About a year ago, Diesel World conducted some mileage testing, using an F-250 as a mule. We systematically explored the benefits of a series of products and tactics. Ultimately, we improved mileage in our 7.3-powered Super Duty to 23.92 mpg, starting with a baseline of 17.2.  At the time, we promised to look for other options, ideas and further documentation of the products we tested.

That original test showed that a number of performance products could also be mileage improvers, and that the cost of purchase could be returned over the long haul. (We think our test protocol was a valid, real-world trial; if you'd like to scrutinize the details, review DW June and July '07 issues.)

Since then, we've looked to confirm our results, and even found some other ways to save money on fuel.

Tuning For Mileage
One of the easiest ways to get better mileage is through electronic tuning. Many, but not all, electronic tuning modules have the potential to deliver improved mileage, depending on how you drive. 

One example we looked into, TS Performance MP-8 module, might be typical. The module improves mileage because with more power, the driver can operate at part-throttle more often. The TS module adjusts in-cab to provide up to 100 hp for towing or daily driving.  The idea is, with more power, you can hold gears longer, and the engine labors less at certain rpm. So there is less hunting in the transmission, less revving, and better mileage, all things being equal. We talked to Denis at TS Performance about the MP-8, and he said the module would definitely improve mileage, estimating somewhere between 2 and 4 mpg. "It's going to help you no matter how you drive, unless you operate continuously at wide-open throttle. And that's not what people do. If you think about it, wide-open-throttle might be great for racing or playing around, but on the street, you're not operating wide open very often. Unless you drive it like you want to go to jail, the MP-8 going to improve your mileage.  As long as you keep the engine in the sweet spot, around 2,000 to 2,500 rpm, you'll be more efficient." 

This is consistent with our own findings. In last year's testing on the Ford Super Duty, we tried a Quadzilla Xzillaraider with Commander performance chip and ended up gaining 2.68 mpg, an increase of 13.6 percent. This was with the chip set at Level 3, with Airaid intake and Magnaflow exhaust already installed.

Almost all tuners have settings designed for towing; which will usually be best for fuel economy. However, not all modules are designed with mileage in mind. Some really are all-out performance devices designed for track use, rather than everyday driving. So, if you're after a little more power, better driveability, and the potential for better mpg, make sure you work with the manufacturer to select the right tuning product.

Synthetic Lubes
It seems clear that synthetic fluids reduce friction significantly enough to provide measurable mileage gains. It's also clear that, despite the higher initial cost of the synthetic lubricants, that cost is returned relatively quickly. Just to recap our own study, we got a 10 percent increase in mileage using Royal Purple synthetic lubes in the engine, in both axles on our 4x4, and in the transmission. True, this was a lot of lube to buy, costing $205.  But this turned out to be an economical upgrade over 12,000 miles, resulting in $243 saved in fewer oil changes and better mileage.  Of all the possible ways to improve fuel efficiency, use of 100-percent synthetic lubes is probably the first/best option.

Optimizing Operating Temps
Another potential mileage enhancer comes in the form of high performance coolant. As you probably know, clutch fans are thermostatically activated, and tend to rob horsepower and mileage when they are drawing air. As work increases, heat loads increase, forcing the fan to move more air, using more horsepower (and fuel) than before. With a higher-performance coolant, like the Evans NGK+ glycol product, you can afford to run hotter without risking overheating in spots, here and there, as coolant boils within the waterjacket. This opens opportunities for better mileage. To get better mileage, use a different thermostat to keep the clutch fan turned off at lower temperatures. Letting the fan clutch actuate at 210?F, or even 230?F, would mean less engine drain and fuel saved. 

Another unlikely fuel-saver would be a block heater. If you use electrical power to bring your engine up to operating temperature every morning, instead of idling, you save fuel. This would be particularly relevant if you do lots of short trips, and don't have a heated garage.

