That's going to be hard (if not impossible) to get passed into law. Maybe you'd have a chance if you put in some loopholes to the truck tax for farmers, construction workers and such. Nobody needs an SUV unless you go off road frequently and need to carry covered cargo or people while you do it, you'd basically kill that market (something I'd like to see happen, but not this way). And like I said earlier, there already is a gas guzzler tax on vehicles (<22.5mpg I think....
confirmed). You could probably get away with raising that number up to 25. There is also the CAFE tax on car makers, but that has had less than stellar results as well. From
wikiThe SUV is the main reason for the decline in avg mpg since 1987. Americans like big cars. Because the Gas Guzzler tax doesn't effect trucks/SUVs, the people that like big vehicles have moved on to them (mainly SUVs). So you're correct to aim your proposed legislation at them, if only there was a way to aim it so that it didn't hit the people that actually use these vehicles for their intended purpose and only hit the trendy douchebags that need a new Yukon because their neighbor just got one.
(if you can't tell, I freaking hate SUVs and other tall vehicles, especially when a 110lb soccer mom tries to park one)