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2008 Ford Super Duty - Diesel vs. Gas

Two super duty pickups, two engines, one winner

Photography by , Trevor Reed

It's the $6,895 question: When buying the '08 Super Duty, should you order the V-10 gas engine or the brand-new Power Stroke 6.4L, dual-turbo diesel V-8? We didn't even bother to consider the standard 5.4L, V-8 gasser. The V-10 is rated with 32 more horsepower at the flywheel than the 5.4L and only adds $600 to the price of a Super Duty, but the $6,895 diesel produces 193 lb-ft more torque and does it at 3,000 rpm or less. Even though we hate to admit it, there is more to life than just horsepower and torque, so we had to know for sure.

In addition to strapping gas and diesel versions of the '08 Super Duty to a load dyno, we tested their fuel economy, raced them down a dragstrip, measured acceleration while towing 8,500 pounds, and observed noise levels with a decibel meter. We also logged hundreds of miles that included elevation changes of more than 4,000 feet and driving conditions that ranged from nasty city gridlock to cruise-controlled highway rides-and we even spent a little time in the dirt.

The test vehicles were two similarly equipped '08 Ford Super Duty F-250, 4WD, extended cab, shortbed pickups with the automatic five-speed Torqshift transmissions. The V-10 gas truck had the FX4 off-road package, while the diesel was a Lariat edition, the only significant differences being more power seat motors, a power rear window, and dual-zone automatic climate controls in the diesel. The gasser weighed in at 7,050 pounds with a full tank of fuel, and the diesel came in at 7,710 pounds. The V-10 tester had 4.30 gears (4.10s are standard), and the diesel is only available with a 3.73 ratio, but as the acceleration runs proved, the gas truck was still at a disadvantage while launching.

The MSRP of the diesel we tested was $51,040, and the gas truck had a sticker price of $45,055. Neither one is cheap, but a Super Duty F-250 XL, 2WD, regular cab with the six-speed manual starts at just $23,305, which means the cheapest V-10 is $23,905, and the cheapest diesel can be in your driveway for about $30,200.

Take a look at the data collected and ask yourself, Would I pay $6,895 more for the diesel? We asked ourselves that question and came to the conclusion that our jobs are not in jeopardy-diesel is now, more than ever, the best choice for powering a heavy-duty pickup truck.

Dyno Test
Scott Clark and the staff at Spectre Performance hooked both of our F-250 extended cab trucks to their Mustang MD-1750 load dyno so we could get baseline horsepower and torque ratings at the rear wheels. The diesel pushed out exactly 200 more lb-ft of torque and 67 more horsepower because the gas truck would not hold Fourth gear long enough to reach the advertised rpm for peak power. The gas engine likely has more horsepower than our dyno data shows.

MPG Testing
The trucks were fueled and weighed (7,050 pounds gas and 7,710 pounds diesel-a 600-pound difference) before the fuel-economy testing began. It consisted of mostly unloaded driving on city streets and highways (oh yeah, and seven passes down the dragstrip), along with towing and hauling with the diesel truck. Why no towing or hauling mpg figures for the gas truck? Only one trailer was available and only one transmission needed to be hauled, and both drivers chose the diesel. Go figure.

6.4L, V-8 Power Stroke dieselCity traffic: 69.10 miles/3.78 gallons = 18.28 mpgTowing 8,500 pounds and highway traffic:351.90 miles/22.47 gallons = 15.66 mpgGrapevine pass with cruise control:72.90 miles/3.82 gallons = 19.08 mpgHighway 99 driving and city runaround: 197.60 miles/10.02 gallons = 19.72 mpgHighway 99 and Grapevine pass with cruise control with Dodge transmission in bed: 174.40 miles/11.08 gallons = 15.74 mpg

V-8 Power Stroke diesel mpg averages865.90 miles/51.17 gallons = 16.92 mpg (including towing)339.60 miles/17.62 gallons = 19.27 mpg (unloaded)

6.8L, V-10 Triton gasolineCity traffic: 123.50 miles/13.80 gallons = 8.95 mpgHighway driving without cruise control:74.60 miles/7.88 gallons = 9.47mpgGridlock traffic: 33.50 miles/3.63 gallons = 9.23 mpgGrapevine pass with cruise control: 73.80 miles/6.18 gallons = 11.94 mpgHighway 99 driving and city runaround:207.50 miles/21.00 gallons = 9.88 mpgHighway 99 and Grapevine pass with cruise control:181.70 miles/14.48 gallons = 12.55 mpg

V-10 gasoline mpg average694.60 miles total/66.97 gallons = 10.37 mpg

Calculating The Savings
One way to answer the $6,895 question is to calculate how long it will take for fuel savings to pay for the diesel option. At the time of testing, diesel was actually cheaper than gasoline-something we haven't seen in California for more than a year. In fact, it cost more than premium 91-octane gas just a few months ago. To make the math easier, we ran the numbers assuming gas and diesel both cost $3 and used the standard estimate of 12,000 miles of average yearly driving. When the calculator cooled off, we learned the diesel Super Duty will take about four years to pay for itself, and every mile after that can be considered part of the diesel bonus round.

