Fuel economy-maximizing behaviours 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.
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.
Various terms describe drivers using unusual driving techniques to maximize fuel efficiency. A few of these are:
- Hypermilers are drivers who exceed the 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 1930s, gas rationing during World War II, techniques that prevailed during 1973 oil crisis, and methods used globally in markets that endure expensive fuel.
- Nempimania is an obsession with getting the best fuel economy (or the best only-electric range) possible from a hybrid car. It is derived from the Japanese “nempi” meaning fuel economy.
- Eco-driving covers similar ground in other European marketplaces.
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.
Key parameters to maintain are proper tire pressure, and wheel alignment, and engine oil with low-kinematic viscosity referred to as low “weight” motor oil. Inflating tires to the maximum recommended air pressure means that less energy is required to move the vehicle. Under-inflated tires can increase rolling resistance by approximately 1.4% for every 1psi (0.1bar) drop in pressure of all four tires. Equally important is the scheduled maintenance of the engine (i.e. air filter, spark plug), and addressing any on-board diagnostics codes/malfunctions in the Engine Control Module and related sensors, especially the oxygen sensor.
Minimizing mass and improving aerodynamics
Drivers can also increase fuel economy by driving lighter and/or lower-drag vehicles and minimizing the amount of people, cargo, tools, and equipment carried in the vehicle. Removing common unnecessary accessories such as roof racks, brush guards, wind deflectors, running boards, push bars, and narrow and lower profile tires will improve fuel economy by reducing both weight and aerodynamic drag. Some cars also use a half size spare tire, for weight/cost/space saving purposes.
Choice of gear (manual transmissions)
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. The optimum speed varies with the type of vehicle, although it is usually reported to be between 50 and 55mph (80 and 89km/h) for an unspecified vehicle.
Engine efficiency varies with speed and torque. The optimum efficiency point is around 1750 rpm, and 90% of maximum torque at that speed, for this turbo-diesel engine. 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. A manual transmission lets the driver choose between several points along the curve. In the turbo diesel 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 thus best efficiency is achieved near the higher gear. In a gasoline engine, efficiency typically drops off more rapidly than in a diesel because of throttling losses, and the trend discussed here is even more dramatic. Because cruising at an efficient speed uses much less than the maximum power of the engine, the optimum operating point for cruising at low power is typically at very low engine speed, around or below 1000 rpm. This is far lower than the above mentioned 1750 rpm. This explains the usefulness of very high “overdrive” gears for highway cruising. For instance, a small car might need only 10–15 horsepower (7.5–11 kW) to cruise at 60mph (97km/h). It is likely to be geared for 2500 rpm or so at that speed, yet for maximum economy the engine should be running at about 1000 rpm to generate that power as efficiently as possible for that engine (although the actual figures will vary by engine and vehicle).
Acceleration and deceleration (braking)
Fuel efficiency varies with the vehicle. Fuel efficiency during acceleration generally improves as RPM increases until a point somewhere near peak torque. However, accelerating too quickly without paying attention to what is ahead may require braking and then after that, additional acceleration. Experts recommend accelerating quickly, but smoothly.
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 freedom and can elect to accelerate, coast or decelerate depending on whichever is expected to maximize overall fuel economy. Traveling 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 early if necessary, far before the signal. For example, a driver who is approaching a red light should adjust vehicle speed in advance, such that the vehicle arrives at the intersection when the light is green. It is also important to account for the time it takes for the stopped traffic at the light to start moving again. In theory, the ideal situation is the driver slowing immediately to the calculated speed that allows the car to be barely behind the car in front as that vehicle is accelerating from the light. If the driver does this the instant the red light is recognized, this will result in the vehicle having maximum speed, and kinetic energy, as it reaches the intersection. This means that energy lost to braking is as little as possible. Instead of coasting up to the light and stopping, the driver will now be traveling 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 from a full stop. Using this practice during periods of traffic congestion may affect other drivers and the overall effect is not obvious.
Another problem with this technique is that some traffic lights (usually on minor roads where they intersect major roads) are not timed but triggered. They will stay red until a car arrives at the intersection. In this situation, the optimum strategy may be difficult to determine.
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, i.e. gliding along without propulsion. Coasting is an efficient means of slowing down, because kinetic energy is dissipated as aerodynamic drag and rolling resistance, which must always 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 engine speed. While coasting with the engine running and the transmission in gear, most cars’ engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required.
