The price of diesel fuel in many regions of the country is outpacing the price of gasoline. This spread may widen as the 2007 on-road diesel emissions standards mandate the introduction of ultra-low sulfur diesel. In addition, the standards will require refined emissions reduction hardware, which could further increase the premium you currently pay for diesel engines.

As a result, you will need to pay more attention to powerplant choice when purchasing new pickups. "It really comes down to a fairly simple mathematical equation of up-front costs vs. the fuel economy difference," says Phil O'Connor, F-Series Super Duty marketing manager at Ford. Savings from the increased fuel economy of a diesel engine must pay for the difference in initial purchase price over the life cycle of the pickup.

The trick is defining the variables in the equation. "The challenge is that a contractor has to estimate the fuel economy difference between a gas and a diesel engine, based on prior experience or talking to other contractors who use the truck the same way they do," notes O'Connor. Next, you have to estimate future fuel prices. Finally, you need to factor in that diesels last longer than gas engines.

"This is what big fleets do all of the time when purchasing trucks," O'Connor points out.

Torque beneficial for towing

Towing will impact engine choice. However, depending upon frequency and the loads being pulled, it might not be the deciding factor.

"A vehicle has to achieve certain levels of performance in order for it to be rated to tow," says O'Connor. "Certainly, power is one of the criteria. So if we rate a vehicle to be able to pull up to 7,000 or 8,000 lbs., it doesn't matter whether it is a gas or a diesel. That vehicle will accelerate up to a level that we deem acceptable with that load being towed."

In the past, gas engines were more responsive when the truck was operated unloaded, and they offered much quieter performance than their diesel counterparts. But this performance gap has narrowed.

In addition, the diesel offers more torque. "It is going to get any load up and moving a little bit easier," says O'Connor. It doesn't matter if it is a load near the truck's GVWR or a relatively light load.

Don't get confused by the horsepower and torque ratings when trying to pull heavy loads. Torque is the twisting force that determines how much load can be moved. "It is not a question of horsepower. It is a question of having enough low-end torque to accelerate the mass of the load and get it up to some reasonable cruising speed," explains Sam Winegarden, executive director of engine engineering, GM Powertrain. "A diesel, with all of that low-end torque, is just tailor-made for the application."

Consider the torque ratings of many of the gas engines available. "If you are looking at V10s and big V8s, eight liters and above, you are probably looking at peak torques around 450 ft.-lb.," says Winegarden. "A really stout one may get in the 475-ft.-lb. region, but none of them walk up anywhere near the 600 ft.-lb. kind of a number that the diesel gets."

Fuel efficiency vs. load factors

Fuel efficiency is another piece of the equation. Even though the price per gallon of fuel may be more expensive, the diesel engine is more efficient and therefore goes farther on a gallon of fuel.

The thermal efficiencies of a diesel engine vs. a gasoline engine are at the heart of the fuel efficiency difference. "A diesel will run a compression ratio of 18:1, 19:1 or 20:1," explains Winegarden. "Across the industry, a typical gas engine will run 10:1. That difference translates into huge thermal efficiency gains. From a fuel economy perspective, it favors the diesel."

"You generally get better fuel economy with a diesel than you do with a gas engine," O'Connor agrees. "The difference depends on any number of different conditions - trailer weight, road conditions, altitude, etc."