I’ve had 205/70-14s on the back of my wagon for eight years now, using 14×6 aluminum wheels with a slight negative offset (they are biased towards the outside). Obviously that tire is about an inch smaller in diameter than yours, but it’s the same section width. Scott Phillips has 215/60-15s on the stock 15″ wheels on his convertible and they also fit fine. Again, about an inch smaller in diameter than yours but about 0.4″ wider in section.
As for measuring, well, tires are flexible. Both actual section width and diameter depend on both air pressure and rim width.
So I found a TH200.. The seller gave me the tag number 84-01-13245, and I think the casting number 3fm320825. I have searched all over , and cannot find any info. Anyone?
3FM320825 is the VIN derivative.
3 = Oldsmobile Division
F = model year 1985
NOTE: the other tag indicates a 1984 model year. Are you sure this is an “F” and not an “E”? E would be correct for 1984.
M = Lansing assembly plant
320825 = last six digits from the VIN of the car it came from.
The stock tire size on 62 Jetfires and convertibles was 7.00 x 13, which was 25.8″ diameter. The fact that some vendor (who is not Oldsmobile) claims that 180/85-13 is the replacement tire doesn’t mean that it’s correct or even the same size as original.
I can tell you from first-hand information that 205/70-14 on 6″ wheels fit. I can also tell you first-hand that 215/60-15s on stock 15″ wheels fit. Both of those combos are about 25.25″ in diameter. Your 205/75-15s are about 27.1″ in diameter. Those would be a good choice for a 1968-72 Cutlass, but are waaaay too tall for the 61-63 cars.
Look, I understand that tire section width will vary with wheel width for a given tire, but common sense should tell you that the exact amount of change will depend on the height of the sidewall and on the difference between the section width and the rim width. This can’t be a single number for ALL tires and ALL rims. In any case, my point above still stands. With my 6″ wide wheels and 205/70 tires, I have more than 1/2″ clearance to the fender lip.