In reality, any 'vehicle' which does not meet both passenger car safety and passenger car fuel economy standards [1] should only be allowed to be licensed as a "commercial" vehicle and should pay "commercial" rate (higher) taxes (and maybe even require a "commercial" stamp on one's drivers license to operate). If the states had done this when SUV's first appeared on the scene, we might not have nearly every car on the road being a hulking monster SUV today.

[1] which no SUV's nor trucks used as passenger cars, do because they are classed as 'utility vehicles' and have lower safety/fuel economy standards -- which is why the auto-makers went whole-hog on making/selling them, it got their CAFE averages down artificially.