Driving two seemingly very similar vehicles back to back gives a good reference point for comparison. Right after we had tested the Westport CNG bi-fuel conversion on a Ford F-250, we got Venchurs Vehicle Systems' bi-fuel package for the Super Duty. Looking at the spec sheet, the two vehicles seemed nearly identical. But driving and living with them for a few days revealed some key differences.

The Westport conversion added $10,950 to the price of the Super Duty, and the Venchurs conversion came in just a hair less at $10,900.

Whereas the Westport package tries to hide its difference from a regular Super Duty, with the CNG gauge residing where one of the two 12-volt outlets used to reside and its CNG tank shrouded in a plain plastic covering, the Venchurs conversion uses a Ford Racing accessory gauge mounted near the base of the A-pillar, and the CNG tank is covered by a buff-looking diamond-plate enclosure. The most important functional difference from the Westport conversion is a fuel-source switch on the dashboard, which allows the driver to choose between natural gas or gasoline on the fly.

The transition between CNG and gasoline on the Westport conversion is largely transparent, due to the fact you have no control over when the transition takes place. When you flip the switch on the Venchurs truck, there is a noticeable momentary hiccup as the truck changes fuels and, when going from gasoline to CNG, a change in the sound and tone of the engine. Running on CNG, the engine makes a very faint clicking sound at idle, like a diesel, but much quieter.

There was a slight drop in power running on CNG, but the 6.2 liters of all-American V-8 took care of that. Yet there's no escaping the 3.5 tons of truck you're hauling around. A little more low-end torque would be welcome, but such is the compromise of a bi-fuel engine, which has to be tuned adapt to the different thermodynamic and energy density properties of gasoline and natural gas.

Most of the time, the driver has control over which fuel is used, but we observed a few occasions when the engine computer overrode our input, and decided to run the truck on gasoline. Generally when cold, the engine would start on gasoline, then switch to CNG after a minute or two , as indicated by the green light illuminating on the switch marked ALT.

Another quirk on the Venchurs system that we didn't notice on the Westport was that at times the engine seemed to take longer than expected to crank over, primarily in CNG mode. The Westport did not show this same hesitation upon startup.

Like Westport, Venchurs is a recognized as a qualified vehicle modifier (QVM) by Ford, meaning the system does not void the factory warranty, and can be installed, sold and serviced through any authorized dealer.

In performance testing, the Venchurs truck was slightly quicker than the Westport, going from 0 to 60 mph in 9.2 seconds, compared with the Westport truck's 9.7. Measuring fuel consumption on CNG vehicles is not as exact a science as running on gasoline, but the approximate observed average of 9 mpg we measured on the Westport truck is probably applicable here as well.

Either conversion gives truck buyers a painless way to get into natural gas vehicles. For those who like a little more of a custom appearance and flavor, the Venchurs package has a certain appeal. Those who prefer a more low-key appearance will probably choose the Westport. Either package will save you a few dollars when you fill up, if you can find a public CNG station close enough.

2011 Ford F250 Lariat Venchurs Bi-Fuel CNG
BASE PRICE $46,385 (before conversion)
PRICE AS TESTED $57,285
VEHICLE LAYOUT Front engine, 4WD, 5-pass, 4-door pickup
ENGINE 6.2L, 385-hp/405-lb-ft 16-valve V-8
TRANSMISSION 6-speed automatic
CURB WEIGHT (F/R DIST) 7320 lb (55/45%)
WHEELBASE 141.8 in
LENGTH x WIDTH x HEIGHT 232.4 x 79.9 x 79.2 in
0-60 MPH 9.2 sec
QUARTER MILE 17.2 sec @ 82.5 mph
BRAKING, 60-0 MPH 151 ft
LATERAL ACCELERATION 0.71 g (avg)
MT FIGURE EIGHT 30.2 sec @ 0.52 g (avg)
EPA CITY/HWY FUEL ECON N/A