I'm not seething with anti-Saudi resentments like various folks on both sides of the aisle, but insofar as Mark Steyn wants to engage debates with his readers about the best ways to stick it to Riyadh, someone might as well explain basic economics to him:
And, besides, the premise is completely false: If you trade in the Expedition for a Honda Civic, that oil you save won't stay in the ground and thus impoverish the Saudis; it will merely be sold to the Chinese and Indians and other fast developing nations who will replace America and Europe as buyers of the cheapest and most easily extractable oil in the world. So the sheikhs will be as rich as ever and funding as many Islamist nutters. But we'll be driving worse cars and feeling virtuous.
Yes, it's true that US reduction in demand for oil won't necessarily reduce the aggregate quantity of Saudi oil reaching world markets (it sort of depends on a lot of factors), but reduction in the demand for oil reduces the price of crude and therefore the revenue of oil exporting countries. It does so, moreover, whether or not the oil exporting country in question specifically exports oil to the United States. And, indeed, even if total world demand doesn't go down because rising Indian and Chinese demands exceeds the reduction in US demand, that still leaves oil prices lower than they would have been had the US not reduced its demand.