We have an AWD and the on board computer says 19.4mi/gallon for mostly freeway driving (90%+.) As I read somewhere EPA results are rarely acheived, as well as from experience, as they have some sort of weird test method. It is interesting to note that you say your other cars have actually acheived greater than the EPA rating. You also stated something else that may be part of the reason, I noticed that EPA rates for the 3.3L engine, and this is shared with other Toyotas, (I think a Camry.) The Camry (3.0L) also shared it's engine with the Sienna and Solara. Could it be the EPA uses an average, which of course would disadvantage the larger and heavier Sienna?