I run a G2500 propane conversion with a 350. Runs great, but on the last tune-up, I thought I would be smart and change my NGK V-power plugs to NGK platinums. While I thought the van ran smoother and quieter, my mileage dropped from about 10 mpg to about 8.5. This was consistent over about 5 fill ups.

Is there something about propane and platinum plugs I wasn't aware of?

 

try using copper plugs and a different heat range. Platinum plugs generally dont do well with propane