($6,895 - $600 V-10) = $6,295 diesel option/$3 per gallon = 2,098.30 gallons12,000 miles per year/10.37 mpg = 1,157.20 gallons of gas per year12,000 miles per year/19.27 mpg = 622.70 gallons of diesel per year2,098.30 gallons/534.50 gallons saved = 3.93 years to break even

Towing Test: Steep Grade 15-65 Mph
One of the most important areas of 31/44- and 1-ton truck performance is the ability to accelerate while hauling a trailer. To simulate merging with highway traffic on a steep grade . . . well, we did just that. A 3,000-pound trailer was loaded with a jungle-gym-rollcaged Jeep Cherokee 4x4 (about 8,500 pounds total), and the trucks were timed in passes that started with a 15-mph rolling start followed by full acceleration to 65 mph. The diesel averaged 7.32 seconds faster than the gas and felt like it could perform the same feat with even more weight in tow.

6.4L, Power Stroke V-8 diesel
Engine type: V-8, iron block, iron headsDisplacement: 390.5 ci, 6,400 ccBore x stroke: 3.86 x 4.13 inchesCompression ratio: 17.2:1Fuel injection:High-pressure, common-railValvetrain:OHV, four valves per cylinderHorsepower:350 hp at 3,000 rpmTorque:650 lb-ft at 2,000 rpmOil capacity: 15 quartsTest vehicle curb weight: 7,710 pounds

6.8l, V-10 Triton Gasoline
Engine type: V-10, iron block, aluminum headsDisplacement: 415 ci, 6,802 ccBore x stroke:3.55 x 4.16 inchesCompression ratio: 9.2:1Fuel injection:Sequential, multiport electronicValvetrain:SOHC, three valves per cylinderHorsepower: 362 hp at 4,750 rpmTorque:457 lb-ft at 3,250 rpmOil capacity: 7 quartsTest vehicle curb weight: 7,050 pounds

Dragstrip Testing
After the trailer was unhitched, the gas and diesel trucks were raced head-to-head on the Los Angeles County Raceway dragstrip. Both trucks were placed in 4WD High with the wheel hubs locked and power-braked at the line for full-throttle launches. Standing on the brake and flooring the throttle in the diesel allowed boost to build to more than 30 psi, which made the Power Stroke pump out torque off the line and build speed faster than the gas engine-and it is a lot of fun. On the final run, we tried to squeeze a little more speed from of the gas truck by taking it out of Overdrive. Unfortunately, the Super Duty only offers Third gear below Drive, so shifting out of OD at the top of the track resulted in instant Third-gear rev-limiting and a slower top speed.

Noise Testing
Diesel engines have a bad reputation for being noisy, but the new 6.4L Power Stroke was designed to be the quietest oil-burner ever sold by Ford. It uses fast-acting piezo electric injectors and high-pressure, common-rail fueling to deliver multiple precise shots of fuel for smooth and quiet injection events. Placement of the injectors under the valve covers and insulation in the engine compartment combat noise from the diesel, along with exhaust tones being chopped up by two turbos. Even with these advancements, the diesel is noticeably louder at idle than the overhead-camshaft, V-10 gas engine. The diesel was a bit quieter at wide-open throttle and while driving on the highway at 65 mph; both trucks scored the same while cruising at 40 mph. While measuring the diesel, it went into its Active Regeneration soot-burn cycle, where engine rpm and post injections are used to increase EGT and cook matter trapped in the diesel particulate filter. Surprisingly, the higher engine speed smoothed out vibrations and quieted the cabin by 2 dB.

6.4L, V-8 Power Stroke diesel
Interior at idle: 70 dB (68 dB during Active Regeneration)40-mph cruise: 82 dB65-mph cruise: 83 dBWide-open throttle: 85 dBOpen-hood idle: 81 dB (86 dB during Active Regeneration)

6.8L, V-10 Triton gasoline
Interior at idle: 61 dB40-mph cruise: 82 dB65-mph cruse: 85 dBWide-open throttle: 88 dBOpen-hood idle: 77 dB

After Driving Both TrucksWould You Pay $6,895 More For The Diesel Option?
Twenty years ago, I can remember daydreaming about buying an '87 F-350 with a 460ci, big-block gas engine. At the time, I had no interest in the 351ci small-block or the 6.9L diesel because I wanted all the horsepower Ford would sell me. My, how times have changed.

