Gas vs. diesel?

A head-to-head trailering test compares two popular

'99 Ford engines in F-2S0 Super Duty trucks

Gas or diesel engine? That's a question faced by many light-truck buyers as they shop for their dream tow rig. All other features being equal, a truck fitted with either of these potent engines will do a good job of towing its factory-designated load. But, when choosing a truck's power plant, there are a lot of factors to consider beyond basic performance.

Diesel engine fuel economy has always been a plus, but what about all the other issues that come into play? There's the extra YOSt of the engine, fuel (diesel is often more expensive than gasoline nowadays), the maintenance and so on to considernot to mention fuel availability. These details should all be part of the shopping package.

We decided to put a pair of Ford trucks-one fitted with the Triton 6.8-liter \10 gas engine, and the other with the Power Stroke 7.3-liter intercooled V-8 diesel, to the test. The setup was simple: Choose trucks with the closest tow ratings possible. hitch them to identical trailers and send them out on the road and see what happens. \Ve also "ran the numbers" on the results to see how one engine type or the other would affect the owner's pocketbook in the long run.

TEST VEHICLES

The subjects of this test were two Ford F250 Super Duty four-wheel drives with cushy occupant-comfort optional equipment and heavy-duty towing packages. One truck, a SuperCab, was fitted with the 6.8-liter -10, automatic transmission and 4.30:1 axle ratio; it is rated to tow 13,800 pounds, and is priced at $32,360 (suggested retail as tested). The other was a standard cab with the 7.3-liter Power Stroke V8 diesel, automatic transmission, 3.73:1 axle ratio and a 13,700-pound tow rating; price tag, $34,045, as tested.

The disparity in the axle ratios was required to keep the trucks' tow ratings as close as possible. A comparison of identical truck hardware was not the issue here: we wanted to know how a diesel truck would stack up against a gas-powered truck with virtually the same tow capacity as rated by Ford. The tow ratings were within 100 pounds of each other and, since curb weights were similar, the test trucks were about as balanced as one can get.

The test trailers were relatively lightweight, compared to the trucks' tow ratings, at about 6,800 pounds each. The Fleetwood Mallard 33Z travel trailers, 1999 models, were big enough to give the trucks something decent to tow, while giving us some interesting test result numbers, even if they didn't push the trucks' towing limits.

TEST PROCEDURE

We drove both truck-and-trailer combos on a 640-mile loop that included flatland freeway driving, hill climbs and a few secondary state highways. Highway speed was maintained at the legal limit for a light truck towing a travel trailer.

When looking for fuel, we stayed away from the cut-rate discount stations and generally kept our eyes open for name brands with quality we could trust. Most stations were adjacent to freeway exits, which could have affected the pergallon price as compared to shopping around off the main road.

TRUCK OPTION COST

There's no doubt, at $4,495, the diesel is a costly option compared to the $350 premium for the V-10. At the same time, the diesel's 3.73 axle is a no-cost item, while the V-lO's 4.30 axle (with limited-slip differential) is an extra $320. Added up, that still leaves an extra $3,825 investment to choose the diesel engine.

The big question among most owners: Will the diesel engine's potential fuel savings offset the extra cost in a short enough time to make the investment worthwhile?

RELIABILITY

Diesel enthusiasts used to be driven by beliefs that their engines were more reliable than comparable gasoline engines. Unlike its gasoline cousins, a diesel has no plugs, points, ignition, carburetor or other parts that frequently need tune-ups. Likewise, diesels were immune from the vapor-lock problems that once plagued gas engines.

But times have changed. Fancy electronics and advanced technology have made today's gasoline engine very tidy and reliable. Service intervals have been stretched, distributor-less coil-on-plug ignition systems and sophisticated direct fuel-injection systems have allowed gasoline engines to run more efficiently for a lot longer. Today's diesels feature computerized electronic fuel-injection systems and emission controls, so to some respect, they're becoming more complicated. The difference in reliability is narrowing all the time, such that it's hardly an issue anymore.

Diesel engines typically have had longer life spans than many gasoline engines, largely because they were originally designed for commercial use and are built extra-heavy duty to handle the high compression ratios and extreme combustion pounding, not to mention the inherent lubricity of the fuel. It's realistic to expect a light-truck diesel in recreational use to run 200,000 miles, or more, without a major overhaul. And although today's gasoline engines are also designed for higher mileage use, they still can't quite measure up to a diesel's projected life span. A diesel will realistically last long enough to make that price payback. The questions is, will owners drive far enough over the years?

