Is there such thing as a gas efficiency graph, the same way we have hp and toruqe graphs? I know there's a gas efficiency graph against mph, but I was wondering if it exists for against RPM

Tips to improve your Gas Mileage <-- fuel efficiency vs mph. I think this graph is not scientific; when it says 15mph i think it implies city driving. Of course cruising on 15mph on 5th gear w/o ever stopping should achieve higher mileage than cruising at 50mph.

 

Yes, it's called Brake-Specific Fuel Consumption (BSFC). It's a three-dimensional graph giving fuel consumption (typically in pounds per horsepower-hour or grams per kilowatt-hour) for a combination of engine speed and either torque or engine load.

Here's an example -- not for a Fit, obviously --