Establishing Economy
Since much of this site is devoted to improving fuel economy, it stands to reason that to be scientific about it, you must be able to measure the results. Otherwise, how would you know if it worked? In short, fuel economy is measured by the distance traveled divided by the fuel consumed.
Miles per gallon = miles/gallons or distance traveled/fuel used.
The math is 3rd grade elementary school stuff; however...
“I heard about some fancy “Fuel Economy Tune-Up” being offered by a garage 200 miles away. A friend of mine is singing their praises. I live along the ocean, and the shop is up in the mountains. My theory is to fill the tank before I leave home, then fill it again before dropping it off at the shop; calculate fuel economy. Let the mechanic do the “Tune-Up”, then top the tank off again before heading home. In my home town, I’ll fill it up again, run the numbers, then compare the “Before” and “After” numbers to determine the effectiveness of the work I just paid for. This is should be a scientific testing method!” You may already guess that the mechanic could do absolutely nothing to the vehicle and the “calculations” would suggest a 10% to 15% gain in fuel economy due to the difference in elevation.
This illustrates the influence of “variables”. Variables consist of things that change beyond the control of the driver. The list includes altitude, temperature, wind, road conditions, traffic, different drivers, and even the fuel itself. Let’s look at them individually.
Altitude Changes
If you drive up hill you get worse fuel economy than if you drive down hill. An old joke is: to get better fuel economy, always drive down hill (with a wind at your back). The above story illustrates just that. Plus, altitude affects barometric pressure, which affects the amount of nitrogen and oxygen in any given volume of air. Engines are “gutless” at the top of high mountains because of this effect. (Look up WWII data regarding altitudes Allied versus Axis planes could fly, and why.) Higher altitudes have lower air density, which also affects wind resistance...
Temperature
At colder temperatures, oils and greases are thicker, causing higher parasitic losses. Cooler air contains more oxygen and nitrogen per volume than does warmer air, which affects performance and AFR considerations. If your ECU uses an Air Temperature Compensation Factor for ignition timing, you get more ignition timing advance in cooler temperatures. As noted earlier, cooler air contains more nitrogen and oxygen — air. Wind resistance has a greater negative impact on fuel economy at cooler temperatures, which of course is worse at higher speeds. Considering all these factors (some of which are Pro and others Con), you are far less likely to see good fuel economy in colder temperatures.
Wind
The higher the vehicle speed, the higher the impact of aerodynamics on fuel economy. In addition to vehicle speed and temperatures, air currents (wind) can work for you (tail wind) or against you (head wind). This certainly affects fuel economy.
Road Conditions
If you put on your jogging shoes and run on the local high school track on a dry sunny day, you can run pretty fast. You must overcome the counter-forces of air, which has little resistance to your efforts. If you run in a swimming pool, you run much slower. The water has a much greater resistance to overcome. Similarly, if you test fuel economy on a dry road, you will get the best possible numbers. If it is raining, you have to plow through the water, which reduces efficiency. Driving through snow is even worse.
Traffic
It is well established that considerably more fuel is consumed during acceleration versus maintaining a constant speed. If you do a mileage run on a back country road at 1:30 in the morning, chances are you can drive any speed you want (up to the speed limit, of course). However, if driving at 4:30 in the afternoon, you are more likely to get behind an old fart (I say that with utmost respect as I consider myself in that category), or worse yet, a school bus that stops at every 4th house. Highway driving is similar, where rush hour invariably results in traffic jams. Driving through town can result in different traffic light conditions (more red lights on one of the tests). Mileage tests should be conducted at a time of day, and on a route that allows for constant speed as much as possible.
Driver
Many years ago my Dad broke his leg. When he drove, the family pick-up truck got 15 MPG. While his leg was in a cast, Mom drove. Somehow she got 18 MPG in the same truck!?! There is a dissertation on driving for maximum fuel economy (Driving Dissertations) that illustrates techniques to get the most from a vehicle. Some folks violate these rules to the point where they should be charged with “fuel economy homicide in the 1st degree”. If evaluating fuel economy technologies, the same driver should be used for both “Before” and “After” testing.
Fuels
Oil companies advertise better fuel economy from their gasoline versus the competition. Different brands of fuel really can affect fuel economy test results. Fill at the same station (and preferably the same pump) for both “Before” and “After” tests. Another consideration is seasonal changes in fuel chemistry. In the hot summer months, oil companies load their gasoline blends with heavier elements (like decane, undecane, and dodecane), which burns relatively efficiently (?) in warmer temperatures, but retards rapid evaporation. In winter months, they replace the large HC ingredients with smaller versions (dodecane is replaced with pentane). This also contributes to different MPG results when testing in January versus July.
Summary
If you casually observe your fuel economy, perhaps you notice that some weeks it takes $45 to fill the tank, while other weeks it’s $55. The price of fuel has some effect, but so do all the other factors mentioned. To do a “scientific” fuel economy test, all variables must be controlled. Both the “Before” and “After” driving conditions must be virtually identical. Any deviation from “same” will deliver different results. In statistics, the more samples you can include in your model, the more accurate your evaluation. If you have 2 years of accumulated fuel economy data (every time you fill the tank you write down the odometer reading and the amount of fuel added), you have an excellent baseline that accounts for variables — averaged out over time. After modifications are made to improve fuel economy, the next year’s data will show extremely accurate results.
FE-NR
MPGenie Basics 051 Training - Establishing Economy
