The standard tire size codes used on automobiles have changed over the years, making it tricky to be sure
you're mounting the correct sized "modern" tire on your vintage vehicle. Prior to the 1960's, tire manufacturers
sold mostly "bias-ply" or "cross-ply" tubeless tires. These tires rode hard, didn't handle well and developed
a "flat spot" when the car sat for several days, resulting in a funky "thump thump thump" ride when you started
out the next morning. But we didn't know any better - so we we were content. But then in the '60s tire
makers started offering radial tires! Man what an improvement! On some cars, like vintage Volkswagens,
radials completely transformed the ride and handling. These days we have a huge choice of quality radial
tires from a multitude of tire manfacturers. But what size is best for your car? Usually it's best to go
with a size that's close to what the car originally came with. But since the size designations have changed
over the years, this can be tricky. So let's explain the "P-Metric" tire size codes you'll see on a modern tire.
example, let's say that you have a tire with P215/70R15 embossed in the sidewall. This tire has a width
of 215mm (measured from sidewall to sidewall, not across the tread). The 70 is the tire's "aspect ratio". Most everyone calls this the "profile" or "series" so in our example we have a "70 profile"
or "70 series" tire. The aspect ratio indicates
the height of the sidewall as a percentage of the width, so in our example the sidewall height is equal to
70% of the width of the tire. Profiles of 70 and above are generally considered to be a "normal" sidewall
height. A profile less than this is often called a "low profile" tire. These low profile tires look
and usually handle better in the turns, than a 70 or above profile. But be warned that as the profile
gets lower (meaning a shorter sidewall height) the sidewalls have less "flex" so you're gonna have a
stiffer ride. If you're using the original wheels it's best to stick with a 75 or 80 series tire.
Ok, back to our example; the 'R' indicates
that this is a radial tire, and the '15' means
that this tire mounts on a rim with a 15" diameter. So using the table below, you should be able
to figure out which modern "P-Metric" sized tire comes closest to your ancient original tires
or to the tire size shown in that tattered glove box owner's manual!