I would guess that there are other infuences that may have gone unnoticed, e.g., elevation decline or wind shift (or driving direction). There are times when my mileage shoots up (or down) and I don't know why - usually it's elevation changing. And you can't see it. You have to use the NAVI or GPS to tell.

If you get out on, say, a 5 mile stretch of road and try different speeds repeating the same stretch in the same direction, it will probably be the best simulation you can get without running something like the WB simulator program.

'05 Accord Hybrid
'05 Odyssey EX-L
'03 1800 Goldwing
'98 Civic EX (16 yr. old daughter's)