I'll just load several things into this same post...
First off, the xB is VERY sensitive to driving style, and while the 'brick on low profiles' shape doesn't do well for MPG at high speeds,
how rapidly you accelerate and how often you change speeds hurts it even more.
Secondly, every extra 100 pounds you carry costs you another 0.1-0.2 MPG.
Third, most synthetic oils have less 'drag' in the engine than most 'ordinary' oils, and give fractionally better milage.
Fourth, too soft tires reduce your milage - running a tire 6 PSI low will generally lose about 5% of your milage.
Fifth, gasoline with alcohol in it will absolutely reduce your MPG. Gasoline contains about 115000 Btu’s per gallon, and ethanol only 84000 Btu. That means that ethanol only has 73% as muich energy per gallon as gasoline, and will only push your vehicle 73% as far per gallon. (I.E. if you get 38MPG on gasoline, you will only get 28MPG on ethanol.) Even just 10% ethanol in regular gasoline (common now in many areas) will reduce your milage.
IMPORTANT: Toyota specifically recommends against using any fuel in our vehicles that has more than 10% ethanol - and that counts anything you might pour into the tank as an additive. If you are using a 10% ethanol fuel (E10) do NOT add anything to your tank that has alcohol in it. NEVER use E85.
Sixth, Toyota specifically recommends only using one of the "Top Tier" gasolines to ensure adequate detergents in your fuel to prevent build-up of deposits. This very much limits the fuels you can use, unless you are willing to chance damage causing deposits in your engine.
Any of these brands is acceptable to Toyota:
QuikTrip
Chevron
Conoco
Phillips
76
Shell
Entec Stations
MFA Oil Company
Kwik Trip/Kwik Star
The Somerset Refinery, Inc.
Chevron-Canada
Aloha Petroleum
Tri-Par Oil Company
SEE: http://www.toptiergas.com/ and
http://autorepair.about.com/od/generalinfo/a/110305.htm and
http://www.caranddriver.com/article....rticle_id=9752
(Personally I try to stick with Chevron, 'cause I like their additive package.)
Seventh, well, seventh is all the rest of the things that can mess with your milage, from a wet road to eroded sparkplugs, to having a bunch of electric stuff on (headlights, fan, high power audio, etc.), from running your air conditioning (or defroster) to having wide tires. Heck, even some brands/models of tires are very much better - or worse - in the milage they deliver. Same for wheels - some slide through the air, some create tremendous drag - just like four paddle wheels churning away on the sides of your box.
Still, overall, our boxes get good milage, and certainly better milage than any other deccent vehicle with the usable space of ours.
Here's how mine does. I can readily see when I was driving 'in a spirited manner' and when I was just cruising on the highway on a trip. (Note also that I correct for the error in the odometer readings.)
I'll go away and let you digest some of that for awhile, OK?
*mutter* damned tpyos!
bad gas mileage?? 
