When advertising the fuel economy of new vehicles, if the advertisement lists both city and highway fuel economies, they must be labeled with “estimated city mpg” and “estimated highway mpg.” If only the city or highway estimate is included in the ad, it must have the corresponding “estimated city mpg” and “estimated highway mpg” label, meaning you can’t list the highway mileage in a way that might make customers believe it’s the vehicle’s city mileage. Also, the estimates provided must match the exact make/model advertised. For example, you can’t advertise a Toyota Camry with the estimates of a Toyota Camry Hybrid. 
Penalties for non-compliance:
Civil penalties can range from thousands of dollars to millions of dollars, depending on the nature of the violation. The FTC can also impose cease-and-desist orders for the deceptive ads and require advertising to correct the deceit communicated in the original ad. 
Please note: This is not legal advice and dealers should always seek the assistance of qualified legal counsel.
From "19 Laws, Rules and Regulations That Can Cost You More Than Money" in the September 2010 issue of Auto Dealer Monthly.