[roosterk0031]

2nd car for days you don't need AWD, I'd pick a Mirage, idle consumption is probably under 1/3 gallon per hour, $.66 an hour to stay warm.

Forget propane, in 20 lbs tanks it's around $4 a gallon, $5.80 gasoline equivalent. (88,000 btu per gallon vs 128,000 btu, from memory could be off some).

When adding a extra fuel to the air stream the car's LTFT(Long Term Fuel Trim) will go negative using less gasoline, you'd really want that amount of fuel to be constant ratio with the air flow so that the LTFT doesn't jump all over the place. The most you can do without getting a CEL is about 20% of fuel demand. Open Loop or Closed loop the LTFT is still used to adjust the mixture.