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1975 GM Tires


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(In response to a similar thread in the General Forum, which was noted to be "moved", but a "You can't view that" message appeared instead)



For the 1975 model year, GM brands debuted their new "efficiency system" for their cars and light-duty trucks.  Key items in this new "system" included the first use of catalytic converters and unique radial tires.


The catalytic converters allowed engineers to recalibrate engine systems for better drivability, general performance, and fuel economy.  A little richer fuel mixtures and better spark advance curves were more like year's-earlier models, but better refined, and used the catalytic converter to do the cleaning function for the exhaust emissions controls.


The radial tires were "special" in that they were designed by a new group at General Motors who dealt with tires and how they interacted with the vehicles they were installed on.  Hence, the GM TIRE PERFORMANCE CRITERIA number for each tire installed on GM vehicles.  Other than radial construction, the tires had a particular GM-spec tread design, which was also the first use of a radial "all season" tire on GM vehicles.


The tires were, typically, a little wider in section width (although rated at the "75" aspect ratio) and also had a little wider tread width (with wheels that were about 1/2" wider in rim width, than had been prior practice).  The GM-Spec tread design featured a solid center tread rib with a more "sectional" tread design from the center outward, which gave it more "all season" capabilities than normal "all-solid-ribbed" tread designs). 


GM engineers also worked with the tire companies to ensure that all tires had the internal guts they needed AND GM engineers determined that the needed for the best combination of ride/handling/wear for the particular GM vehicles they would be installed on.  FIRST time that had ever been done on that scale in the industry.  Then, it didn't matter which company built the tire, they would look the same, act the same, and wear the same as the OEM tire on the vehicle when it was built by GM.  Different brands could be installed on the same axle, if needed (by the customer, should a tire failure occur and the particular brand of tire was not readily available).  It was a great consumer-focused orientation, which would ensure continued customer satisfaction in that the GM vehicle would continue to perform "as designed" as it aged and tires needed replacement (according to GM literature).


As for class judging in the present time, these tires are not, to my knowledge, being reproduced by anybody.  Some judges might not know of these tires' existence, either, as this was something that few might remember happening.  Most GM tire literature (think Owner's Manual items and 1975 literature which detailed this new "system approach") might have a rendering of the tire and tread design. 


Now, interestingly, I don't recall BFGoodrich having a GM-spec tire back then.  Their tires usually appeared on Ford products, but had what I termed a "Ford-spec" tread on them.  It was similar to orientation to the GM-spec tread design, but was different.  UniRoyal, Firestone, General, and Goodyear all were GM OEM suppliers back then and had the GM-spec tread designs on their OEM-spec tire sizes (many with the appropriate T.P.C. number cast into the outer sidewall.


I drove many GM pickups back then and also had a '77 Camaro (PosiTrac-equipped) with the GM-spec tires and I can attest to their snow capabilities (at least here in North Texas) and wet weather capabilities (with proper inflation pressures).  The "ride" was more radial than bias ply, but was not quite the same "radial ride" as Michelins had back then.  Therefore, I'd rate it more like a quality bias-belted tire (with radial tendencies) rather than a "pure radial ride".  They were good tires and were the pre-cursor to the more aggressive "all season" tires which came in the earlier 1980s (when the true GM-spec tread was phased out of production, although the Tire Performance Criteria contined to live on into current times.


Those 1975-era tires would have been alpha-numeric in size designation (i.e., H78-15) as the P-Metric designations were didn't happen until about 1979 or so model year.  Which might further complicate tire size criteria for class judging situations.


I suspect that the GM-specs were quietly incorporated into the tire OEM's normal production stock.  With the volume of tires that GM purchased, it probably made economic sense to do that.  Some could get the tires with the GM TPC number on them as others had the blank area such a number would have been in on their sidewalls.  All with the same tread design and other cosmetic similarities.


The GM TPC Group is still around and functioning as in the past.  GM-OEM-use tires still have particular TPC Numbers on their sidewalls, which means that what goes into these tires are approved by GM and built to GM quality/performance/durability specs.  I personally suspect this was one reason that (a particular brand of tires) tires on GM vehicles typically did not have the same durability issues as (the same brand) tires used by other vehicular manufacturers.  That company's tires used on GM vehicles had a slightly different name designation than the other OEM's tires that had durability issues.


I hope this might explain "tire things" for 1975 (and later) GM vehicles.



Edited by NTX5467 (see edit history)
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General tread pattern would be like . . .


-- // // II \\ \\ --    with "--" being the tread's edge blocks, "//" and "\\" being the center tread blocks on each side of the "II" center rib.



The similar Ford-spec tread had differently-shaped tread blocks of the same general orientation.  If you ever saw them side-by-side, the similarities would be obvious.


Pirelli P77 is of the same GM-spec orientation, but with a more aggressive tread block design.  Inner "half" tread was of a more "all season" compound and had more aggressive tread than the outer section (designed for better cornering and high-performance" characteristics.  Rubber compounds also varied from the inner and outer sections to further enhance performance potential.



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  • 4 weeks later...

I never saw the OP that NXT5467 refers to in his 'reply' above, but the topic title caught my eye and I am happy to be able to add a little specific info on the OEM tires that came on my 1975 LeSabre convertible. While I've put four new tires on the car, I still have the originals which are in pretty good shape after running for about 15k miles. The original spare, and the jack, have never been out of the trunk! As can be seen in the photo below, NXT5467's little tire tread 'character sketch' is spot on.


The OEM UNIROYAL tires are prominently marked...


Inside the whitewall, close to the tire bead, are the following markings...


Hoping this additional info might help someone out there! ZT

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