Since the 90's there have been tire 'calculators' that try to produce
tire dimensions based on the standard nomenclature, such as 245/40-18.
There *is* a mathematical meaning to those numbers, but the problem
is that they are like the viscosity labels on oil. They cover a range of
real, differently dimensioned tires. If you have lots of room for differences
and don't need to ensure that tires have identical diameters etc, then
the 'calculators' will probably come close enough, but if you have tight
clearances to contend with, and/or need the tires to match with diameter
like us, then you really have to read the individual tire spec charts to
get the truth, or as close as you can get. I have seen incorrect data
in published vendor-produced tire specs too.
I do recall the diameter of two 245/45-16 tires I used to race, being
over a half inch difference in diameter, *from the same manufacturer*,
just different models. Street tires are converging slowly to being more
uniform, but if you are at all involved with high performace tires, all bets
are off. I just got off the phone with a vendor that is discontinuing a
305/30-18. The replacement is a 315/30-18. *The tire dimensions will
be identical*. They just decided that the tire had always been closer to
the 315/30-18 meaning, so after years of production, they are just
changing what they print on the side...
Joe

 

Great information. thanks