09 RK with ~6K. I check it every time I get gas. I average 43 mpg. Some more, some less. I try to use shell 91 and fill it the same way. Gas nozzle in the tank until it stops pumping then I pull the gas nozzle up until it is about 1/8" below the sheetmetal hole inside the tank. I barely squeeze the handle and watch for the fuel bubbles right at the small vent hole. If you don't try and get it at the same point, your calculations will be off a bit at each tankfull however, if you average the gallons used and the mileage over 5 or 6 fill ups, you will see about what your average mileage is. Number of miles divided by number of gallons = MPG
But, like someone said...WHO CARES? If you like riding a HD, it is what it is.
And if you like to see how your bike is running and if a problem is developing, a great way to know that is to keep track of your mileage. If the mileage drops for no apparent reason, ie. driving 90 mph, fully loaded, up hill for a few hours, you might have a fuel or ignition problem. At least that's what the 'ol man taught me. BTW, it works!