Michelin invented the steel-belted radial tire, which it introduced in 1946. That turned out rather well for them. In the early seventies, anticipating the trend to ever lower-profile tires, Michelin made a bold gamble to reinvent the genre, with a proprietary metric wheel-tire combination that was not interchangeable with conventional wheels and tires. Although it made considerable inroads for a few years, including a lot of American Fords, in the end the TRX concept went flat.
Michelin’s ideas behind the TRX tire were very valid. As tires moved to lower profiles, their smaller sidewalls naturally resulted in substantially stiffer rides. Michelin, being based in France and so associated with the smooth-riding French cars, looked for a compromise, to gain the performance benefits of a low aspect ratio tire and maintain good ride quality.
The solution was the TRX flange and tire (the TDX was a similar design but also included limited run-flat capability). The wheel flange is angled outward, and not so deep. That allows a more gradual transition from the tire bead to the sidewall, maximizing its ability to flex, even with a limited sidewall height.
The TRX was made in three wheel diameters: 365 mm (14.37″), 390 mm (15.35″) and 415 mm (16.3386″). They generally replaced 70-series tires more than one inch smaller in diameter. In the case of the Mustang and Thunderbird Turbo Coupe, their 220/55 390 TRXs replaced the standard 205/70 R14 tires. That was a big jump in diameter size and aspect ratio, but the TRXs did the job admirably, in terms of no noticeable reduction in ride harshness.
TRX tires were used by a number of European manufacturers, including BMW, Audi, Citroen, Peugeot, Ferrari, Alfa Romeo, Renault , Saab and possibly others. Ford of course, adopted them not only in Europe, but made a major push with them in the US, during the late seventies and early eighties.
In addition to Michelin, Avon and even Goodyear made TRX tires; but not for long (in the case of Goodyear). As tire technology improved with conventional rims and radials, the advantages of the proprietary TRX system soon became insignificant, and the TRX system petered out as OEM tires in the mid eighties or so. Most owners with TRX tires have switched to modern conventional wheels and tires, as replacement TRX tires became ever more expensive. At some point, Michelin sold the molds to Coker, who continues to make them for purists, at a price.
But in 1983, the distinctive graceful large wheels and thin TRX tires were not only desirable from an aesthetic viewpoint, they also functioned superbly. All things must pass, except perhaps for conventional inch-size wheel rims, it seems.