I'm thinking that it is more about trail-etiquette. My '75 didn't come with a cat. That means that if I'm scoring 12mpg, and the same truck fifty-years-ago, was making it to 16, then you wonder what happened to twenty-five percent of the fuel along the way? Is it making carbon monoxide at traffic stop lights? The exhaust kinda fills the cab when the wind is just right, especially when running without a hard-top. I live in the desert, and my FJ40 leads the group because of the bad body and tub seals, and the trail dust gets in, you can see it glued to the recesses of the steering wheel and gear-shifter knob from my sweat. I don't want smelly exhaust, but I know I'm gonna get at least a small amount in this rig.