Just walking past the TV and there was a dark silver MC filling the screen with the mpg noted underneath."What, a MINI Cooper ad on TV?" Then a Cross over Chevy/GMC SUV pulls in front blocking all view of the MC. The ad continues about the equal mpg blah, blah, hybrid with room etc... I think this will back fire.

I donno but is it legal to mock another car in an AD? Can BMW sue them?

The comparison made appears to be true. Comparative advertising is legal in the USA.

That's one difference with UK and US laws, you can have a dig (as long as it's seen as "fair play") in the US, not so in the UK.