Minimizing ancillary losses
A driver may further improve economy by anticipating the movement of other traffic users. For example, a driver who stops quickly, or turns without signaling, reduces the options another driver has for maximizing his performance. By always giving road users as much information about their intentions as possible, 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.
Using air conditioning requires the generation of up to 5hp (3.7kW) of extra power to maintain a given speed. A/C systems cycle on and off, or vary their output, as required by the occupants so they rarely run at full power continuously. However, the alternative, opening the windows, also creates drag losses that can often exceed the losses from air conditioning, depending on the speed of the car. Using the passenger heating system slows the rise to operating temperature for the engine. Either the choke in a carburetor-equipped car or the fuel injection computer in newer vehicles will add more fuel to the fuel-air mixture until normal operating temperature is reached, decreasing fuel economy.
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. All vehicles built since 1996 are equipped with OBD2 and most will have knock sensors that will automatically adjust the timing if and when ping is detected, so low-octane fuel can be used in an engine designed for high octane, with some reduction in efficiency and performance. If the engine is designed for high octane then higher-octane fuel will result in higher efficiency and performance under certain load and mixture conditions. 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.
Modern hybrids come with built-in trip computers which display real-time fuel economy (MPG or L/100km), which helps the driver adjust driving habits. 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.
Be safe and considerate using advanced fuel economy-maximizing behavioral techniques
The fuel economy-maximizing behavioral advanced techniques are less broadly applicable. Please remember that most of them may compromise safety and be illegal.
Burn and coast is also known as pulse and glide. This method consists of rapid accelerating to a given speed (the “burn” or “pulse”), followed by a period of coasting (the “coast” or “glide”) down to a lower speed, at which point the burn-coast (AKA pulse-glide) ` sequence is reiterated. Coasting is most efficient when the engine is not running, although some gains can be realized with the engine on (to maintain power to brakes, steering and ancillaries) and the vehicle in neutral, or even with the vehicle remaining in gear. If a manual transmission vehicle coasts with the engine off, it is typically re-started by disengaging the clutch. The engine control units of some vehicles command a richer fuel setting immediately after the starter is activated, so the bump-start manual transmission vehicle will typically achieve the best fuel economy gains. Some hybrid vehicles are well-suited to performing the burn and coast.
For coasting in gear, a later-model vehicle with a fuel-injected engine will realize more gains from the burn and coast technique than older carbureted engines because the engine control units in most fuel-injected engines will cut fuel to the engine when the car is in gear, the throttle is closed and the engine is running faster than idle speed. This is sometimes referred to as “deceleration fuel cut off”. This will often engage while a car is coasting down a hill and is common in both automatic and manual transmission vehicles, although the particular engine speeds at which it will engage vary.
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 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.
Drafting is where a smaller vehicle drives (or coasts) close behind a vehicle ahead of it so that the vehicle in front shields the vehicle behind from the headwind. Wind tunnel test of a car (model) ten feet behind a semi-truck (model) showed a reduction of over 90% for the wind force (aerodynamic drag). The gain in miles/gallon however is only 20-40%.
There are a few anecdotal reports on the internet of persons who claim to have coasted for a long distance (without using any engine power most of the time) behind a larger vehicle. It these are true, it must mean that in some cases the aerodynamic drag is no longer drag but pushes the behind vehicle forward so as to overcome its rolling resistance. One should presume that this drafting is on a level road since otherwise a downgrade of a little under 1% is enough to overcome rolling resistance.
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 55mph (89km/h) right behind a Freightliner tractor trailer. As they got closer their results ranged from a baseline (no truck) figure of 32 to 35.5mpg-US (7.4 to 6.63L/100km, a 11% greater distance covered per unit volume of fuel or a 10% reduction in fuel consumption) at 100feet (30m), and then progressively up to 44.5mpg (5.29L/100km, a 39% greater distance covered per unit volume of fuel or a 28% reduction in fuel consumption) at 10feet (3m), as a result of decreased drag consequent of drafting
Geoff Sundstrom, director of AAA Public Affairs, notes that “saving fuel and conserving energy are important, but so is safety, and preventing crashes.” In Ontario, optimal highway speed for fuel-efficiency often lies between the legal minimum speed and the legal speed limit, typically 80 to 100km/h. However, these legal speeds may actually be slower than average traffic speed. 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 recognized risk.
Drafting a big rig at close distances is life-threatening and extremely dangerous. They recommended a minimum safe driving distance of at least 150feet (46m) from a big rig.
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. While one function of the driving laws is to help increase safety, the attendant safety issues are not always clear cut, and often neither are the laws. Coasting in neutral is illegal.
In addition, 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.