Tire size's

["Wm. R. Ulman" <twolaneblacktop@xxxxxxxxxxx>]

With all this talk about tires, width, diameter, etc...it makes me
curious as to how to actually read a tire size.  I guess I just never
really understood what it all meant.  For example, back in the day when
these cars were new, and even till I was driving for a few years tire
sizes were labeled completely different.  When new a '66 had 915-15 bias
ply tires.  I think around '69-'70 that changed as well.  Tires were
then labeled as such: G78-14, or L78-15. Now I do know that the 14, or
15 meant the rim size, but I always thought that meant height, no width
of the rim, but I am now thinking that is not so.  That it actually
meant the width of the rim, as the height must be well over 14, or even
15 inches.  All U.S. cars, or tires bought in the U.S. were sized like
this.  Come the late '70's when Jimmy Carter and his administration
tried their ill-fated attempt to get the U.S. population to ease into
the metric system rather than our stand alone measurement system (what,
the foot/gallon system?), one of the only lasting results was the
conversion of tire sizes to metric, and tool sizes too I guess.  Oh
yeah, and soda bottles, but I digress.  

With the introduction of metric tire sizes, everything just went to hell
I think.  It is my understanding that the biggest tire you could get for
an American car after that point was a P235, which most of us run on our
Imperials now as radials.  But these are actually shorter tires than
what was a G, H, or greater size of yesteryear, thereby making our cars
sit lower than they were intended.  I just get lost when my friend Bill
starts talking about height to width ratios.  I always thought that for
example - I had G78-14 tires on my Olds 4-4-2 convertible back in '79.
When I bought my first set of radials in about '82 I got a set of "70"
series raised white letter tires.  I always thought that '70, '60, or
even smaller numbers meant the width of the tread.  Is that not so?  My
Crown Convertible Coupe is running on P235-75R15.  Now the way I read
that is: P235 means the height from tread to rim, 75 means width of the
tire over the tread, which is 75 percent of the tire height, and R15
means rim size.  Is that correct, or can someone explain, simply, how to
read and understand tire size.  In a earlier post today, someone was
talking about getting P245, or P255's, which I read as a taller tire,
which is also a better size for our Imperials 1975 and older, at least
back to the 1950's.

Thanks in advance, and feel free to respond to me individually so as not
to bore the entire list.

Bill
Seattle, WA