The "tire comparators" are very good tools to use to compare tire sizes as to OD and width, one to another. Back when the alpha-numeric tire sizing system went into effect in about 1968 model year, a CAR LIFE article detailed this move as "consumer focused" so they would not be sold tires smaller than was needed for their car. The "Minimum Size" tire pressure decal on the B-pillar was a part of this initiative. That article also stated the sizes of these new-sized tires had a governmental spec tolerance of 7% from the minimum specs in the chart in the article.
If you go into the various "Specs" charts for the various tire lines at
www.TireRack.com, it becomes obvious that the same size of tire indifferent brands are not all exactly the same as to sizing. BTAIM
Rather than tire OD, that spec is listed along side a "Revs/mile" spec which is much more accurate. Of course, with some math the OD can be converted to "revs/mile", too.
When I was researching the old BFG Advantage TAs for my '67 Newport, that was one of the first places I came into contact with that spec. Little did I know that that spec had been around for decades! Where had it been hidden from view? In the old "A.M.A. Specifications" which all OEMs published and submitted to the Automobile Manufacturers Association (called Motor Vehicle Manufacturers Association in later times) that was a sort of over-seeing group for the OEMs. When I discovered where I could find those old submission lists, I was amazed at the specs in them, including "Tire Revs/Mile specs"! For the old bias-ply tires, the speed for these specs was "@45mph".
For comparisons, the "tire comparator" utilities can be great tools, just that sometimes, how things are in the production world might vary a bit from their "ideal" specs.
Enjoy!
CBODY67
@commando1
