Some vehicles actually do get different mileage from different tires, at least from my experience.
I had a 2001 Honda Insight...it got 50 MPG with one brand of tires and 40 MPG from another. And other owners noted this too.
I would think that with other cars a variation of 20% in mileage may not be and unreasonable variance between tire design.
Of course the higher mileage tires I experienced was not any claim that the tire manufacturer made; it was noted by end users, a better measure I think.
Sure you could sue a manufacturer for making false claims ... but you would need to prove it, very hard to do. Complain to your Attorney General's office may be a better option.
I sold my 2001 Honda about 18 months ago for 5K, 1/3rd of what I paid for it ... not too shabby.
Awaiting the new Elio
The owner of the Elio is being somewhat of a prick from what I understand and has caused unneeded delays ... he has investors will to pump $$ into the company but he wants to give them nothing in return. How's that working in the USA ? Not good. And people put deposits down on this car ... they are foolish for doing so IMO.
I got another 18-24 months before I need to purchase a new vehicle. If all-electric ones (Nissan Leaf for example) prices drop like a rock, I may pick one of those up for local hops and save some gas $$. The only thing that really should go wrong with the drivetrain on this is the batteries. (I have had to change my Honda battery ~ cost me $500 with dealer picking up other costs).