Decades ago, harkening back to the oil shocks of the 1970′s, the US Congress instituted what would seem to be a reasonable idea of discouraging gas guzzlers. Vehicles below 22mpg are slapped with a special tax, usually referred to as the gas guzzler tax ( the legislation was called: The Energy Tax Act of 1978). The tax is collected by the manufacturer or importer, and is paid by the first buyer.
An economist would refer to such a tax as a disincentive to guzzling gas (actually a disincentive to producing/buying gas guzzling vehicles).
As is usual, what would seem to be a good idea is destroyed by spineless politicians and weaselly lobbyists.
Therefore, SUVs (light truck), of course, are exempt
In 2005, about 170M was collected. There are untold hundreds of millions of dollars of foregone revenue due to the SUV loophole.
Source: TRANSPORTATION ENERGY DATA BOOK: EDITION 26–2007, Tables 4.20 and 4.21
(a treasure trove of energy data. Also interesting is the studies on speed vs. mpg — generally vehicles peak around 50mph).