Here in the UK pump diesel (#2 diesel) is between 135,000 and 140,000 Btu/gallon.

I'm wondering if your yank diesel is different, esp summer / winter fuels?

Apart from the 1.2:1 ratio of gallons, which will mean a 1.2:1 ratio of MPG (while is why kilometres per litre is a better method) a difference in fuel quality will also affect MPG, or kilometres per litre.

Biodiesel (B100 or 100%) is around 127,500 Btu/gallon, so obviously running on bio will also drop your MPG.

#4 diesel is basically home heating oil.