Improving Gas Mileage

(Without all the Gimmicks)

Topics In this Article:
  
Terminology
Techniques used to maximize fuel economy
Basic techniques
Maintenance
Minimizing mass
Efficient speeds
Choice of gear (manual transmissions)
Acceleration and deceleration (braking)
Coasting or gliding
Anticipation
Fuel type
Trip computer
Advanced techniques
Burn and coast
Auto-stop, forced stop, and draft-assisted forced stop
Drafting
Energy losses
Safety
Tailgating
Drafting
Coasting in neutral

Improving Gas Mileage and Maximizing Your Fuel Economy

Fuel economy-maximizing behaviors describe techniques that drivers can use to optimize their automobile fuel economy. The energy in fuel consumed in driving is lost in many ways, including engine inefficiency, aerodynamic drag, rolling friction, and kinetic energy lost to braking (and to a lesser extent regenerative braking). Driver behavior can influence all of these. The city mileage of conventional cars is lower than highway mileage due to: 1) a high proportion of idling time, 2) operation mostly at very inefficient low-output engine operating points, and 3) more frequent braking (and more frequent resultant acceleration).

Terminology
Various terms describe drivers using unusual driving techniques to maximize fuel efficiency. A few of these are:

Hypermilers are drivers who exceed the United States Environmental Protection Agency (EPA) estimated fuel efficiency on their vehicles by modifying their driving habits. The term 'hypermiler' originated from hybrid vehicle driving clubs and noted hypermiler Wayne Gerdes and combines current technology (e.g., real time mileage displays) with driving techniques innovated historically with events such as Mobil Economy Run during the 1930's,, gas rationing during World War II, techniques that prevailed during 1973 oil crisis, and methods used globally in markets that endure expensive fuel.

Nempimania (also Nenpimania) is an obsession with getting the best fuel economy possible from a hybrid car. It is derived from the Japanese "nempi" a contraction of nenryōshōhiryō meaning fuel economy, and mania, meaning "craze for." Nempimania is exhibited by owners of the Toyota Prius and other hybrid owners by various habits aimed at maximizing fuel economy: slow starts, "Pulse and Glide", timing stoplights, etc.

Techniques used to Maximize Fuel Economy

Techniques used to improve fuel economy include basic techniques that can be used by most drivers, and advanced techniques that are more specialized, but can be used to achieve extremely high mileage.

Basic Fuel Economy Maximizing Techniques

Maintenance

Key parameters to maintain are high tire pressure, tire balance and wheel alignment, and engine oil with low-kinematic viscosity referred to as low "weight" motor oil, which is filled just to the low-level mark. Inflating tires to the maximum recommended air pressure means that less energy is required to move the vehicle. Under-inflated tires can lower rolling resistance by approximately 1.4 percent for every 1 psi drop in pressure of all four tires. Equally important is the proper maintenance of the Engine Control Module and the sensors it relies on to control engine operation, particularly the oxygen sensor.

Minimizing Mass

Drivers can also increase fuel economy by driving lighter-weight vehicles and minimizing the amount of luggage, tools, and equipment carried in the vehicle.

Efficient speeds

Maintaining an efficient speed is an important factor in fuel efficiency Optimal efficiency can be expected while cruising with no stops, at minimal throttle and with the transmission in the highest gear (see Choice of gear, below). The optimum speed varies with the type of vehicle, although it is usually reported to be in the range of 35 to 55 mph (56-89 km/h). For instance a 2004 Chevrolet Impala had an optimum at 42 mph (70 km/h), and was within 15% of that from 29 to 57 mph (45 to 95 km/h). Drivers of vehicles with fuel-economy displays can check their own vehicles by cruising at different speeds and monitoring the readout.

