We've gone up and down on this. We typically get 34-36... and then out of the blue two tanks dropped to 25 mpg. We couldn't find any reason.

Now, we are very careful to fill up with only tier 1 gas... and we avoid the station where we got the two 25 mpg tanks. I don't know is this is fact or myth... or why our MPG changed... but we are back to getting 34-36 mpg.