Polk County, FL, has about 2,000 vehicles in its fleet. Subtracting the roughly 100 passenger cars and 300 pieces of heavy equipment from that total leaves 1,600 Class 1 through Class 7 trucks. Bob Stanton, director of fleet management for Polk County, performed an analysis of operating expenses for the trucks in his fleet, and made the decision that all future purchases of vehicles under 25,000 lb. GVWR would be equipped with gasoline engines.

This appears contrary to conventional wisdom, but Stanton’s research upholds his decision. "I’ve been in the business 22 years, and prior to my coming here seven years ago, all my experience was with heavy-duty trucks," he said. "I was very surprised three or four years ago when I ran our first analysis," Stanton continued.

"I was surprised when I began to pay attention to what we were paying as a premium in the purchase price for the diesel option when we bought these vehicles originally. I wondered whether we were getting our money’s worth."

The Diesel Option is Expensive

Stanton said that if you look at the $4,000 premium the manufacturers charge for a diesel engine over the same truck with a gas engine, it’s going to take a number of years and something more than 250,000 miles to pay it back, and that’s not including interest. "As a government agency," he added, "our premium isn’t quite that high, more like $3,500. I just ran some numbers this morning, and looking at the Florida state contract for purchase of vehicles, the diesel option ranges anywhere from $2,200 to $3,000."

Stanton noted that in the Class 3 to Class 6 range of vehicles, there is a lot of choice in gasoline-powered models. The Chevrolet/GMC C-Class, for example, is offered with a 7.4L gasoline engine that develops 210 or 270 horsepower, depending upon configuration. Ford offers models from the F-350/450/550 line that can be equipped with the 6.8L V-10, developing 275 horsepower and with GVW ratings up to 19,000 lbs. Finally, although Stanton has no experience with it, Dodge offers the 3500 series with GVW ratings up to 11,000 lbs. and power from 5.9L V-8s that put out 245 horsepower and an 8.0L V-10 that develops 310 horsepower.

Stanton says he feels that these choices are ample to serve in any task he needs. He runs his trucks between 125,000–200,000 miles before he turns them. In terms of age, he says that "If we’re talking a light truck, it’s eight to 10 years" before he replaces it.

Fuel Mileage Not an Incentive

Polk County is in central Florida, with the nearest big city being Tampa, some miles to the west.

"The county we operate in is geographically about the size of the state of Delaware," Stanton noted. "It’s a fairly large county in terms of land area. So our vehicles travel a considerable number of miles during a year."

He added that he wouldn’t classify even those distances as over-the-road driving, but would still consider it as mainly stop-and-go.

Stanton said that he found there was no great fuel economy advantage with the diesels. "Unless what you’re looking for is a mile or a mile and a half per gallon better with diesel. I can tell you that the Ford salesmen here, who calls on us all the time, tells us that unless you’re driving more than 25,000 miles per year, the diesel is not a viable option."

He said he did find that the diesels did have a slight advantage in fuel mileage, but it was only about 1.2 miles per gallon. "So," he added, "taking the low end of that range and using the advantage of 1.2 miles per gallon, we’d have to operate the vehicle 146,000 miles before we paid that off. And that’s irrespective of anything we might have paid in additional maintenance costs or anything else."

He concluded that now, with little difference between the price of gas and the price of diesel, you’re already in the bank for 146,000 miles with the diesel before you get anywhere near paying off the premium. At the mileage he runs his vehicles, Stanton is just about using up a gasoline engine. He noted that he’s not having to replace many engines.

"We don’t do it very often," he said. "It’s a rare situation when that occurs." Stanton said that even though diesels might be more economical if he ran them much longer than 200,000 miles, that doesn’t work in his situation.

He added, "I’ve never experienced a Ford 7.3L that’s gone much longer than 200,000-250,000 miles before it’s had to be overhauled or replaced. That’s my experience here, maybe nationwide you’ll be able to find other folks that have better experience than that."

Stanton also noted that he hasn’t replaced any of the GM 6.5Ls. "We’ve only been using them for about three or four years, while we’ve had the 7.3Ls for 11 years in various applications." Stanton noted that he has a good population of the 7.3Ls running, both the turbo introduced in 1994 and the non-turbo that goes back to 1989, using them in almost identical applications. Those are what he used to make his comparisons. "We’re showing that in maintenance costs alone, a diesel is costing us anywhere from 20 percent to 28 percent more than the equivalent gasoline engine over the same period of time."

Parts Cost Adds to the Problem

Stanton said that although he doesn’t buy a lot of his parts through a dealership, the county does quite a bit of dealership business, so he gets a price break from dealers.

"I asked my parts manager to give me parts cost comparisons between commonly used parts in both the gasoline and diesel engines: oil filters, air filters, fan belts, radiator hoses and so on.

"Looking at the comparison of parts prices, against the GM 6.5L V-8 turbodiesel, I’m using a comparative gas engine, the 5.7L V-8. With the Ford 7.3L V-8 turbodiesel, I’m using the Ford 5.8L V-8 gas."

On the Ford 7.3L, the oil filter, which is required to be replaced every time the vehicle goes in for service, costs $14 more than the equivalent part on the gas engine. "Fortunately," Stanton commented, "on the GM side, they’re the same price, but the owner’s manual recommends service intervals on the diesel that are twice as frequent." Doing twice as many services on a diesel as you would have to on a gas engine means you need twice as many filters.

For the Ford, the air filter is $7 more for the diesel, on the GM it’s $8 more than the equivalent part on a gas engine.

Stanton noted that on a diesel you replace the fuel filter more often than you ever do on a gas engine. "On a gas engine, it might be once a year," he said, "while on a diesel, some people change the fuel filter every time they do a service. On a fuel filter change, using Ford’s numbers, they recommend every 15,000 miles.