The Real World

During the 1992 oxygenated gasoline season, Chevron received a number of calls from drivers reporting fuel economy decreases of 8% to 10% and a few reporting decreases of 15% to 20%. Using the above example of 23 mpg, this would be a decrease to 20.7 mpg (-10%) to 18.4 mpg (-20%). The California Air Resources Board received similar complaints.³

To explain why a few drivers experienced greater decreases in fuel economy than expected for oxygenated gasoline, we must look at the other factors which affect fuel economy. The weather, gasoline composition, the driving cycle, and the mechanical condition of the car are four important factors which might act negatively during the oxygenated gasoline season.

Weather

The oxygenated gasoline season begins October 1 or November 1, depending on the area. This coincides with changes in the weather -- lower temperatures and rain or snow -- which reduce fuel economy.

* Summer or winter, a car's fuel economy is lower while it is warming up, a process that takes 5 to 10 miles or urban driving. Factors which make the effect greater when temperatures are lower include: more time spent at idle to defog/defrost windows of cars parked outside, more time spent using a "fuel rich" warm up mixture, and the additional energy needed to overcome higher viscosities of the cold fluids: engine oil, differential oil, and transmission fluid. The effect is magnified by short trips. The fuel economy of a five mile trip can be 5% to 15% lower in winter.4
* One might assume that the energy saved by not running the car's air conditioner would counter-balance some of the low temperature warm up effects. But the air conditioners of many cars operate when the climate controls are on the "defrost" or "defog" setting to improve defogging performance by reducing the humidity of air blown on the windshield.
* Rain or snow on the roadway decreases fuel economy because it offers more resistance to the forward motion of the tires. Loss of traction (tire spinning) also decreases fuel economy.
* Finally, bad weather often results in more bumper-to-bumper travel during commute times. Stop and go driving exacts a heavy fuel economy penalty.

Gasoline Composition

We have already discussed two related changes in gasoline composition which decrease fuel economy: adding oxygenate and adjusting the octane of oxygenated gasoline. Adjusting gasoline volatility for seasonal ambient temperatures also requires changes in gasoline composition. Summer gasoline is given a lower volatility to avoid vapor lock and minimize evaporative losses. Winter gasoline (both conventional and oxygenated) is given a higher volatility to facilitate starting and warmup. The compositional changes required for the shift from summer gasoline to winter gasoline decrease the gasoline's energy content. Depending on the magnitude of the volatility change, the fuel economy of winter gasoline will be 0.5% to 1.5% lower than of summer gasoline.

Driving Cycle

Terrain, traffic conditions, and driving habits all effect fuel economy. The following list gives examples of changes in these factors which will decrease fuel economy. Some of these changes are more likely to occur during the oxygenated gasoline season.

* Winter driving, on the average, tends to consist of shorter, urban trips, while summer driving is more likely to include some longer highway trips. The fuel economy for urban driving is 25% lower than for highway driving at moderate speeds.5
* Stop and go commuter driving decreases fuel economy.
* Highway driving at speeds greater than 55 mph decreases fuel economy.
* Driving up and down hills decreases fuel economy.
* Greater loads (more passengers or more baggage) decreases fuel economy.
* A driver with more aggressive driving habit (e.g., faster acceleration and more frequent braking) than another driver will obtain a lower fuel economy when both drive the same car. At the extreme, the difference my be as large as 25%.

Car's Mechanical Condition

A car which is not properly maintained is likely to have poorer fuel economy. Factors which reduce fuel economy are:

* Wrong timing (spark retard)
* Fouled spark plugs
* Wrong oil grade (too high viscosity)
* Misaligned wheels
* Underinflated tires

These factors have a cumulative impact on fuel economy. It is difficult to estimate the impact's size because the contribution of each component depends on how far it is out of adjustment. The impact can be large; decreases of 10% to 20% in fuel economy are possible.6

Measuring Fuel Economy

Finally, measurement errors can make fuel economy changes appear larger than they actually are. To determine the size of a change, one needs good fuel economy values for the present time and for some (reference) time in the past.

Most people measure fuel economy by filling a car's gas tank and noting the odometer reading, driving for a period of time, and then refilling the tank and noting the new odometer reading. It's not easy to fill the tank to the same level every time: vapor bubbles can get trapped against the top of the tank and the automatic nozzle shutoff system may not bring the gasoline level to the same point in the filler spout. A filling difference of just 0.2 gallons in a 10 gallon purchase represents an error of 2%. Because of this problem, the accuracy of a fuel economy calculation is increased greatly if it is measured over several tankfuls.