Squeezing More MPG From a Super Duty: Part III
In the first two parts of the story, we set up our baseline testing route, changed the fluids to Royal Purple (synthetic), upgraded our exhaust to Magnaflow’s turbo-back system, installed a new intake system from Airaid, added the Xzillaraider programmer from Quadzilla as well as adding a hard tonneau cover from Truck Covers USA.
With these mods, fuel economy increased from 17.2 to 23.92 mpg. That’s a whopping gain of 6.72 miles per gallon. Along with boosting fuel economy, we had increased horsepower from 250 at the rear wheels to 411 hp, which essentially improved the truck’s efficiencies, hence leading to the increase in miles per gallon, only if we could keep our foot out of the throttle.
Just as we were putting a wrap on the project, satisfied with the results and drivability, we received a challenge from Master Sergeant Tom Wanamaker of the Minnesota Army National Guard who said he’d like to see us reach a 50 percent improvement over stock. Not ones to shrink from a challenge, we headed back to the drawing board to see how to squeeze even more mpg from our 2004 Super Duty.
With 3:73 gears in the rear, we figured a slight drop in cruising rpm, to get closer to the engine’s torque peak, would help since it’s the most efficient spot in the rpm band. The easiest way to do so without swapping rearend gears is with a slight increase in tire size. The folks at Firestone also suggested using a taller tire to aid in the quest to gain some additional fuel economy.
On their recommendation, we swapped out our original rolling stock of 255/75R16s, with an overall diameter of 31.1 inches, for a set of 305/70R16 Firestone Destination A/Ts which have a total overall diameter of 32.8. The flip side is whenever you change tires from the stock size, it will always have an impact on your speedometer reading, and eventually, your miles per gallon calculation since tire size changes also affect the odometer.
Our upsizing resulted in a 1.7-inch increase in tire size. This represented a speedometer error of 5.6 percent, meaning the speedometer (and odometer) is reading 5.6 percent low. So, when we hit the open road and started conducting our fuel economy testing, we filled up where we had been and stopped where we normally did. Not surprisingly, this time there was a difference in odometer reading thanks to the tire size increase.
Our test look was usually almost 351 miles, but this time, our odometer said 333.2. After correcting for the speedometer error, (5.6 percent) we calculate based on the actual mileage driven, not the odometer reading, and found an increase of .5 percent. The tire size change brought the total slightly over 24 mpg.
Next, we installed a Gear Vendors overdrive which essentially turns the truck’s 4-speed automatic into an 8-speed transmission with overdrive available in every gear. In all practicality, and for fuel economy purposes, we only use the Gear Vendor unit in top gear, with the torque converter locked. However, for those of you who tow heavy loads, having the additional gearing available in all forward gears can help improve fuel efficiency and reduce EGT on those long hill climbs.
The Gear Vendor Overdrive is actually a gear splitter which provides overdrive. This could very well push us toward our goal of 50 percent better fuel economy. This extra overdrive would lower our cruise rpm and could yield another 20–28 percent fuel economy increase, according to the folks at Gear Vendor.
Sounds reasonable enough, so we took the truck over to Manual Auto Group to have the Gear Vendors unit installed. Once it was installed, we needed to drive 20-plus miles to break in the unit as per the instructions before we could engage the overdrive. With the break-in complete, we went back to our test area, filled up, and hit the road, hoping to hit that magic 50 percent level.
Unfortunately, after the final run on our test look, we filled it up and noticed that we actually put in a bunch more fuel than what we had used previously: an additional 5.7 gallons. So much for the 50 percent goal — but what the heck happened? After lots of data pondering and head scratching, we finally figured it out. When we originally set this test up, we slowed the truck down to 67 mph cruise speed to keep the rpm nears the engine’s torque peak sweet spot. Using the Gear Vendors overdrive actually dropped the rpm range down around 1,300 and 1,400 rpm, well below the torque peak. In theory, this drop in rpm was actually increasing the load on the engine and causing the truck to consume more fuel.
So, we decided to conduct additional testing based on our new information and knowledge. We went back to our driving route, and didn’t use the overdrive but sped up to 80 mph (where most people drive on the highway anyway). After spending the better part of a day running around, we finally came to our end point and filled up with a little more than 22 gallons. This time, we logged 15.63 mpg down from 24 mpg at 67 mph. Now, with the sun only a few hours from setting, we hit the road again to see how much of a difference the overdrive would actually make at the right rpm. All said and done, we rolled back into the fuel station and filled up with 17.8 gallons. The results were much better than the 22 gallons before, but this yielded an increase of 22.8 percent for 19.3 mpg.
