[Diesel_Dave]

Yes, assuming your goal is economical, then you need to compare cost improvement (cents/mile). This will include the cost of diesel + the cost of the LPG. If you wish you could calculate an "equivalent fuel economy" on a cost basis like this:

MPG_equiv=M/(Gd+C*Gl)

where
M = miles travelled
Gd=gallons of diesel used
C= cost ratio of LPG to diesel, e.g. (price of LNG per gallon/price of diesel per gallon)
Gl=gallons of LNG used


That way if the cost per gal for LNG is half that of diesel it "counts" as half a gallon of diesel.

If you objective is technical equivalence, then you'll have to do something similar with energy content instead of price. If your objective is carbon dioxide related that you can weight it by the carbon content.