Owners/service Manual problems of the 2012 Toyota Yaris

When I purchased this car in April from carmax , it was advertised as making 28 mpg in the city and 32 on the highway. I am only getting 20-21 mpg (combined, but mostly city), which is what I got with my old datsun that I just sold. This is false advertising. I called carmax and they said those numbers don't come from them, but from Toyota. So I called Toyota and they said I would have to take it to a Toyota dealer to see if something was defective. But as we all know, dealers charge ridiculous prices. I wanted to take it to my mechanic, but Toyota said that they don't recognize outside mechanics. The mileage had so much to do with my purchasing this car that this really makes me mad. When I re-checked the 2012 Yaris mpg on carmax, it showed them as making 32/38 mpg (these were automatics, mine is manual, but I would think manual would be even better mileage than automatic?). Anyway they didn't have any manual 2012's listed. If it were off a mile or 2 or even 3 then okay, there are variances, but to be off a good 7-8 miles/gallon is way too much to not scream "false advertising". Is there anything that I can do - any recourse that I have? yes, for the last 3 gas fill-ups I have recorded my mileage and the amount of gas it took to fill, so I could accurately figure out what mileage I'm getting.