What was quite surprising about this entire test was that even at 80 mpg our rpm were still only at 1,600 or so with the overdrive on. We assume that if we had taken our truck to West Texas where the speed limit is 80 mph, we might have actually been able to see better mpg, while going a little faster (85). Trucks with higher gear ratios (4:10, 4:30, 4:88) would see a much higher benefit to using a Gear Vendors overdrive at a lower speed. Another factor when speeds climb is the aerodynamics of the truck, or lack there of. A flat-faced Super Duty is not terribly “aero-efficient” so sometimes the faster you go, the harder the engine has to work to overcome aero drag and resistance.
Since we were in fuel-saving mode, we opted to do our own version of Myth Busters by testing the rumor that upgrading the injectors in a Power Stroke can also help increase fuel economy. However, with all of the modifications done to our truck, we didn’t want to skew the results, so we decided to use a completely stock 6.0-liter for the test. Now think about this, upgrading injectors that flow more fuel, can save you fuel? Interesting in theory, but does it work in real life?
Dynomite Diesel has developed a set of 90-hp injectors that feature a 6-hole injector tip, instead of the factory 5-hole set up. Ross explained that with the additional sixth hole, they were actually able to increase the atomization of the fuel, which equates to a better, more efficient burn. As the computer is telling the injectors what to do, it will sense the increased power and actually back off on how much fuel it is calling for, which, in theory, means increased fuel economy.
So, we took the stock 6.0 over to Dr. Performance and set the truck on the rollers. The truck put down 254 hp. Then, we took the truck out to our fuel testing ground, and set the cruse control. The truck averaged 17.38 mpg. Next, we took the truck over to Crede Young at Diesel Dynamics to swap out the injectors. Approximately, 45 minutes later, we were back on the road to break in the injectors. After a few days of playing around with the new power, we took the truck back to Dr. Performance to the dyno to see what the injectors did. The truck almost put down exactly 90 hp more over stock at 344.3 hp. After realizing that this truck is truly getting the power advertised, we headed out to our test area and filled up. We hit the road again and conducted the test exactly as our original test had been set up; going 67 mph on a set driving course.
Previously, the truck had been receiving 17.38 mpg, and now, we were up to 19.69 mpg. That’s an increase of 2.31 mpg by only changing the injectors and nothing else. With our testing complete, we have determined all of the products we installed did indeed work to increase mpg. Each product, of course, performs better under specific circumstances, so choose wisely based on your application and driving style when looking to save money at the pump. Overall, increasing the overall efficiencies of your power plant usually results in more power and improved fuel economy. However, when it comes down to it, it’s the small things that count — proper tire inflation, driving style and slowing down often matter the most.
In the never-ending quest for more miles per gallon, we opted for a larger tire size to brin gthe truck’s rpm down at cruise speed without a gear change. A set of 305/70R16 Firestone Destination A/T’s replaced our original 255/75R16 tires.
Prior to the Gear Vendors installation, a two-piece driveshaft was utilized to connect the transfer case and rear end.
The Gear Vendors overdrive unit installed onto the back of the transfer case. This install also allowed us to switch from a two-piece driveshaft to a single-piece unit.
We also tested what upsized injectors do for fuel conomy on a stock 6.0-liter Power Stroke. To get at the injectors, Diesel Dynamics easily had the valve covers off and stock injectors removed in no time.
The holes on top of the injectors yousee is where the oil manifold goes. This manifold holds and directs the high-pressure oil driven by the high-pressure oil pump which drives the injectors.
The new injectors from Dynomite Diesel are 90-hp injectors and yielded exactly a 90-hp gain on the dyno.
Bridgestone Firestone North American Tire, LLC www.firestonetire.com (800) 367-3872
Diesel Dynamics (817) 558-3673
Dr. Performance www.drperformance.com (888) 417-8547
Dynomite Diesel www.dynomitediesel.com (360) 794-7974
Gear Vendors www.gearvendors.com (619) 562-0060
Manuel Auto Group www.ManuelAutoGroup.com (817) 367-4000