Minivan Mileage vs. Trip Computer

We have a green minivan that has a trip computer in it that, among other
things, tells us our average fuel economy, current fuel economy, and
time in the car with the engine running. We usually reset these
functions at each fill up.

The van needs two new tires, so I drove it to work this morning to drop
off at the tire store. But first, the van needed gas. So I put in
16.075 gallons at our local Costco, at $2.699/gallon, the same price as
earlier this week. The van had gone 254.0 miles, making for 15.80 mpg.

The trip computer indicated our fuel economy for this tank was 16.4.
About a 4% difference in numbers. If I stopped the pump after the first
click, mileage would have worked out to be 15.98 mpg, still lower than
the trip computer.

Almost all the miles we put on the van are "around town" miles, trips of
less than ten miles, and frequently less than five miles. There are
times we go three weeks between fill ups, so I guess 15.8 is okay for
this sort of driving in a minivan.

The trip computer showed fuel economy of 29.4 mpg for my commute to work
this morning in the very same minivan, but with the 4% error, my mileage
may be as low as 28.32 mpg for the trip.