As a matter of clarifying metric tire sizes . . . which is "historic", possibly a little farther back than some might realize . . . the original metric sizes were in millimeters rather than inches (as we used in the USA, for section width designations). These were what I'd consider "true dimensions" rather than otherwise. An 8.25x14 tire size (in the pre-section width designation-use times) would have been a 205x14 tire size. When the USA "feds" mandated tire sizes (physical for each tire size, allowing for a 7% variation from their specs) and also then designated the "letter designation" (for better alleged consumer understanding and shopping results), that's when the section width was added to the mix. The "old metric" 205x14 became a 205R78x14 size and then "GR78x14".
When the need for lower rolling resistance tires (and higher inflation pressures, up to 35psi for normal passenger vehicle tires, "standard load"), this newer design orientation led to the "P-metric" sizing, circa 1979. This new change caused MUCH confusion of what older size equated to the newer P-metric sizing. Initially, as the newer P-metrics used higher total inflation pressures and slightly increase load carrying capacity, one prevalent orientation was to use equivalent weight capacities for such cross-over sizes. Unfortunately, such an orientation usually resulted in tires being used which were physically smaller in outside diameter (and the resultant "revs/mile" spec). This was not good, appearance-wise or for speedometer accuracy.
The "P" in "P-metric" meant it was for passenger vehicle use, plus containing other letter and designations we hadn't seen previously. In the case of Corvettes, Z/28s, and TransAms, they used the GR70x15 size in 1978, then the P225/70R-15 size in 1979. Both of those tire sizes were direct equivalents in revs/mile specs and other size aspects. GM used the same speedometer gears for both sizes, too.
In 1973, due to the performance orientations of the G70 sizes (14" and 15"), almost ALL of them were "white letter" tires, not blackwall, although there were also some in that size that were whitewalls, from the factory or otherwise. If you had a performance tire, you wanted others to know about it! Buick termed them "Billboard White Letter" tires, or similar, in their sales literature of the time. Such "white letter" tires were just as much of a status symbol, back then, as whitewall tires were an upscale or luxury orientation from the basic, standard level (in many cases) blackwall tires. It was not until the 1980s that we started to view "blackwall" tires as more or a "Euro-influence" situation and whitewall tires seemed to totally disappear from vehicle option lists.
In the past several years, I've noticed that some are now using "205/75R-15" tire sizes to really mean "P205/75R-15" tires. The "P-metric" is a federal US tire sizing system. I suspect that the sidewall will still reflect the "P-metric" sizing although the sales literature might not, for some reason, especially as those listed without the "P" are of the same exact size specs as those same tire sizes with the "P" in the sales literature listings.
At the factory equipment level, and related replacement tire level, the GR70x15 tires were either white letter or whitewall. Same for the earlier bias ply sizes, as I recall, especially in OEM brands, although some other non-OEM brands might have also had them in blackwall sizes.
One reason why "reversed" tires don't work is that the inside sidewall was where lots of additional things happened. For example, the tire serial number is stamped, there, plus other production-related things which are NOT on the outer sidewall's surface. IF they were the same inside or outside, that would be different, but they are not and never have been that way. In this case, "cosmetics count".
In the case of your Buick, I strongly suspect the OEM tire brand would have been UniRoyal, or possibly Goodyear.
The GR70x15 size was not only used on Corvettes, Z/28s, and TransAms, but also on Monte Carlos and Grand Prix, back then. It's on these other cars that they were available from the factory with "white stripe" whitewalls other than white letters, due to both the performance and luxury orientations of those two cars.
Also, in the 1973 orientation, I strongly suspect your Buick probably had bias-belted tires (not reflected in the G70x15 tire size, as that was a build-orientation rather than a size designation issue) rather than the prior "bias ply" tires. Getting that type of tire can be even more problematic, especially in a blackwall tire. Therefore, although I haven't looked at the AACA Judging manual, I feel that your best alternative for modern, currently-available tire is the P225/70R-15 size. That tire has appeared on many Corvettes, Z/28s, TransAms and such with good performance and cosmetic results AND was what the factories used to replace the earlier GR70x15 tire size. The sizing is correct, too. IF you might prefer a whitewall tire, they are available in some brands, too, as you can find on TireRack.com. For a GS, though, the white letter tire would most probably have been the tire that came on it as a part of the GS package.
Just some thoughts and clarifications . . .
NTX5467
Edited by NTX5467(see edit history)
