Auto Manifesto

March 3, 2008

60 MPG Hummers?

Generally speaking the lighter the car the less fuel it uses, and the heavier it is the more it consumes. We typically measure that as miles per gallon (MPG). But just exactly how would different vehicles compare if they were measured on the basis of weight moved for a given unit of fuel? This is the most accurate metric because it shows how far one ton of weight can be moved with one gallon of fuel by a particular vehicle.

Below is a chart of different current model vehicles and their ton-mile results. Weight data was mostly sourced from the March 2008 issue of Road & Track magazine, as well as Wikipedia (Hummer, Jeep Grand Cherokee), and (Smart ForTwo). MPG information is from the EPA’s new 2008 ratings, except for the Hummer H2 which is not rated. I used this article to obtain the estimate for the H2. According to EPA, the Hummer H3 gets 16 MPG highway so 13 for the H2 is not unreasonable.

For comparison I threw in ballpark tractor-trailer figures. Regular ones are allowed to weigh up to a total of 80,000 lbs GVWR. But not all trucks haul the maximum at all times so I estimated 60,000 lbs and corresponding MPG figures.

It turns out the most efficient vehicle on a ton-mile per gallon basis is, in fact, a tractor-trailer. No surprise there since they’re designed strictly for business use. Passenger vehicles on the other hand are not rational designs. They also have to meet subjective requirements. The most efficient vehicle there is the Toyota Prius.

Taking the efficiency of a Prius and applying it to all the other vehicles (next to last column), we arrive at theoretical MPG figures all the other vehicles might attain. If we really go out on a limb and compare the truck MPG with light vehicles, we’d find that if a Hummer H2 were as efficient as a real truck it would achieve around 60 MPG. Obviously this is not entirely scientific, and trucks use higher efficiency diesel fuel, but it gives us an idea of the kind of room we have for improvement using largely existing technology. It can be done, and I’m convinced the contenders for the Automotive X-Prize will prove it.

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