PERFORMANCE

This is an area where there's little doubt. Both trucks are smooth, relatively quiet runners. They start easily, exhibit excellent throttle response and provide the kind of driving excitement and gutsy hauling ability that make both engines popular with the RV trailering crowd.

Climbing a serious 6-percent grade, the trucks stayed neck and neck. The gas truck dropped to second gear and climbed the hill at 54 mph at 4,000 rpm, running smooth and strong at that speed, and the diesel was right beside it in third gear at 2,500 rpm.

Dropping down a 7-percent grade, the gas truck held its load back to 55 mph in third gear at 2,500 rpm, while the diesel ran up to 62 mph in second gear at 2,500 rpm and slowed back to 50 mph in second gear at 2,800 rpm. Some kind of exhaust or compression brake would definitely help the diesel truck's downhill compression braking ability.

Acceleration runs produced very similar numbers. The gas-truck lashup recorded 22.4- and 13.0-second 0- to 60-mph and 40- to 60-mph runs, respectively, while the diesel truck came in at 21.5 and 13.2 seconds.

Both trucks were able to cruise comfortably in overdrive gear, although the gas truck downshifted to third somewhat sooner than did the diesel truck.

FUEL PURCHASING, ECONOMY, COST

As a rule, highway service areas with two or three gas stations will have at least one that also offers diesel. There's no guarantee of this, of course, and we made several false-alarm cruises through gasstation clusters in search of diesel fuel. We never ran so low that exhausting our supply was an issue because we started looking well before the last needle-width of fuel was left in the tank, but it's not going to be available at every place you choose to stop. This is especially true if you're looking for a specific brand. Plan ahead, and minimize problems.

Many drivers complain about dieselfuel smell. In many locations, there's a sort of a dirty stain of spilled diesel fuel near the pump. Unlike gasoline, which evaporates quickly and leaves no wet residue, diesel can leave an oily mess. This is particularly true of stations with less-than-perfect housekeeping practices. So, a driver of a diesel tow rig needs to keep an eye out for the mess to avoid tracking wet diesel residue into the vehicle after the fill-up.

There was a serious difference in fuel economy during our road test. The V-10 gas truck averaged 7.6 mpg while towing and 12.1 mpg solo, and the diesel used 10.9 mpg towing and 18.4 mpg solo. That would create a significant difference in long-term fuel cost.

As it turned out, diesel and gasoline fuel prices averaged out to $1.30 per gallon each. This is surprising, since diesel is far costlier than gasoline at many stations in most parts of the country, but at two of the five fill-ups we made during the towing part of the test, the diesel fuel was actually cheaper. (All fill-ups were at traditional consumer gas stations.) These prices, naturally, will vary a great deal, and the test took place in Southern and Central California, where the fuel costs seem to be as high as anywhere in the country.

The gas truck burned 84.8 gallons of fuel, costing $109.91, on the towing trip, and the diesel used 58.4 gallons, costing $75.76. That represented a $34.15 fuel savings-or 45 percentfor the diesel truck driver. Obviously, that's a sizeable savings!

Over the duration of the 640-mile test loop, the gas truck cost about 17.1 cents per mile in fuel, and the diesel truck was about 11.8 cents per mile. That's a 5.3cent-per-mile fuel-cost savings while driving the diesel truck.

Projected out to a 100,000-mile towing life span, which is entirely reasonable with today's vehicles, the diesel truck would bum $11,893.70 in fuel and the gas truck would use $17,149.60, for a savings of $5,255.90 in fuel by driving the diesel truck; in this scenario, the diesel is 69 percent cheaper to run over the 100,000 miles. Given the fuel prices we encountered, that five grand-plus would more than pay for the difference in price due to the diesel engine option, but fuel is just one expense.

MAINTENANCE

Lower maintenance costs, other than lubrication, have been an alleged advantage for diesel engines. Ford does not recommend any tune-ups for the V-10 engine until it reaches the 100,000-mile mark, so we haven't included any gas engine tune-up parts or costs in our calculations. While there are no tune-ups to do, per se, on a diesel, there are other costs to consider.

For example, the 6.8-liter V-10has a seven-quart oil capacity, including filter, and the 7.3-liter Power Stroke diesel uses a whopping 15 quarts per oil change, including filter.

Ford has two recommended oil change intervals for these trucks, and since we're using the towing figures for our cost calculations, we'll use the company's 3,000-mile "severe-duty" oil change interval as a baseline. Using the 100,000-mile distance again as a reference, the user will do approximately 33 oil changes. Those 33 oil changes translate into 231 quarts of oil in the V-10 and 495 quarts in the diesel.

