I find my mileage based on "trip meter odo reading divided by difference in fuel between complete refills" is generally higher by at least 3 and as much as 5 mpg than what's given on the trip computer. Then again I know my trip computer is not very accurate as to how much fuel is left in the tank, as my fuel warning lights come on sooner than they're really supposed to - filling up just as the last bar starts blinking, the last stage of empty, usually sees me putting in about 4 to 4.2 gallons out of a tank that's supposed to be 5.3 gallons in size.
As for odo-based mileage, I suspect there's variation based on the shutoff valve at different pumps, and possibly due to elevation. Earlier this summer I went on a road trip from NYC to Boston to the White Mountains (Kancamaugus Highway) in NH to Montreal and back. I filled up at a local gas station before I left, and these were the mileage readings and locations of my subsequent fuel stops. Most of the time I was in freely moving traffic, going about 70-75 MPH on the speedo, with me as a ~200 lb. rider and about 20 lbs. in gear and luggage with me and a Large CalSci windshield:
Providence, RI - after 182.5 miles, filled with 2.75 gallons
Portland, ME - 165.2 miles, filled with 3.12 gallons
Swanton, VT - 241.6 miles, 4.1 gallons
Chestertown, NY - 228 miles, 4.3 gallons
Briarcliff Manor, NY - 211.9 miles, 4.2 gallons
(Then another 40-ish miles to get home)
Odo based average for my trip: 1029.2 miles over 5 refueling stops totaling 18.47 gallons for 55.72 mpg. The trip computer said 52.2 mpg.
I was amazed to only put in 2.75 gallons at Providence, that gave me a mileage of over 66 mpg for that leg of my trip, and that was going on I-95 between NYC / New Haven, CT / Providence, RI mostly at around 70-75 MPH (with a little bit of local city riding mixed in)! But I was surprised the other way to get much worse mileage riding back down on I-87 and then the Taconic State Parkway in NY between Chestertown and Briarcliff Manor, putting over 4 gallons in after only 212-ish miles of the same kind of riding (for just over 50 mpg).
I'm guessing the pump in Providence didn't fill my tank up nearly as much as the other fill-ups, and/or I jammed the neck of the pump in deeper or at a different enough angle that the shutoff valve kicked in early. If the configuration shutoff earlier by 0.5 gallons, it wouldn't be much for a car but would be right in line with the mileage I got overall otherwise.
Last edited by robardin; 09-05-2012 at 03:37 PM.