In a perfect world I do think that a US version of Top Gear could work. The issue of "no good American cars to test" is easy to get around. First, there are plenty of fun tests that they could do with domestic cars...classics vs retro-mods, "performance versions" vs based models, etc, etc. Second, Top Gear is not exclusive to UK cars, they test everything under the sun and the US version could do the same.
Then, I think that the conversation over who to use as the US Stig would be interesting as well.
But Alas, it's the States circa 2008 and commercial TV will ruin it. I will tune in to the BBC America Monday nights for the "orginal".