Hi, I have used Fuelly faithfully for a few years now. What I'd like to see, is an option is filter/search/browse vehicles by fuel type. I'm going to be buying a truck that runs on propane, and there's no way to find the few vehicles on Fuelly that might be running on propane or natural gas, for comparison. They could even be lumped together as alternative fuels?
Decades ago the US Air Force experimented with propane fuelled vehicles, during one or another fuel crisis. They found that, on a volume basis (gallons), that the fuel mileage dropped about 10% from gasoline. That is because propane contains less energy by volume than gasoline. Bear in mind it isn't as easy to find propane stations on the road, should you travel.
In those days (late 1970s) substantially all engines were carbureted. It is easy to use a propane mixer in place of the carburetor. I have no idea how the present generation of fuel-injected engines would handle propane.