While up to 95% of the efficiency limits at city speeds are intrinsic to the construction of the vehicle, 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 (or "
spoilers", when designed for downforce and not enhanced flow separation), 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. Optimal efficiency can be expected while cruising at a steady speed and with the transmission in the highest gear (see Choice of gear, below). 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 . '' 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. 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 (
kinetic energy and
gravitational potential 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 (via coasting) 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 (or accumulated) fuel-mileage efficiency.
Anticipating traffic A driver may improve their fuel efficiency by anticipating the movement of other vehicles or sudden changes to the situation the driver is currently in. For example, a driver who stops quickly, or turns without signaling, reduces the options another driver has for maximizing their 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 (as well as increase their safety). Similarly, anticipation of road features such as traffic lights can reduce the need for excessive braking and acceleration. Drivers should also anticipate the behaviour of pedestrians or animals in the vicinity, so they can react to a developing situation involving them appropriately.
Minimizing ancillary losses Using air conditioning requires the generation of up to 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. Switching off the A/C and rolling down the windows may prevent this loss of energy, though it will increase drag, so that cost savings may be less than is generally anticipated. Using the passenger heating system slows the rise to
operating temperature for the engine. Either the
choke in a carburetor-equipped car (1970's or earlier) or the fuel injection computer in modern vehicles will add more fuel to the fuel-air mixture until normal operating temperature is reached, decreasing fuel efficiency. This driving strategy has been found and experienced by drivers to save fuel for a long time, and some experiments also validated its fuel-saving ability. In the PnG operation, 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. with up to 20% fuel saving. In the PnG strategy, the control of the engine and the transmission determines the fuel-saving performance, and it is obtained by solving an optimal control problem (OCP). Due to a discrete gear ratio, strong nonlinear engine fuel characteristics, and different dynamics in the pulse/glide mode, the OCP is a switching nonlinear mixed-integer problem. Some hybrid vehicles are well-suited to performing pulse and glide. In a series-parallel hybrid (see
hybrid vehicle drivetrain), the
internal combustion engine and charging system can be shut off for the glide by simply manipulating the accelerator. However, based on simulation, more gains in economy are obtained in non-hybrid vehicles. The efficiency of a combustion engine in a hybrid vehicle can be determined using its
consumption map, battery capacity, battery level, load, and gear ratio, and it also depends on acceleration, wind drag, and speed.
Causes of pulse-and-glide energy saving Much of the time, automobile engines operate at only a fraction of their maximal efficiency, resulting in lower fuel efficiency (or higher specific fuel consumption (SFC), which is the same thing). Charts that show the SFC for every feasible combination of torque (or Brake Mean Effective Pressure) and RPM are called
Brake specific fuel consumption maps. Using such a map, one can find the efficiency of the engine at various combinations of
rpm,
torque, etc.
Drafting or slipstreaming Drafting or slipstreaming is a technique whereby a smaller vehicle drives or coasts close behind a vehicle ahead of it so that it is shielded from wind. Aside from being illegal in many jurisdictions, it is often dangerous. Real-world tests of a car driving ten feet behind a semi-truck showed a 90 percent reduction of aerodynamic drag (wind force) and as a result, a 39 percent increase in efficiency.
Safety There is sometimes a tradeoff between saving fuel and preventing crashes. == Hypermiling ==