I always tell my passenger who are curious about the gas mileage that the gauge is off about 3 mpg. So if its reading 58 mpg I would say that its 55 mpg true....
Have you ever gotten 68+ MPG? Even on a single trip? If so, you can always answer, quite truthfully, something like, "At times I can get over 68 MPG, but my average is closer to 65 MPG."
I'm fairly sure I've attained 68+ calculated MPG on a single trip (I've gotten 95 mpg on the 2 mile drive home from the gas station, for example), but that isn't the point. For what it's worth, my displayed mpg (lifetime) is 63 mpg. My calculated mpg (lifetime) is 60 mpg. So when people ask me what type of mileage I get, I state I get about 60 miles per gallon. I'll round off to a whole number, but I don't like reporting 3 mpg off if I know that isn't the case. For some people who I know care about mileage (like my brother), I'll give them the tenths place too (59.8)
So... I'm guessing that you guys also do things like calibrate your monitor, double-check your thermostat, independently verify the temperature of your fridge and oven, check your utility meters...
I also calibrate every clock in the house to the national bureau of standards time stations in Fort Collins. Colorado. I also use pattern generators to calibrate my TVs too. It drives my wife and kids nuts.
My husband doesn't use the national bureau...he is into Greenwich mean time. I completely "get it" don't do it but get it!! LOL