Figuring $1.25 per quart average, which may be conservative on a national basis, the gas engine uses $288.75 worth of oil, and the diesel uses $618.75. Oils designed specifically for diesels may run higher in price than a comparable oil that's suitable for a gasoline engine.

Oil filters add to the equation. Aftermarket filters are less expensive, but using the official Ford parts prices, the V-10 oil filter is $8.29 and the diesel filter is $24.42 per our local Ford truck dealer Multiplied times 33 oil changes, that's $273.57 for gas filters and $805.86 for diesel filters.

Diesel trucks also use fuel filters, which must be changed on a regular basis. The factory recommends the filter be changed at 45,000, 75,000 and 90,000 miles (within our 100,000-mile measuring life span for the test purposes). These fuel filters cost $26.25 at our local dealer, for a total of $78.75.

Added up, the gas truck oil changes cost $562.32 and the diesel oil service costs $1,503.36. That makes the diesel $941.04 more expensive to maintain for those 100,000 miles. (The difference in oil change labor costs for the diesel engine is about $10 at Ford dealerships. Thirty-three oil changes mean an additional $330 in diesel service labor.)

DIESEL PAYBACK?

To sum up all these figures, the combined cost of operating the diesel truck for 100,000 towing miles would be approximately $13.396.36, or about 13.3 cents per mile. The gasoline-powered truck would cost about $17,711.92, or around 17.7 cents per mile. Using these basic figures, the diesel would offer a savings of approximately 4.4 cents per mile to operate.

How long would it take to recoup the cost of the diesel engine using these figures? You'd have to tow your trailer 86,932 miles to cover that extra $3,825 diesel option cost at a fuel-and-maintenance savings of 4.4 cents per mile. That's an entirely reasonable driving life span for the average owner, but the figures were calculated using our equal fuel cost numbers, which are not typical for most of the country. It is also worth noting that not all owners drive their gas or diesel pickup exclusively for towing. Therefore, the number of miles to recover the initial investment will vary among owners. Noting the diesel's 18-mpg solo capability, it's actually feasible that the diesel cost can be amortized quicker if the truck is driven solo most of the time. Your ability to find and use less-expensive fuel can also play a major part in how fast the engine pays for itself.

In most parts of the country, the cost of diesel fuel seldom, if ever, equals that of gasoline, and it can be significantly higher. A spot-check of five stations in Indiana revealed a disparity of approximately 18 cents a gallon between automotive diesel fuel and regular unleaded gasoline.

Just for the sake of comparison, we ran the above fuel economy numbers using an 18 cents per gallon difference with the diesel fuel at $1.30 per gallon and gasoline at $1.12 per gallon. That resulted in a gasoline operating cost of $15,337.36 for 100,000 miles, or about 15.3 cents per mile. Under these conditions, the diesel would only be about 2 cents per mile cheaper to operate. At that figure, it would require a considerable 188,750 miles of driving the diesel to amortize the cost of the diesel option without taking into account the additional costs for maintenance.

RESALE VALUE

Last but not least, there's resale value to consider. In the real world, this will vary a great deal, particularly in areas such as ranch country where diesel pickups are in high demand. If you're trading in, many dealers will apply Kelley Blue Book values. For a typical example, Kelley Blue Book rates a 1996 Ford F250 with the 7.3-liter Power Stroke engine as having $1,845 more trade-in value than a comparable truck with the 7.5L gas engine.

Some years ago the situation was different. Back in 1994, when Ford made a midyear changeover from the old nonturbocharged indirect-injected (IDI) 7.3L diesel to the turbocharged IDI model, the non-turbo truck was listed as $195 lower in trade-in value, but with the turbocharged engine, the value was $485 greater. Today's turbocharged diesels, at trade-in time, are clearly viewed more favorably among dealers.

GAS OR DIESEL?

Which way should you go? That depends on a lot of factors, as detailed above. If you plan on relatively shortterm ownership of the truck and trade off every few years, the diesel probably isn't worth it. If you keep your rig until the wheels fall off and make serious use of it, the engine may be able to pay for itself within the truck's useful life span. But that again depends on the percentage of time the truck is used for towing.

As a final thought, some people simply like the idea of owning a diesel and listening to the rumble and roar of the compression-ignition engine. In that case, there's no sense placing a dollar value or cost payback on owner satisfaction. Ditto on the Triton V-10. TL

Copyright T L Enterprises, Inc. Apr 1999
Provided by ProQuest Information and Learning Company. All rights Reserved