As I drove the Ram, I eyed the CNG gauge like a hawk. The needle seemed to tick away like the second hand on a Rolex, but I was surprised to find I had enough fuel to take a 160-mile road trip with plenty of therms left over to run errands around town for a couple of days.

With nearly 200 miles on the odometer, I finally ran out of CNG. I observed no sudden loss of power, just an LCD readout that silently switched to "Gas" from "CNG."

Using my iPhone, I found a station seven miles from my office. I inserted a credit card and grabbed the pump nozzle. The CNG port on the truck is in the same nook as the gasoline filler, which is a nice thought, but there is not enough space to get your hands easily around it.

The nozzle locked in place, I turned a ball valve to open the fuel line—whoosh! In seven minutes, I was done. The pump read 16.59 gallon for a total of $35.69. Try doing that with $4-per-gallon gasoline.

How was the fuel economy? It depends on how you look at it. By my crude calculations, I hit 11.8 mpg in the CNG mode alone. That doesn't sound great, but consider two things: I paid only $2.15 per gallon, and I have a heavy right foot.

As they say, your mileage may vary.