I knew going into this test that if Ford's V-10 Super Duty was even close to being as powerful and fuel-efficient as the 6.4L Power Stroke, we'd all be in trouble. Sure, we're biased toward the diesel engine because we write for Diesel Power magazine, but on paper, the 362hp V-10 looks like it should do everything the 350hp 6.4L diesel can-especially when optioned with the 4.30 axle gears (compared with the diesel's 3.73s) our test truck had. I'm pleased to report that when it comes to towing, acceleration, and fuel economy, the gas engine doesn't even come close.

When driving the two trucks back-to-back, the biggest difference I noticed was how much lighter (and somehow smaller) the V-10 truck felt. The V-10 does have better throttle response than the diesel, and after driving it, I would have sworn it was quicker than the 6.4L. I think the dragstrip numbers we recorded favor the diesel because we were able to spool both its turbos before launch. In a traffic-light drag race, the two trucks may be a closer match than our data show.

The 6.4L Power Stroke definitely felt more powerful overall, which is great for our job security. The torque advantage is impressive when towing or just cruising up grades at highway speeds. I was also pleased to see the diesel's fuel-economy advantage still shines through all the new emissions equipment. It remains to be seen how the aftermarket is going to modify these trucks, but I have no doubt we'll see 100hp upgrades by the end of the year.

Would I pay $8,385 for the diesel/automatic options? Without a doubt, I would. I think if you're in the market for a truck this size, the diesel engine just makes more sense. Ford puts more engineering resources into the diesel, the Torqshift transmission gear ratios are better suited to the diesel's power curve, and the diesel truck's resale value is higher. The days of the big-block gas engine are truly over.-David Kennedy, Editor

At first, $6,895 sounds like a lot of money. Add $1,490 onto that for an automatic transmission (the only thing I'd want behind a turbodiesel), and you've got $8,385-almost enough to make you consider a life with spark plugs. Thoughts like that are caused by sticker shock, and the only cure is to get behind the wheel of the dream truck that's haunting your wallet and see if it feels worth the extra cash, and the 6.4L Power Stroke diesel passes that test.

It's quiet but performs like a hot rod. When both turbos are spooled, it feels like the Super Duty should be wearing an SVT badge on the tailgate. The only bummer is the sluggish operation of the electronic throttle control when aggressive inputs are received from your right foot, even though there's almost always 20 psi or more of boost on tap. It's no surprise because the 6.4L Power Stroke has to produce 650 lb-ft of torque while emitting no black smoke and getting good fuel mileage. This is achieved through a slow build of fueling after you mash the pedal that only seems to peak right before it's cut off in preparation for the First-to-Second-gear shift. Full fuel is available (except during shifts) until the truck reaches 86 mph when the speed limiter kicks in.

The V-10 gas truck does not suffer from a very noticeable lag between input and power delivery, but it's a lot easier to make gasoline burn clean when EFI and 10 relatively small cylinders are used. Thanks to less torque, the gas Super Duty doesn't cut power very much between shifts except when the transmission is cold, so it's easy to play with the truck's entire powerband.

After driving both trucks on the same mountain routes and dodging through city traffic, I noticed the diesel's lag (don't call it turbo lag, though) faded into the background. Both of these 31/44-ton trucks can be squirted through lane changes and up steep grades with ease, but the diesel always has more power in reserve, especially while towing. Add in much better fuel economy and the fun factor of a chipped truck, and it would seem foolish to choose the gas V-10 when the new dual-turbo, 6.4L Power Stroke is available-even if the diesel/automatic option costs $386 more than a Kawasaki Jet Ski STX-12F.-Trevor Reed, Feature Editor

From a purely monetary standpoint, the difference in Fuel mileage alone will save you $7,000 with the diesel after only about 50,000 miles of driving if diesel and gas both stay at around $3 a gallon. Diesels also hold their resale value better than gas vehicles, and they last longer. The diesel makes sense.

As far as driving impressions go, the gas truck had great throttle response and had a broad powerband all the way up to 5,000 rpm, which was surprising when considering the engine's size. Once I climbed into the diesel, however, it felt like a rocket ship, with power available at virtually any rpm in any gear once the turbos were spooled up. The only downside was the diesel still had lag from a dead stop, but other than that, throttle response was good, even at low speeds.

Would I pay the extra money and buy the diesel? You bet. The only situation in which I would choose the gas truck over the diesel would be zipping in and out of city traffic. In all other situations-freeway driving, accelerating, and towing-I feel the diesel is the better choice.-Jason Sands, Associate Editor

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