Toyota and Ford hybrids have a threshold speed—around 42 mph (68km/h) in the case of the Prius—above which the engine must run to protect the transmission system. Below this model-dependent speed, the car will automatically switch between either battery-powered mode or engine power with battery recharge. These hybrids typically get their best fuel efficiency below this model-dependent threshold speed. Coasting can be achieved by using Neutral transmission range. The Honda IMA vehicles have a limited, battery-only, powered capability, although after-market modding has made the Insight capable of running in electric only-mode . They achieve higher fuel economy. The GM hybrids have an engine auto-stop when halted. As of January 2007, they have no battery-only, powered capability.


Choice of gear (manual transmissions)

Engine efficiency varies with speed and torque, as can be seen in a plot of brake specific fuel consumption.  A typical optimum efficiency point is around 2000 RPM and 75% of full throttle. However, for driving at a steady speed, one cannot choose any operating point for the engine--rather there is a specific amount of power needed to maintain the chosen speed. Constant power curves are shown as dashed lines in the map. A manual transmission lets the driver choose between several points along the curve. In the typical example, one can see that too low a gear will move the engine into a high-rpm, low-torque region in which the efficiency drops off rapidly, and the best efficiency is achieved near the highest gear. Because cruising at an efficient speed uses much less then the maximum power of the engine, the optimum operating point for cruising at low power is typically very low engine speed, around 1000 RPM, even though the highest efficiency point occurs with higher power acceleration at around 1000 RPM.


Acceleration and deceleration (braking)

Fuel efficiency varies with the vehicle, but generally acceleration is most efficient at 75% to 100% throttle openings However, accelerating too quickly without paying attention to what is ahead may require braking and then after that, additional acceleration.

Generally, fuel economy is maximized when acceleration and braking are minimized. So a fuel-efficient strategy is to anticipate what is happening ahead, and drive in such a way so as to minimize acceleration and braking, and maximize coasting time.

The need to brake in a given situation is in some cases based on unpredictable events which require the driver to slow or stop the vehicle at a fixed distance ahead. Traveling at higher speeds results in less time available to let up on the accelerator and coast. Also the kinetic energy is higher, so more energy is lost in braking. At medium speeds, the driver has more "degrees of freedom", and can elect to accelerate, coast or decelerate depending on whichever is expected to maximize overall fuel economy. Travelling at posted speeds allows for best civil planning and should allow drivers to best take advantage of traffic signal timing.

While approaching a red signal, drivers may choose to "time a traffic light" by easing off the throttle, or braking if necessary, far before the signal. For example, a driver who is approaching a red light should adjust vehicle speed such that the vehicle arrives at the intersection when the light is green to minimize braking. Instead of coasting up to the light and stopping, the driver will now be travelling at a slower speed for a longer time, allowing the light to turn green before he arrives. The driver will never have to fully stop, as accelerating from just a few MPH is much more efficient than a full stop. It should be cautioned that doing this during congestion (when a car will need to stop for the same signal more than once) will likely lead to more congestion; the individual driver may save fuel, but much more fuel will be used overall.

Conventional brakes dissipate kinetic energy as heat, which is irrecoverable. Regenerative braking, used by hybrid/electric vehicles, recovers some of the kinetic energy, but some energy is lost in the conversion, and the braking power is limited by the battery's maximum charge rate and efficiency.


Coasting or gliding

The alternative to acceleration and braking is coasting. Coasting is an efficient means of slowing down, because kinetic energy is dissipated as aerodynamic drag and rolling resistance, which always must be overcome by the vehicle during travel. When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle speed.


Anticipation

A driver may further improve economy by anticipating the movement of other traffic users. For example a road user stopping their vehicle sharply or failing to indicate a change in direction can reduce the options a driver has for maximizing their performance. By always giving road users as much information about their own intentions a driver can help other road users reduce their fuel usage. Similarly anticipation of road features such as traffic lights can reduce the need for excessive braking and acceleration.


