The largest variable is the operator when he fills the tank.

The diesel fuel will foam significantly..........and, the only way you can get consistent mpg readings is to fill it right to the top of the filler pipe. This takes another two minutes at the end to allow the foam to settle.

I'm meticulous about filling to the cap and the SD has a perfectly calibrated odometer. It travels the same roads at the same speeds each week. It still varies from 26.5-28.5 mpg on the winter fuel

I'm looking forward to the summer fuel..........the SD climbs to 28.5-30.5 under the exact same driving conditions.