[California98Civic]

No race or measurement dimensions, all too hard to "enforce" and the X-Prize already exists. Instead, generate media attention for cheaper and cleaner alternatives to current US fleet through a nationwide "game." All cars converge on Detroit on a given date, and then fan-out on a vast scavenger hunt of sorts, taking pictures of one's car at some site. Sites could be museums, or a factory, or a race track, or other things less topical but just fun. Roll into small cities with local press alerted that this odd squad of modded cars involved in a nation-wide game are on their way. You'll get reporting if your timing is right, and the reporting will raise the issue (usually). In fact, one item on the hunt list could be a pic with a news team. let contestants track and report their MPG in any reasonable way. With the pressure off that factor as the heart of the competition and redirected as the substance of a media message I think you'd avoid the "cheating" on logs.