32 mile per gallon of what? You need to do the energy BTU calculations to determine if there is any increase in energy utilization efficiency. Physics says there won’t be.
Propane has a higher octane rating than gasoline (104 to 112, compared with 87 to 92 for gasoline) and potentially more horsepower, but its lower British thermal unit (Btu) rating per gallon results in lower fuel economy.