Intake/Exhaust
Replace a small-diameter, poorly configured OEM diesel exhaust with a larger-diameter, mandrel-bent aftermarket exhaust. Combine that greatly reduced restriction with a well-engineered aftermarket air filter or intake system, and the improved efficiency of the engine can yield a significant fuel-economy benefit under normal driving conditions. This is another one of those upgrades that combines performance and mileage benefits, but only if the driver takes advantage of the opportunity to get both. Drive hard in all-out performance mode, and you won't use any less fuel. Drive using the tach and gears, keeping the engine close to the torque peak or just above it, and you go farther on every gallon. But that choice is up to you. How much you gain will depend on how you drive, and how poorly configured the OEM systems were in the first place. On our F250, our steady test driving gave us a combined return, both intake/exhaust, of 1.3 mpg.

Aerodynamics
Especially on a big truck, aerodynamic improvements can yield measurable mileage gains, but these can be hard to quantify and test definitively. There has been conflicting urban legend, for example, as to whether something as simple as leaving a tailgate down can improve mileage.

The scientific way to measure these gains would be to conduct a "coast down" test, which involves testing how fast a truck loses momentum as it rolls along without power. This kind of testing would be well beyond the capabilities of most truck owners, so most evidence as to what works is anecdotal. Still, there have been some revealing studies.

Possibly the best-documented mileage gains involve improving air flow by covering the bed. Tonneau covers are generally sold to improve appearance and provide cargo security and weather protection. But the mileage benefit can be real--under certain circumstances. We were able to locate a study supplied by SEMA, the specialty vehicle manufacturers organization, that focused on pickup bed covers.

A number of full-size pickups were tested in a wind tunnel with various tonneau covers to assess their effect on the vehicle's coefficient of aerodynamic drag. The report stated that a reduction in drag could result in an improvement in fuel economy of about 6 percent if all else remains the same and the tonneau cover didn't significantly add weight. Generally, the faster the vehicle was driven, the more valuable aerodynamic improvements became.

OEM tests have shown similar results. "Tonneau covers on pickup boxes reduce aerodynamic drag," according to a quote we found from Ford's Jack Williams. How much? "We've seen reductions of about 8 to 10 percent on the F150."  This is roughly consistent with our own testing, in which we observed a 7 percent improvement on our 100-mile test loop after installing an American Roll Cover tonneau on our test F 250.

For those who tow, other aerodynamic improvements may also make a big difference-as long-haul truckers know well. Trailers can be big and boxy, so they kill fuel economy.  Use of a wing or aero top will save fuel over the long haul.

It should go without saying things like little flags, roof racks, and roof-mounted equipment will hammer your fuel economy. Add an array of big, flat off-road lights on top and you'll make matters worse. We saw one guy (car owner) who calculated a roof rack cost 12 percent mpg on a recent vacation trip, and putting a mountain bike on the roof rack had a penalty of 27 percent.  

There is a catch: Cleaning up air flow is only going to work at higher speeds. Highway driving is where the best gains will be seen, while around-town stop-and-go driving will yield less improvement, if any. So the bottom line here is, the more you tow and drive the highway, the more aerodynamic improvements will pay off.

So what about the tailgate, up or down? According to a recent Discovery Channel Mythbusters show-probably as good a test as any-no gains were registered with the tailgate down, but replacing the tailgate with netting (reducing weight and wind resistance) yielded measurable mileage benefits.

Weight reduction and rolling resistance
Unlike aerodynamic improvements, which only work when the car is moving fast at a steady state, weight reduction will show MPG improvements when the engine is working hard. In urban stop-and-go driving, weight reduction becomes important. On highways, every time you roll on throttle to maintain speed up a hill, excess weight forces you to burn more fuel.  On level ground at steady speeds, weight becomes a lesser factor, and wind resistance becomes more important.

How much? We're told OEMs actually have formulas that assign a specific theoretical fuel-economy improvement to a given amount of vehicle-weight reduction, but there are so many variables involved here, gains would be especially hard to quantify. We found plenty of formulas that show how lighter weight can make a drag racer go faster, and by looking at those, it seems that every 100 pounds counts when you get on the throttle. If you're trying to go really fast, as in racing, every single pound makes a difference.

