My source gives diesel 132,000 BTUs per gallon, 92 Octane gasoline has 115,000 BTUs. The difference is 17,000 BTUs, or about 14.7% of the gasoline value, so I did make a mistake. Looks like I was looking at the figures for 89 octane to get the 20% value, 89 octane is 110,200 BTU.
I don't have the figures for weight per gallon, but I don't really see how that would pertain here. Fuel tanks on these trucks are rated by volume, not weight and the fuels are also purchased by volume, not weight.