DOD can really help only the highway mileage. Tahoe 4wd is rated at 15/19 which is pretty good considering 5200 pound curb weight. But it has better aerodynmaics, taller gearing, and all season street tires. Tahoe is not a Hummer.

You want a fast and economical SUV, check out the upcoming Toyota Rav 4 with the 268 hp V6. But Rav 4 is not a Hummer.

It would be interesting to see how a 5.3L V8 could be crammed into the stubby front end of an H3. Maybe a custom builder could do it to build a sleeper road SUV. But I can't imagine what that lump of cast iron would do to the offroad dynamics of the H3.