Ditto.
Ditto again; I use one I got from this list which computes the difference between manually computed gallons/odo readings mpg and the HAH mpg readout (non-Navi). Last tank was dead on--both 34.0; more typically, math mpg is a bit lower (.05 to .1), though I'd say at least a third of the time the HAH readout is lower than the math, and occasionally the difference either way is larger, around .2. Pump variations and fill levels could easily account for these differences. I accept the auto shutoff at each pump and don't add more fuel, a "constant variable," I guess. I agree, that over 10 or 20 or 100 tanks, the overall average should be accurate enough to let me know what car and driver(s) have done.
"Two sailboats heading in generally the same direction is always a race, even if one of them doesn't know it" may apply to the mpg discussion here. The fellow who sold us our HAH is a neighbor, and he will drive our HAH in for service and bring it back, saving us a chunk of time since we live 40 minutes from the dealership. He drove it in today for 15K service (already!!). My wife just said "you're kidding, right?" when I mused that I'd lose 5 mpg on this tank by letting him drive the HAH in for service. Have to admit that the Greenhybrid database and my struggle to get to and stay in the 34 mpg group was as great a concern as overall fuel economy. Still happy to have free service delivery and return... I surely can make up the lost mpg on a planned road trip to Tucson :=).

To return to the sailing analogy, I'd say what we have here are several boats heading the same general direction; some of the skippers are just cruising, and some are applying every sail and rig tweak they know to squeeze out another tenth of a knot in boat speed. The database contains people with a mixture of purposes. If I understand Jason's position, the database is more a cruise than a regatta, reflecting a variety of driving conditions and purposes, resulting in a big picture of hybrid performance. Drivers within the database can, if they like, set performance targets by observing what other drivers post, or not. I've been motivated by some of the mpg reports and hypermileage discussions; my mpg has improved because of them. When someone in a comparable boat cruises right past me, I want to know "how are they doing that?" I may decide that the effort is not worth the benefit and keep on cruising, or I may join the regatta and see whether there's any way to match xcel and other hypermilers ( the answer is "no," by the way). I see nothing wrong with encouraging drivers who are posting impressive numbers to conform to the math mpg norms used by the other drivers who have a regatta going inside the big cruise. The emphasis would be on "encourage" given the broader, non-competitive purposes of Greenhybrid.
Lewis