My tires say 40 MAX very clearly on the side lol..yours may differ.
The code may be thrown if the gas cap wasn't tight at some point. The light will go off when it's retightened, but the code may have been stored. See if it comes back after you clear it.
But yeah, I'm jealous of the 30mpg lol. I average 19
Maybe your tires are the wrong size and your odometer is reading higher than actual lol