I used a home made propane system on a idi I had. When I turned on the propane to maintain speed I would have to let off on the pedal. Yes you use less diesel but now you have to buy the propane. With diesel at $3 and it just cost me $15 to fill a BBQ tank we are screwed. With running dual fuels you may get a slight increase with a better burn but unless you can get propane for free I don't see any big gains