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Truck Engines: Gas vs. Diesel

Which is best for you?

Pickup truck diesel engines of a decade ago were noisy, smelly and shook like a washing machine with an out-of-balance load. Today's pickup truck diesel engines are much more civilized, but are they really any better than their gasoline counterparts? To answer this, you need to understand the good, bad and ugly of each engine. You also need to seriously consider how you'll use the truck because each engine is better suited for certain duties. And remember, you can only get a diesel engine in 3/4- and 1-ton pickups—there are no 1/2-ton diesels. Let's take a look at the most important criteria.

Noise & Vibration

Advantage gasoline. Despite huge improvements in noise isolation and engine noise technology in pickup trucks in the past 10 years, diesels are still noisier and shake more than their gasoline brothers. However, a recent back-to-back drive in two Ford trucks; one equipped with a 5.4L gas V-8 and the other fitted with the new Powerstroke 6.0L diesel V-8, showed us that diesels aren't that far behind. At idle, the clatter and shake of the diesel is clearly noticeable, while it's tough to tell if the gas engine is even running. Under low-speed acceleration, the diesel still makes more noise. But, once you're up to speed, there is little difference between the two even when accelerating on the highway.

Power & Torque

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 tow a trailer or haul a load often. If this is the case, then you'll want a gas engine. By design, gas engines rev up faster and are able to reach higher rpm peaks than diesels. This allows them to attain higher horsepower numbers and quicker 0 to 60 mph times.

However, if towing capacity and brute pulling force are your game, then a diesel is for you. The torque advantage diesels have 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 vs. 9:1 gas), a diesel makes all of its torque and power low in the rev range. As an example, the 8.1L gasoline V-8 found in Chevrolet and GMC pickups puts out 340 horsepower and 455 lb.-ft. of torque, while GM's 6.6L V-8 turbo diesel makes slightly less horsepower at only 300, but makes up for it in torque with a healthy 520 lb.-ft. of grunt.

Initial Cost

Advantage gasoline. Because of the high compression ratios and resulting high cylinder pressure found in diesel engines, they must be built to withstand more punishment than gas engines. Beefed up parts include a thicker block and cylinder heads, and stronger connecting rods, pistons, crankshaft and valves. These parts can be costly. As an example, if you upgrade a 2003 F-250 from the standard 5.4L V-8 to the new 6.0L turbo diesel V-8, you're going to spend around $5,000. However, to go from the 5.4L to the 6.8L V-10 gasoline engine, the price is a more manageable $600. Another diesel disadvantage that comes as a byproduct of needing heavy-duty components is increased weight. A diesel engine can weigh several hundred pounds more than a comparable gas model.

Fuel Economy

Advantage diesel. 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 that 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 used in gas engines where gas is mixed with incoming air in the intake manifold, the diesel system has very little wasted, or unburned fuel. Diesels also use about 1/3 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 6-8 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.

Short-Term Care

Advantage gasoline. Regular maintenance on a diesel is more costly thanks to several items, including the larger volume of oil in the engine, and the fact that fuel filters and water separators need to be serviced more often. Modern gas engines have an even bigger advantage thanks to extended service intervals on things such as spark plugs, engine oil and antifreeze.

Long-Term Care

Advantage diesel. The flip side of a diesel engine's expensive initial cost is its excellent durability. Dodge, Ford and GM learned long ago that they were better off buying diesel technology from experts such as Cummins, International and Isuzu than spending tons of money developing it themselves. These manufacturers all have years of experience building heavy-duty, over-the-road diesels that have to log 100,000 miles a year for years on end, routinely haul heavy loads and may have to idle for days at a time. Think of the diesel engines found in GM, Ford and Dodge pickups and SUVs as mini big-rig engines. The average gas engine is only good for around 125,000 miles before needing a rebuild and isn't designed to constantly pull a heavy load. A diesel can go more than three times this amount before needing an overhaul.

Cold Weather

Advantage gasoline. Anyone who has tried to start a diesel engine on a cold winter's morning knows the winner in this category. Diesels don't have spark plugs like gasoline engines do. The fuel is ignited spontaneously once it's injected into the cylinder that's already under pressure. When it's cold (below 30 degrees Fahrenheit), the air isn't hot enough to ignite the diesel fuel. To help counter this, today's diesel pickup truck engines use a computer, which senses cylinder temperature and injects the fuel later in the engine rotation. By injecting the fuel when the piston is closer to top dead center, the cylinder is under more pressure and the air inside is hotter which aids combustion. As an added measure, most modern diesels come equipped with a 110-volt heating element designed to keep the engine block warm when the truck is parked.

Smell & Emissions

Advantage gasoline. Despite petroleum companies best efforts at producing diesel fuel with lower sulfur levels, burned diesel fuel still smells much worse than burned gasoline. Beyond the smelly tailpipe, diesel still lags behind gasoline in the areas of oxides of nitrogen (NOx) and particulate matter emissions. It's the particulate matter that causes the black soot seen emanating from diesel vehicle tailpipes, while NOx is one of the components of smog. The next generation of diesel fuel is generally thought to improve on this situation.

Fuel Cost

Advantage diesel. Because diesel fuel is easier to refine, taking less time to get from raw petroleum to final product than gasoline, it's usually priced lower than gas. However, occasionally in the U.S., diesel is priced the same or more than 87 octane gas. This is often attributed to the fact that diesel isn't as desirable in some areas leading to higher diesel prices. However, diesel advocates say that if more people drove diesel light trucks and cars, the price would drop dramatically in these areas—and possibly throughout the country.

Fuel Availability

Advantage gasoline. The lack of fuel availability is the reason we hear most often as to why people don't choose a diesel engine. Only about two percent of the nation's cars are diesel powered, compared with 25% for some European countries. The number is larger for light trucks and SUVs in the U.S., but the overwhelming majority are gasoline-powered. It's kind of like the chicken vs. the egg scenario. The car manufacturers say they'll build more diesels if people will buy them. Consumers say they'd consider diesels if there were more fuel stations that dispensed diesel. Fuel companies, in turn say they'd produce more diesel if consumers wanted it. Diesel pumps can be found in most areas that have a large amount of commercial truck traffic.

Conclusion

Choosing between a gas and diesel engine really comes down to what you'll do with the truck and where you live. If you use your truck like a car, desire quick, quiet acceleration, rarely haul a heavy load and you don't plan on keeping it past 100,000 miles, then you may want to consider a gas engine. They run smoother, fuel is easier to find and they're easier to start in cold weather. However, if you use your truck for towing, value good fuel economy, and plan on racking up loads of miles, then a diesel is for you. In the end, the leading disappointment regarding diesels is that the price to add a diesel to a 3/4- or 1-ton pickup is still quite high versus a more powerful gas engine. But, you'll make this back in fuel savings over time. On the flip side, we were pleased to find that manufacturers continue to develop diesel technology, especially in the areas of cold starting, combustion smoothness, and emissions. Now we need diesels in 1/2-ton pickups and midsize SUVs.