Fuel type

It is commonly believed that efficiency of a gasoline engine is related to the fuel's octane level; however, this is not true in most situations. Octane rating is only a measure of the fuel's propensity to cause an engine to "ping"; this ping is due to "pre-combustion", which occurs when the fuel burns too rapidly (before the piston reaches top dead center). Higher-octane fuels burn more slowly at high pressures. For the vast majority of vehicles (i.e. vehicles with "standard" compression ratios), standard-octane fuel will work fine and not cause pinging. Using high-octane fuel in a vehicle that does not need it is generally considered an unnecessary expense, although Toyota has measured slight differences in efficiency due to octane number even when knock is not an issue. Most vehicles equipped with emissions systems have sensors that will automatically adjust the timing if and when ping is detected, so low-octane fuel can be used even if the engine is designed for high octane, at some reduction in efficiency. If the engine is designed for high octane then higher-octane fuel will result in higher performance (with full-open throttle), but not necessarily fuel cost savings, since the high octane is only needed with the throttle fully open. For other vehicles that have problems with ping, it may be due to a maintenance problem, such as carbon buildup inside the cylinder, using spark plugs with the improper heat range or ignition timing problems. In such cases, higher-octane fuel may help, but this is an expensive fix; proper repair might make more long-term sense. There is slightly less energy in a gallon of high-octane fuel than low-octane. Ping is detrimental to an engine; it will decrease fuel economy and will damage the engine over time.

Trip computer

Modern hybrids come with built-in trip computers which display real-time fuel economy (MPG), which helps the driver adjust driving habits. However, most gasoline powered vehicles do not have this as a standard option (although some luxury vehicles do). However, most vehicles produced after 1996, have one of three standardized interfaces for "on-board diagnostics", which provides information including the rate of fuel consumption, and the vehicle speed. This streaming data is sufficient to calculate the real-time fuel economy.

Generic aftermarket or "add-on" products are available, such as the "ScanGauge" or "DashDyno SPD", which will connect to a vehicle's onboard computer, read the real-time information, and calculate and display the instantaneous fuel economy. This information assists the driver by displaying the fuel consumption. This provides a general indicator to the driver who can then infer in real-time how driving techniques affect gas mileage. This can help the astute driver to learn how to drive more efficiently, However, such a device does not do all the work for the driver. The device only measures fuel consumption, and fuel economy. It does not indicate braking statistics, for example, nor does it teach a driver methods to minimize fuel consumption.


Advanced Fuel Economy Maximizing Techniques

These are less broadly applicable, and some may compromise safety.

Burn and coast

Burn and coast is also known as Pulse and glide. This method consists of accelerating to a given speed (the "burn" or "pulse"), followed by a period of coasting (or "gliding"), and then repeating the process. Coasting is most efficient when the engine is not running. Because some cars inject extra fuel when the starter is activated, this was originally best accomplished with a manual transmission. Hybrid vehicles are ideally suited to performing this technique as well: the internal combustion engine, as well as the charging system, can be shut off for the glide by simply manipulating the accelerator. Production fuel injected engines shut off the fuel when the throttle is closed and the engine is running faster than idle speed for instance while coasting down hill with the vehicle in gear, in a car with a manual transmission. This is due to the ECU being linked to the ignition and the throttle. This is not the case in older cars that use carburettors which will still drag fuel into the engine. The optimum acceleration that is used in the burn phase is not necessarily full throttle.


Auto-stop, forced stop, and draft-assisted forced stop

In the auto-stop maneuver, the vehicle's transmission is put in neutral, the engine is turned off (a "forced stop"), and the vehicle coasts to a stop. It is possible to coast in neutral with either a manual or automatic transmission. Modern automatic transmissions/transaxles depend on an engine driven fluid pump for lubrication and coasting with the engine off may lead to damage or failure of the transmission. To perform the maneuver, the driver shifts into neutral, and then keys the ignition back to the first position, referred to as "IG-I", to shut off the engine and electronics. The driver then keys forward to IG-II to start the electronics and continue coasting. The key should remain in the ignition in the IG-II position, and not the IG-I position, in order to avoid engaging the steering wheel lock. The driver recovers normal operation by starting the engine in the normal way, by turning the key to IG-III to crank the starter motor, and then releasing the key back to IG-II. Before putting the transmission in gear, if necessary, the driver may "rev" the engine to match the vehicle's gear and speed. The fuel economy from this advanced technique is increased noticeably over any short distance trip, largely because there are no engine idling losses (see figure below). Most modern automatics' computer systems do a very good job at keeping the transmission in the proper gear while coasting in neutral, and the driver should not be conscious of the tachometer when re-engaging, but rather just press half-way down on the accelerator when re-engaging.

