Light-truck diesel engines of five to 10 years ago weren't so good. They were noisy, vibrated, had smelly exhaust emissions, and were more costly to build than their gasoline-burning counterparts. Today's diesels are much more civilized, but are they any better than their gasoline cousins? This is one of the questions we're asked most frequently regarding 3/4- and 1-ton pickups. To answer this, you must understand the positive and negative attributes of each engine, while considering what you'll use the vehicle for. Let's look at the important criteria you should use before making a final decision.
Before we start, it's important to note there are currently no 1/2-ton pickups available with diesel engines, even though there are some small "baby" diesels under development for use in 1/2-tons and midsize SUVs. This story concentrates on 3/4- and 1-ton trucks, but the concepts will be equal once the next generation of smaller diesels arrives.
Advantage: Split Decision
Typically, gas engines make more horsepower, while diesels produce more torque. Are you looking for off-the-line acceleration of an unloaded truck? Maybe you use your truck around town where quick starts are important and you don't often tow a trailer or haul a load. If this is the case, then you'll want a gas engine. By design, gas engines rev faster and are able to reach higher rpm peaks than diesels. This allows them to attain greater horsepower numbers and quicker 0-60-mph times.
However, if towing capacity and brute pulling force are your game, a diesel is for you. The torque advantage of diesels is perfectly suited for pulling heavy loads up steep grades. Because of the relatively high-compression ratio necessary to ignite the diesel fuel (17:1 diesel versus 9:1 gas), a diesel makes all its torque and power low in the rev range. As an example, the GM 8.1L gasoline V-8 in Chevrolet and GMC pickups puts out 340 hp and 455 lb-ft of torque, while GM's 6.6L V-8 turbodiesel makes slightly less hp at only 300, but makes up for it in torque with a healthy 520 lb-ft of grunt.
Diesel fuel has a higher energy density than gasoline. One gallon of diesel contains approximately 147,000 BTUs of energy, while a gallon of gasoline only has 125,000 BTUs. This means it takes more gasoline to equal the power output of diesel, making diesel engines more efficient per gallon of fuel burned. Also, because diesel engines use the more efficient direct fuel-injection method (fuel injected directly into cylinder) compared to the port fuel-injection setup in gas engines where gas is mixed with incoming air in the intake manifold, the diesel system has little wasted or unburned fuel. Diesels also use about one third as much fuel at idle as gasoline units. Even though there are no official EPA-mileage figures for 3⁄4-ton and bigger trucks, we've seen diesels get six to eight more mpg than similar-weight gas pickups. Over the life of the truck, this advantage could be significant, especially if you drive a lot of miles.