Pop quiz: What do the Chevrolet Volt and the 2012 Ford F-250 XLT Westport CNG have in common?
Answer: Both have a gasoline-powered range-extender. Though that's where the similarities between the four-seat, plug-in hybrid compact and the 7000-pound-plus heavy-duty truck end, for all intents and purposes, the F-250 CNG's engine functions much the same way as the Volt's when running on gasoline. In the Ford's case, the engine also runs on a different fuel: compressed natural gas.

The main similarity between the two vehicles is that there's no "switch" to select between fuels. The Westport F-250 operates on CNG until the tank runs out, and switches to gasoline only when the CNG tank is empty. The precise fuel switch point is difficult to determine, even when looking at the gauges. During the truck's nearly weeklong stay with us, it probably burned about a half-gallon of gasoline, running the rest of the time exclusively on CNG.

What's it like driving this beast on CNG? Not a whole lot different than your typical Ford Super Duty. The truck is large, with a typical bouncy ride and loose steering. But there are no strange sounds, smells, or sensations that could be associated with CNG operation. The truck just goes about its business like the workhorse it is.

Power decreases by approximately 15 percent when running on CNG, meaning that the Super Duty's 6.2-liter V-8, which normally produces 385 hp and 405 lb-ft of torque, is probably making somewhere in the neighborhood of 330 hp and 345 lb-ft of torque. That much of a power hit is going to affect performance, especially when the vehicle in question is a 7300-pound truck. Though the truck rarely feels strained or underpowered except for aggressive passing or driving uphill, our as-tested 0-60 time of 9.7 seconds is a full 1.4 seconds slower than the gasoline-only model we tested during the 2011 Truck of the Year competition. The Westport also needed an extra 1.2 seconds to finish the quarter mile, taking 17.6 seconds to do so. To be fair to the CNG truck, it is about 200 pounds heavier than the gasoline-only version thanks to the extra hardware.

There are a few other catches with the CNG conversion. First is the availability of publicly accessible CNG refueling stations, of which there are fewer than 1 percent as many as gasoline stations nationally, on average. Thankfully, Southern California has a relative abundance of them, with more than a dozen clustered in the Los Angeles metropolitan area. Refueling must be done in a precise sequence. You start by removing the dust cap on the CNG filler head. Then, you lock the fuel hose onto the filler head and lift the pump handle, an action that creates a sudden "WHOOSH" sound. As the tank gets close to its maximum pressure, it starts to make a groaning noise and then the flow stops. After you pull the pump handle down and unlock the fuel hose, there is a hiss caused by a small amount of pressure being released upon de-coupling the fuel hose from the filler neck. You do get a small characteristic whiff of natural gas as a result, but it's no more offensive than the smell of gasoline you normally notice when filling up. There is also the issue of pumps having various maximum pressure ratings, with some stopping well short of the truck's recommended 3600 psi limit. As a result, it can be a challenge to determine just how much CNG range you have left.

The biggest advantage of the CNG conversion materializes at the pump. We observed gallon-equivalent prices ranging from $1.62 at a municipally subsidized station to $2.05 at Clean Energy Fuel pumps in gasoline stations. While the price difference of more than 40 cents per gallon equivalent is a little higher than the variation you'd typically see with gasoline, it's still significantly cheaper per gallon than gasoline and will likely maintain a significantly lower price margin. With two brief exceptions, during the past decade the price of natural gas has been consistently lower than that of oil, and given the current supply glut, it will probably stay low for some time to come.

The downsides? First, you give up roughly one-third of the available bed space to make room for the CNG tank, which is housed in a plastic shroud that looks like a toolbox. Second, you give up an extra $9750 for the Westport bi-fuel option or $10,950 if you get the optional 24 GGE tank our tester had. Thanks to pump differences and lack of a fuel mode indicator, it was difficult to establish the truck's CNG fuel economy. We managed about 216 miles on a fully pressurized tank before it switched to gasoline, resulting in fuel consumption of about of 9 MPGe. Not great, but not unheard-of for this class of vehicle. Keep in mind that CNG also has a lower energy density than gasoline.

When it comes to towing, even with the significant fuel price disadvantage, the Super Duty's available Powerstroke diesel is the better choice, largely because it has double the torque. Interestingly, an increasingly common aftermarket trend with diesel truck owners is adding a supplemental propane or CNG system that will increase diesel fuel economy and power and decrease exhaust temperatures. Unlike the bi-fuel gasser, these diesel trucks burn diesel and the supplemental fuel simultaneously.

Should you buy an F-250 Bi-Fuel? If your primary usage patterns do not involve a great deal of heavy towing, or carrying long cargo such as plywood or 2x4s, and there are plenty of places to fill up with CNG in your area, the potential fuel cost savings can make a compelling case, provided you don't mind the compromises. If you don't, it's a relatively painless way to get into the alt-fuel game.

2011 Ford F-250 Westport CNG
BASE PRICE $37,855
PRICE AS TESTED $53,145
VEHICLE LAYOUT Front engine, 4WD, 6-pass, 4-door truck
ENGINE 6.2L/385-hp*/405-lb-ft* SOHC 16-valve V-8
TRANSMISSION 6-speed automatic
CURB WEIGHT (F/R DIST) 7320 lb (0/0%)
WHEELBASE 141.8 in
LENGTH x WIDTH x HEIGHT 232.4 x 79.9 x 79.5 in
0-60 MPH 9.7 sec
QUARTER MILE 17.6 sec @ 79.0 mph
BRAKING, 60-0 MPH 134 ft
LATERAL ACCELERATION 0.73 g (avg)
MT FIGURE EIGHT 30.2 sec @ 0.52 g (avg)
EPA CITY/HWY FUEL ECON Not rated
ENERGY CONS., CITY/HWY N/A
CO2 EMISSIONS N/A