Energy-efficient driving
Energy-efficient driving techniques are used by drivers who wish to reduce their fuel consumption, and thus maximize fuel efficiency. Many drivers have the potential to improve their fuel efficiency significantly. Simple things such as keeping tires properly inflated, having a vehicle well-maintained and avoiding idling can dramatically improve fuel efficiency. Careful use of acceleration and deceleration and especially limiting use of high speeds helps efficiency. The use of multiple such techniques is called "hypermiling".
While these techniques can be applied by any driver, energy-efficient driving has become a major focus of modern fleet management. As a key part of fleet digitalization, companies use telematics to automatically monitor and manage fuel economy. A fleet telematics system collects data on behaviors that waste fuel, such as harsh acceleration, speeding, and idling. This information is then used in driver scoring applications to identify and coach drivers. This is often combined with dedicated fuel-management systems that use high-precision fuel level sensors to get exact fuel consumption data and prevent gasoline theft.
Simple fuel-efficiency techniques can result in reduction in fuel consumption without resorting to radical fuel-saving techniques that can be unlawful and dangerous, such as tailgating larger vehicles.
Cause of energy losses
Most of the fuel energy loss in cars occurs in the thermodynamic losses of the engine. Specifically, for driving at an average of, approximately 33% of the energy goes into exhaust and 29% is used to cool the engine; engine friction takes another 11%. The remaining 21% is split between rolling friction of tires, air drag, and braking. Since no miles are gained while idling, or when the engine is in standby, efficiency increases when shutting off the engine when the car is stopped.Techniques
While up to 95% of the efficiency limits at city speeds are intrinsic to the construction of the vehicle, wide variety of techniques contribute to energy-efficient driving.Maintenance
Underinflated tires wear out faster and lose energy to rolling resistance because of tire deformation. The loss for a car is approximately 1.0 percent for every drop in pressure of all four tires. Improper wheel alignment and high engine oil kinematic viscosity also reduce fuel efficiency.Mass and improving aerodynamics
Drivers can increase fuel efficiency by minimizing transported mass, i.e. the number of people or the amount of cargo, tools, and equipment carried in the vehicle. Removing common unnecessary accessories such as roof racks, brush guards, wind deflectors, running boards, and push bars, as well as using narrower and lower profile tires will improve fuel efficiency by reducing weight, aerodynamic drag, and rolling resistance. Some cars also use a half size spare tire, for weight/cost/space saving purposes. On a typical vehicle, every extra 55 pounds increases fuel consumption by 1 percent. Removing roof racks can increase fuel efficiency by up to 20 percent. Reducing on-board fuel to a lower value can also benefit fuel reduction in a town traffic setting.Maintaining an efficient speed
Maintaining an efficient speed is an important factor in fuel efficiency. Optimal efficiency can be expected while cruising at a steady speed and with the transmission in the highest gear. The optimal speed varies with the type of vehicle, although it is usually reported to be between. For instance, a 2004 Chevrolet Impala had an optimum at, and was within 15 percent of that from.At higher speeds, wind resistance plays an increasing role in reducing fuel economy in automobiles. At 60km/h, the global average speed, energy loss due to air drag in fossil fuel cars is approximately 5% of the total energy loss. Friction, exhaust, and cooling the engine account for the rest. Above 60km/h, wind resistance grows with approximately the square of speed, becoming the dominant factor at high speed.
Hybrids typically get their best fuel efficiency below this model-dependent threshold speed. The car will automatically switch between either battery powered mode or engine power with battery recharge. Electric cars, such as the Tesla Model S, may go up to at.
Road capacity affects speed and therefore fuel efficiency as well. Studies have shown speeds just above allow greatest throughput when roads are congested. Individual drivers can improve their fuel efficiency and that of others by avoiding roads and times where traffic slows to below. Communities can improve fuel efficiency by adopting speed limits or policies to prevent or discourage drivers from entering traffic that is approaching the point where speeds are slowed below. Congestion pricing is based on this principle; it raises the price of road access at times of higher usage, to prevent cars from entering traffic and lowering speeds below efficient levels.
Research has shown that mandated speed limits can be modified to improve energy efficiency anywhere from 2 to 18 percent, depending on compliance with lower speed limits.
Choice of gear (manual transmissions)
Engine efficiency varies with speed and torque. 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 powerband. For a turbo diesel 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. 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 1500 rpm for gasoline engines, and 1200 rpm for diesel engines. This explains the usefulness of very high "overdrive" gears for highway cruising. For instance, a small car might need only to cruise at. It is likely to be geared for 2500 rpm or so at that speed, yet for maximum efficiency the engine should be running at about 1500 rpm or 1200 rpm to generate that power as efficiently as possible for that engine.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 to a greater than necessary speed without paying attention to what is ahead may require braking and then after that, additional acceleration. One study from 2001 recommended accelerating briskly, but smoothly before shifting in manual cars.Generally, fuel efficiency 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 is sometimes caused by unpredictable events. At higher speeds, there is less time to allow vehicles to slow down by coasting. Kinetic energy is higher, so more energy is lost in braking. At medium speeds, the driver has more time to choose whether to accelerate, coast or decelerate in order to maximize overall fuel efficiency.
While approaching a red signal, drivers may choose to "time a traffic light" by easing off the throttle before the signal. By allowing their vehicle to slow down early and coast, they will give time for the light to turn green before they arrive, preventing energy loss from having to stop.
Due to stop and go traffic, driving during rush hours is fuel inefficient and produces more toxic fumes.
Conventional brakes dissipate kinetic energy as heat, which is irrecoverable. Regenerative braking, used by hybrid/electric vehicles, recovers about 50% of the car's energy in each braking event, leading to perhaps 20% reduction in energy costs of city driving.
Coasting or gliding
An alternative to acceleration or braking is coasting, i.e. gliding along without propulsion. Coasting dissipates stored energy against aerodynamic drag and rolling resistance which must always be overcome by the vehicle during travel. If coasting uphill, stored energy is also expended by grade resistance, but this energy is not dissipated since it becomes stored as gravitational potential energy which might be used later on. Using stored energy for these purposes is more efficient than dissipating it in friction braking.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.
Coasting with a vehicle not in gear is prohibited by law in most U.S. states, mostly if on downhill. An example is Maine Revised Statutes Title 29-A, Chapter 19, §2064 "An operator, when traveling on a downgrade, may not coast with the gears of the vehicle in neutral". Some regulations differ between commercial vehicles not to disengage the clutch for a downgrade, and passenger vehicles to set the transmission to neutral. These regulations point on how drivers operate a vehicle. Not using the engine on longer, precipitous downgrade roads, or excessively using the brake might cause a failure due to overheating brakes.
Turning the engine off instead of idling does save fuel. Traffic lights are predictable, and it is often possible to anticipate when a light will turn green. Some cars accomplish this with a start-stop system, turning the engine off and on automatically during a stop. Some traffic lights have timers on them, which assist the driver in using this tactic.
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 fuel-mileage efficiency.