Practical advice would be to lighten up when you drive the truck around town. Put the toolbox back in the garage, don't leave junk in the bed, take the hitch out of the receiver and be aware that weight from tow hooks, heavy bumpers and winches can add up. If you're doing repair or customizing, you can replace metal body parts, like the hood, with a fiberglass piece, and save a fair amount of weight.

Another big factor in rolling resistance is tire contact patch. There are multiple reasons why you should want your tires to be fully inflated to the sidewall maximum, including safety and tire wear. But mileage is also at stake. When you add a heavy load without adding air to the tires, they get wider as they bulge a little more, they become less perfectly round, and end up creating drag. At highway speeds, as you tow all day long, that drag works your engine harder.  Don't forget your trailer tires, either.

Auxiliary Fuel Tank
Adding fuel capacity does add weight as you carry more fuel. But if you are towing state-to-state, having the opportunity to take advantage of lower price opportunities can be a benefit. In Diesel World's home state of California, for example, fuel stations along busy fuel stops on Interstate 5 often charge 35 cents more per gallon than our local station.  If you buy 80 gallons at a time, at a price 35 cents per gallon less, you can save $28.  As we mentioned, weight is more of a factor around town, and less of a factor on cruse control, so this is one of those nice-to-have strategic options that suits long-haul rigs best.

Transmission Efficiency
On automatic transmissions a torque converter is used to transmit power from the engine through the transmission and ultimately to the wheels. Especially with an engine that has better-than-stock output, the stock torque converter is in fluid mode much of the time, and may even slip when it finally does move to lock-up mode. When in fluid mode, heat is generated and you waste fuel. Obviously, no fluid coupling can be highly efficient, but a better torque converter setup-one with enough clutches to lock up quicker and hold without slipping-will transmit power far better, without wasting fuel.  With your torque converter spending more time in lockup mode, transmitting power in a 1:1 ratio, your mileage goes up.  You might expect to gain 1mpg in the real world, depending on how much highway driving you do, and how well you keep the engine close to peak torque rpm.

Make Yer Own
Something we're seeing more and more of is people making their own fuel. Usually this involves collecting waste vegetable oil from a few local restaurants.  There are numerous ways to go about equipping yourself to take advantage of this possibility, but two we stumbled on stand out in our minds.

One company, Extreme Biodiesel, offers a tank system in kit form that processes the used oil, removing glycerin and impurities, so that you end up with a biofuel you can pump directly into your fuel tank.  Depending on the season, this can be burned as B100 or mixed with a percentage of #2 diesel for better low-temperature flow.  The process involves use of methanol and lye to extract glycerin, so there is some cost to it, but generally the final product can be produced for about $1 per gallon or less.  (www.extremebiodiesel.com).

Another way to burn used fryer oil is by filtering it and using it in a supplementary fuel system that heats it enough for proper flow and complete burn. This two-tank system allows for burning either WVO or #2 diesel as the opportunity permits.  One company we know of offers a "Vegistroke" system for Fords that takes advantage of the Ford "dead head" induction system for heat, and uses a pressure-differential system to switch back and forth from waste vegetable oil to #2, automatically. (www.dinofuelalternatives.com)
Either strategy can be used to replace significant amounts of expensive #2 with much less costly (or practically free) waste fryer oil.  The cost of the equipment in both cases is significant, but for those who don't mind taking the time to gather fuel, the cost could be returned in the first 1700 gallons, or roughly 30,000 miles.  That would be based on conservative assumptions that  your rig gets about 15 mpg, that #2 diesel costs $3, and your used fryer oil costs $1/gallon to gather, filter and process.

Bottom Line
It looks like higher fuel costs are here to stay, and in fact, may even go higher. With that in mind, making long-term adaptations to get the most out of every gallon make sense. We'll continue to look at ways to beat fuel costs as new topics arise.  And we invite readers who have done their own mileage testing to contact us at dieselworldeditor.com to share their insights.