Well, I finally took the time to read the above mentioned document. While many of the items that affect milage are pretty obvious, there were a number of them that are pretty subtle. Following are some excerpts from the document.
I have highlighted what I think are the interesting items:
We know that our trucks are more fuel efficient in the summer, and we attribute this to "summer blend" fuel. While this is true, it appears that there are several other factors too.
Worn tires are more fuel efficient than new tires. Who would have thunk?
Water on the road cools the axles and transmission fluid, which lowers it's efficiency. If cooler gear box temps contribute to lower efficiency, what is the effect of Mag-Hytec transmission and diff. covers that cool fluid temps by adding more fluid capacity?
Please keep in mind that all of the following statements are from Cummins, not me. I am only responsible for drawing attention to the items i find of interest. If you have issue with the content, take it up with Cummins, not me.
Enjoy.
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Rock-Solid Rules:
- Every 2% reduction in aerodynamic drag results in approximately 1% improvement in fuel economy.
- Above 55 mph, each 1 mph increase in vehicle speed decreases fuel economy by 0. 1 mpg.
- Worn tires provide better fuel economy than new tires, up to 7% better fuel economy.
- Used lug drive tires can get up to 0. 4 mpg better than new lug tires.
- Ribbed tires on the drive axles provide 2–4% better fuel economy than lugged tires.
- Every 10 psi that a tire is under-inflated reduces fuel economy by 1%.
- The break-in period for tires is between 35,000 and 50,000 miles (OTR Trucks).
- Tires make biggest difference in mpg below around 50 mph; aerodynamics is the most important factor over around 50 mph.
- The most efficient drivers get about 30% better fuel economy than the least efficient drivers.
- Idle time is costly. Every hour of idle time in a long-haul operation can decrease fuel efficiency by 1%.
Lubricants:
While the efficiency of drivetrain components is largely fixed by design, gross efficiency losses can be minimized through proper selection of lubricants.
Synthetic base lubricants are manufactured in the laboratory to exhibit superior high temperature stability and low temperature fluidity. Since these fluids are created to exhibit less thickening at low temperatures, pumping losses are reduced and substantial reductions in spin losses can be realized at low operating temperatures.
Test results indicate no significant difference in engine efficiency between synthetic and mineral base lube oils at normal operating temperatures. Since the synthetics are more expensive and, in an engine crankcase, are subject to the same contaminants as mineral based oils, they may not be cost effective.
All oils thicken at low temperature, causing increased fuel consumption. The synthetic oil is less affected by temperature. This makes synthetic oils more fuel efficient at lower ambient temperatures.
The high temperature stability and low temperature fluidity of synthetic lubricants make them ideally suited for drivetrain components. In this environment the lubricant is not subjected to combustion byproducts. This means the lubricant, with its higher oxidation resistance can last substantially longer. Drain intervals of 250,000 to 500,000 miles more than offset the higher purchase price of the lubricant.
Tire Tread Depth and Pattern:
According to Bridgestone, the tire tread accounts for 60–70% of the tires’ rolling resistance. Not only do the tires differ in rolling resistance when new, but as the tread wears, the rolling resistance of the tire changes.
A 7/32 tread wear represents ~10% reduction in rolling resistance (5% better mpg) compared to a new tire. Rib tires at all wheel positions will provide greatest fuel efficiency.
Tread pattern is important because lugs have deeper tread (more rolling resistance) than ribs. If we take a new ribbed tire as the standard, a new lugged tire is less fuel efficient by about 6%. A worn tire is about 7% more fuel efficient than a new tire.
Engine Operating Temperature (Coolant and Lube Oil):
Low coolant temperatures indicate an engine that is too cold for efficient combustion. Fuel liquefies on the cold cylinder walls and fails to burn. Of course, excess heat causes engine failure.
Lube oil below the ideal temperature is more viscous and harder to pump. Oil above the ideal temperature is too thin to lubricate properly. Either way, the engine suffers.
