car, a tire reasonably close to the original size is the 165/80R15. Most folks elect to put wider (summer) tires on. On my car I run 195/70R15 tires. These produce almost no speedometer error. Others have reported running tires up to 205mm with no problems.
I should note that there was a time when tires were sized following the metric convention but the sizing was prefixed with the letter 'P'. I have no idea what the P stands for; passenger? I don't know if P-metric tires are still around or not. I haven't seen any on the tire racks for awhile now. I do see tires marked 'LT' for Light Truck.
--
Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 52 year old fat man. ;-)
|