Some, but not all, hypermilers use this maneuver, and some may use it more safely than others. The technique is used for general coasting, or as part of the pulse-and-glide maneuver, or when going down hills or in other situations when potential energy or momentum will propel the vehicle without engine power. Some hypermilers may use this maneuver while going downhill, around a corner, and without braking; however, that practice is in all likelihood more dangerous than an auto-stop on a level and straight road, where stopping distance is shorter and visibility is greater. Vehicle control may be somewhat compromised, and this can be more or less dangerous or safe depending on the situation. Turning the engine off will cause the power brake assist to be lost after a few applications of the brake pedal. Power steering is instantly lost, although it is not needed at high speed, only at low speed. Steering is still possible at low speed, but can often require considerably more arm strength to turn the wheel.

For safety reasons, the maneuver is not recommended for use in traffic, since the driver will want the car to be in gear if sudden acceleration is needed as an evasive maneuver. The driver should first look for traffic behind the vehicle before attempting the maneuver. It can be considered more courteous to not coast if another vehicle is closely following.  The proper etiquette and acceptable driving practices are controversial, and is worsened by a lack of communication between drivers. Both sides of the debate are often argued passionately, yet sometimes neither of the proposed driving methods is in complete accordance with the rules of the road. Both hypermilers and regular drivers may at different times violate the same rule yet blame the other type of driver.

Despite the potential risks, it does in fact save fuel to turn the engine off instead of idling. Traffic lights are in most cases predictable, and it is often possible to anticipate when a light will turn green. Some traffic lights (in Europe) have timers on them, which assists the driver in using this tactic.

Draft-assisted forced stop, a variation of the forced (auto)stop (sometimes abbreviated as D-FAS), involves turning off the engine and gliding in neutral while drafting a larger vehicle, in order to take advantage of the reduced wind resistance in its immediate wake (This practice is illegal in some areas due to its danger); while tailgating itself is inherently risky, the danger of collision is increased with D-FAS as hydraulic power for power brakes is used up after a few applications of the brake pedal, and there is a loss of hydraulic pressure that provides power steering, however, there is less need for power steering at high speed.

Some hybrids must keep the engine running whenever the vehicle is in motion and the transmission engaged, although they still have an "auto-stop" feature which engages when the vehicle stops, avoiding waste. Maximizing use of auto-stop on these vehicles is critical because idling causes a severe drop in instantaneous fuel-mileage efficiency to zero miles per gallon, and this lowers the average (or accumulated) fuel-mileage efficiency.

Drafting

The US television show Mythbusters (Discovery Channel), in their June 6, 2007, episode, took a series of measurements where they drove a Dodge Magnum Station Wagon at 55 mph right behind a Freightliner tractor trailer. As they got closer their results ranged from a baseline (no truck) figure of 32 mpg, to 35.5 mpg (11% improvement) at 100 feet, and then progressively up to 44.5 mpg (a 39% increase) at ten feet, as a result of decreased drag consequent of drafting.  It should be noted that according to "Mythbusters", drafting a big rig at close distances is life-threatening and extremely dangerous. They recommended a minimum safe driving distance from a big rig is 150 ft.

Energy losses

Example energy flows for a late-model midsize passenger car:
(a) urban driving; (b) highway driving. Source: U.S. Department of Energy

Understanding the distribution of energy losses in a vehicle can help drivers travel more efficiently. Most of the fuel energy loss occurs in the thermodynamic losses of the engine. The second largest loss is from idling, or when the engine is in "standby", which explains the large gains available from shutting off the engine. Very little fuel energy actually reaches the axle. However, any mechanical energy that doesn't go to the axle is energy that doesn't have to be created by the engine, and thus reduces loss in the inefficiency of the engine.

