<VV>Tires

[Padgett]

 >I'm quite sure that the last two digits of the old "X.XX" sizes had 
nothing at all to do with aspect ratios.

Well sorta. (see the chart about 3/4 the way down the 
http://clubs.hemmings.com/clubsites/chevylist/tech/tire_size_conversion.htm 
page. Note: I do not think the charts or dates are wholly accurate but are 
a good starting place.

Apparently around 1964 "something" happened and profiles got lower 
(apparently from about a 90 to an 80). To mark the change the previous 
sizes (mostly x.00 and x.50) got oddball endings (x.35, x.95) ending in 
x.x5. So *in general* when you see a size ending in "0" that is a 90 aspect 
ratio and ending in "5" is an 80. The next change was when the bias-belted 
tire (sort of a compromise between old style bias ply and radial tires 
which had been coming in from Europe since the '50s and were known for two 
things: incredible tire life at a time when a new set every 10,000 miles 
was common, and unpredictable handling). These were the first tires to 
receive an aspect ratio and letter designation, first 78, then 70, 60, and 
50.  (Probably the belted "wide ovals" were following racing tires which 
had become "low profile" by1965 (see the Car and Driver "2+2 vs 2+2" article).

For a Corvair, this does not really matter since the cars had "old style" 
tires EM 6.50x13 and LM 7.00x13 which were probably "90"s though the actual 
dimensions, particularly tread width,  could vary between manufacturers.

One point above I had forgotten. That the letter designations were 
introduced to designate bias-belted tires, a decades-long kludge that went 
away when true radials took over. I wonder what construction the repro 
letter tires (e.g. F60x15) use today.

Padgett