I've had discussions about milage, and how to track it, is the mpg on the on board computer accurate, etc. Oddly, questions about milage are one of the most frequent things I hear from people when they come up to ask about Ruby.
I've been tracking for a few hundred miles now, Ruby is well broken in (8K miles), has good tires, proper pressure, regular oil changes, and I just serviced the angle drive.
The last loop of 102 miles, 45 were across town, stop lights, 45 mph avg. The rest were country driving, winding 2-lane roads where I kept my speed between 55-65.
As you can see here, the on board computer shows me at 31.4 mpg while Fuelly.com & my calculator show 30.9 mpg. It should be noted that my new rear tire is a 305/35 R20, slightly bigger (+1.6%) than the stock Kenda at 305/30 R20. Among other things, my speedometer is now 1.6% low, when it says 60, I'm doing 62 (or if you prefer, when Ruby says 66, I'm doing 69!) I think this also means I'm traveling farther by 1.6% than the computer thinks I am. That means Fuelly.com is probably 1.6% low, its adjusted milage would be 32.7 mpg.
In any case, the on board computer is pretty much vindicated in my book!