In this respect, the data for fuel energy wasted in braking, rolling resistance, and aerodynamic drag are all somewhat misleading, because they do not reflect all the energy that was wasted up to that point in the process of delivering energy to the wheels. The image reports that on non-highway (urban) driving, 6% of the fuel's energy is dissipated in braking; however, by dividing this figure by the energy that actually reaches the axle (13%), one can find that that 46% of the energy reaching the axle goes to the brakes. Also, additional energy can potentially be recovered when going down hills, which may not be reflected in these figures.  Any statistic such as this must be based on averages of certain driving behaviors and/or protocols, which are known to vary widely, and these are precisely the behaviors which hypermilers leverage to the full extent possible.



Safety

Geoff Sundstrom, director of AAA Public Affairs, notes that "saving fuel and conserving energy are important, but so is safety, and preventing crashes." In the US, optimal highway speed for fuel-efficiency often lies between the legal minimum speed and the legal speed limit, typically 45 to 65 mph. However, these legal speeds may actually be slower than average traffic speed. The hypermiler thus avoids the danger of higher speeds, however, the speed differential created between cars can be problematic in some cases. Driving at speeds much lower than other vehicles may promote other problems; namely, aggressive drivers may choose to tailgate a slower vehicle. Coasting in neutral with or without the engine off may lead to reduced control in some situations, and drafting at any closer than 3 seconds to the vehicle in front is a recognised risk.


Tailgating

On some roads, the norm is to drive above the speed limit, and other drivers may become enraged when encountering a driver travelling at a legal speed. (Note: In the US driving in the left lane at the "legal" speed is still illegal on a multi-lane highway in many jurisdictions.) In particular, slower driving may lead to faster drivers tailgating the slow vehicle, which is a dangerous situation, particularly at high speeds.

There are many reported accounts of road rage and tailgating by aggressive drivers, when hypermilers drive in a manner that other drivers are unaccustomed to, such as coasting to a stop.

The risk of tailgating is largely caused by the accident avoidance time being reduced to much less than the driver reaction time. For maximum safety, driving instructors advocate using the "3 second rule" (the distance between your car and the car in front of you should be 3 seconds of driving time at your current speed), regardless of speed. In the US, if an accident occurs due to tailgating, the tailgater is liable for injury and damages in some states.

The risk of severe road rage may be lessened by permitting aggressive drivers the opportunity to pass when it is safe to do so.

Drafting

According to "Mythbusters", drafting a big rig at close distances is life-threatening and extremely dangerous. They recommended a minimum safe driving distance from a big rig is 150 ft.
 

Coasting in neutral

Those who warn that coasting can be dangerous claim that the driver has less control of the vehicle, and will take longer to react in an emergency.

In a collision-avoidance emergency, the safe technique focuses entirely on controlled braking, and not at all on acceleration. The proper technique is to use threshold braking (maximum deceleration without skidding), then to wait one second for the weight to shift onto the front wheels in order to increase vehicle cornering stability and to increase the maximum lateral acceleration that is possible without skidding, and then to turn the vehicle rather quickly and sharply to avoid the object. If the lead vehicle initiates an emergency stop, the trailing vehicle is likely to need 3 seconds to avoid a collision.

One function of the driving laws is to help increase safety. However, the safety issues are not always clear cut, and often neither are the laws. A driver legally does need to know how to control the vehicle safely when the car is in neutral. The general practice of coasting in neutral is against the law in many American states, yet there are exceptions to this law, and some places advocate its use in certain circumstances, for example: "If you are on ice and skidding in a straight line, step on the clutch or shift to neutral."  Also, in a stuck throttle emergency, the safe procedure is to put the transmission in neutral, and if that is ineffective, to turn off the engine. Also, a driver legally needs to have the ability to bring the vehicle to a stop under any circumstances, including when the engine stalls during normal driving. In the event that there is a loss of engine power, decelerating to a stop is recommended as the safest action. As a safety feature, vehicles are designed to retain some limited ability to steer and brake even when all engine power is lost.

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