"In nineteen hundred and sixty ought nine the plymouth horseless carriage company built this car, and that car being what it was. Way to fantastic for just one name! So they gave it 4 names, the Belvedere, Satellite, Roadrunner, and GTX."
GTX's although they did sometimes get a coat the super fast green, they also had a black stripe added across the bottem which was well known to reduce the horsepower to that of your average orange roadrunner when equipped with the only engine that they shared the 426, then and only then if said green GTX was also equipped the a 426. I suspect the black lower srtipe was one of the first emissions add on's as everyone knows that stripes added above the wheel wells add hp and those below decrease hp. There was also sissified with stuff like Sound deadening Insulation, and rear armrests with the ashtrays facing up in GTX's.
The GTX badges were made with big letters unlike the small and uncapitalized font of the road runner unable to handle wind sheer that resulted from things like 6-packs, or fiberglass hoods so equipping these on a car with such large badges was strongly avoided to the point that they just didnt do it.
GTX stands for "Gooder Than Xroadrunners" but it wasnt always true. Sometimes it stood for notGooder Than Xotherb-bodyplymouthsthatareexactlythesamecaridiot. So in a way, you friend was right.....