17/37 is stock and what my 550's have that get 45-50MPG on good gas. (I haven't measured with the ethanol contamination. I'd expect 3% down for that.)
Ethanol is not contamination.
Ethanol is ethyl alcohol.
Ethanol BTU=76,000
Gasoline (summer)=114,000 BTU
Ethanol makes gasoline burn cleaner.
Sometimes a 3% loss in MPG, and sometimes up to 40% less MPG!
Makes you wonder. Cleaner air though.
It takes .5 to 1.5 gallons of gas to produce 1 gallon of ethanol.
But direct fuel injection systems now use 65:1 ratio of air to gas!
Our old Hondas used about 14:1 air to gas.
Things are getting better.