Coolant and lube oil operating temperatures can contribute greatly to fuel efficiency. Typical cooling system operating temperatures are above 180° F. A 0. 4% fuel economy loss is associated with every 30° decrease in temperature.
Lube system operating temperatures run above 225° F, and a 1% fuel economy loss is associated with every 30° decrease in lube temperature.
Intake and Exhaust Restriction:
An engine that is starved for air (intake restriction) or unable to expel exhaust (exhaust restriction) will lack power and waste fuel. The extra fuel burns inefficiently at best because it takes air to completely oxidize the fuel and extract all of the power that it contains.
Engine Lube Oil Levels:
Exceeding the recommended engine oil levels can result in significant oil churning/spin losses. The effects of these excessive churning/spin losses (greater than -2% impact on mpg) include the following:
- Reduced engine efficiency
- Reduction in performance
- Deterioration of critical oil properties (lubrication and heat transfer)
Axle Alignment:
Tires need to point straight ahead in order to roll with the least possible resistance. A tire that deviates only ¼ degree from straight ahead will try to travel 10 to 15 feet sideways for each mile the vehicle travels forward. Scrubbing the tires in this fashion is bad for fuel economy and also bad for tire wear.
Tire Inflation Pressure:
Proper inflation pressures critically affect tire performance. Under-inflation can detrimentally affect tire performance and durability. Specifically, it:
- Reduces fuel economy
- Increases tire wear rates
- Creates irregular tread wear
- Reduces casing durability
Every 10 psi of under-inflation represents approximately 1% penalty in fuel economy.
Road Surface:
Even road surface has a documented effect on fuel economy. Using new concrete as the standard baseline, worn or polished concrete is even better. All other road surfaces are worse, some substantially.
Tire Rolling Resistance, Pavement Type and Condition, versus Relative Rolling Resistance:
Concrete, polished (best mpg) –12%
Concrete, new – (Baseline) 0%
Asphalt 1% - 8%
Chip and Seal Blacktop (worst mpg) - 33%
(Editor's Note: The above percentages represent relative rolling resistance, not fuel economy).
Road roughness can increase rolling resistance up to 20% due to energy dissipation in the tires and suspension (10% loss of mpg).
Weather and Seasonal Conditions:
You can’t control the weather or the seasons, but they definitely affect your fuel economy. Running only on sunny days with moderate temperatures is very impractical, but you have to take the weather and seasonal variations into account when checking fuel economy.
Ambient Temperature:
Air becomes more dense as temperatures drop, which increases air resistance. For every 10° F drop in temperature, aerodynamic drag increases by 2%. Thus, fuel efficiency will drop by 1%. Overall, fuel economy tends to be higher in the summer than the winter. According to North American Truckload Fleet Data, driving in the summer increases fuel mileage by 8 to 12% over driving in the winter months.
Temperature also affects the tires’ inflation pressure. Tire inflation tends to fall when the temperature drops. Running tires low on air pressure in hot weather is more of a safety issue than a fuel economy problem. And heat is the tire’s worst enemy. For safety and economy, check inflation pressures frequently with an accurate tire gauge. When seasons change and temperatures fluctuate, increase the frequency of inflation pressure checks.
Wind:
Headwinds and crosswinds can significantly increase aerodynamic drag and reduce fuel efficiency. For every 10 mph of headwind or crosswind, mpg is reduced by nearly 13%. You cannot cheat increasing wind resistance.
Rain and Snow:
Precipitation such as rain or snow increases rolling resistance because the tires must push their way through the water, slush or snow on the pavement. Also, water is a more effective coolant than air, so the tires, transmission lubricant and axle lubricant operate at cooler (less efficient) temperatures. Rolling resistance and drivetrain friction in light rain increase fuel consumption by 0. 2 to 0. 3 mpg, per SAE testing.
Fuel Blends:
While blended fuels provide better startability and protection against fuel gelling than standard #2 diesel, fuel efficiency decreases. “Summer” fuel improves mileage up to 3% more than “winter” fuel.
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