Nerys,
Diesel only has 10% more energy per gallon than Gasoline.
Read the LHV for Gasoline/Diesel.
Higher compression increases the power by increasing the thermodynamic efficiency. Usually it's mixed with a high revving sports cars because high rpm cams bleed off compression which means they don't make very much torque at low RPM. A good example of high compression in an economy car is the First Gen Honda Civic Hybrid, 88ft-lbs of torque at 2800RPM from a 1.3Liter and 10.8:1 Static Compression on regular grade gasoline. Peak efficiency for a spark ignition engine is around 17:1 and cannot be done on Gasoline with a spark plug. Diesel engines run leaner than Gasoline and use compressed ignition but they usually need a turbo to keep generating power past 3k RPM.
It looks like washing Gasoline is working for you. The change in MPG is ridiculously higher, 16% more MPG?