Jason, 
>>My question
>>is this: Who the hell cares? Does all of this calculation and discussion
>>of driving habits make any difference? You get however many miles you
>>get out a gallon of gas on a given day or in a given traffic situation.
>>Period. You fill up. You Drive. You fill up again when needed. Simple. 

I believe that checking your mileage from time to time is a good way to find problems with you van. If you average the mileage for 3 or 4 tanks of fuel and compare that to past calculations, then you have a tool to judge the condition of your motor. If the mileage drops more than a couple of miles/ gallon then it may be time to check the "tune" of your motor or check for other problem like dragging brakes, a plugged muffler, poorly inflated tires etc. 

I like to keep track on trips of three or four days because variables like wind, hills and traffic will even out. This is particularly true if the journey is a "round trip". 

George 

'85 Westfalia