Idling for More Than Ten Seconds Uses More Fuel Than Restarting

Restarting a car engine uses about as much fuel as ten seconds of idling. That figure, attributed to a 2000 Canadian engine study, undercuts one of the most durable beliefs in driving: that it is cheaper to leave the engine running than to switch it off and start again. It is the reason recommendations have been made to shut the engine off after just ten seconds of sitting still.

Engine idle: what the engine is actually doing

Idling means running the engine while the vehicle is not moving, or while the engine sits at its resting speed. It happens at red lights, outside a shop or a house, or anywhere else a driver waits with the engine on. At idle the engine runs with no load except its own accessories, and it receives no extra fuel through the accelerator pedal. If a car does move while in gear and idling, the idle speed is set wrongly and needs adjusting.

Idle speed is how fast the engine turns once it is disconnected from the drivetrain and the throttle is untouched. In a combustion engine it is measured in revolutions per minute (rpm) of the crankshaft. Think of it as the slowest speed at which the engine can sustain itself. It makes enough power to run smoothly and to drive ancillaries such as the water pump, the alternator and, in many cars, the power steering, but usually not enough to move the car, unless the idle is set too high. Its opposite is redline, the top speed the engine can reach before serious damage becomes a risk.

For a passenger car, idle is customarily between 600 and 1000 rpm. For medium and heavy trucks it is about 600 rpm. Accessories change the picture. If the engine is operating many of them, air conditioning above all, the idle speed has to rise so the engine still runs smoothly, and most air-conditioned cars handle that automatically through the carburettor or fuel injection system, which raises the idle whenever the compressor runs. Engines modified for power at high revs, such as racing engines, tend to idle roughly unless the idle speed is raised significantly. And in a car with an automatic transmission, the term can also refer to its idle creep, the gentle movement when you release the brake.

A restart costs about as much fuel as ten seconds of idling.

Is idling bad for your car, and does it waste fuel?

Most of the effort to reduce idling is driven by fuel economy and emissions, although some engines can also be damaged if they idle for long periods. The belief that stopping and restarting costs more than idling is, according to the Environmental Defense Fund citing the Canadian report, mistaken. A restart uses about the same fuel as ten seconds of idling, so the logic of switching off holds for anything longer than that.

The pollution side is just as clear. Running an engine and idling it both produce several pollutants that the US Environmental Protection Agency monitors. The health effects relate to engine exhaust. Short-term effects include irritation of the eyes, throat and bronchial tubes, nausea, cough, phlegm congestion, allergic or asthma-like reactions and a greater risk of cardiac events, as well as cancer. Chronic effects include bronchitis, decreased lung function and damage to reproductive function, such as low birth weight and damage to sperm chromatin and DNA.

The harm is not spread evenly. People with heart disease, asthma or other lung problems suffer more. Children are more susceptible because they breathe faster and their respiratory systems are still developing, and the elderly are disproportionately affected because they have limited physiological reserve to compensate.

How to cut idling: start-stop, hybrids and truck units

The heaviest idlers are commercial. In the United States, idling heavy-duty truck and locomotive engines burn about a billion gallons (3.8 billion litres) of fuel a year. Many newer semi-trucks now carry a small auxiliary power unit (APU) that runs the accessories more efficiently while the truck is parked.

For cars, the answer is the hybrid. Hybrid vehicles typically shut their combustion engines down when stopped, and the larger batteries they carry can power the car's electronics, which lets the engine switch off and avoid prolonged idling. Some conventional vehicles now have start-stop systems that cut the engine when it would otherwise idle. At a wider level, governments can reduce reliance on motorised transport by investing in public transport and encouraging transit-oriented development.

Anti-idling laws: where idling is an offence

Rules differ widely in how strictly they are written.

Toronto's bylaw led the way in Canada. The city's health department plans to ask for a limit of one minute and no exemptions for the city fleet, including Toronto Transit Commission buses. Other municipalities followed, among them Vancouver in 2006, Hamilton in 2007, Red Deer and Ajax in 2009, and Mississauga, Oakville, Waterloo, Ottawa and Vaughan in Ontario. Sudbury ran a campaign without a law in 2010.

Hong Kong's Motor Vehicle Idling (Fixed Penalty) Ordinance lets both police traffic wardens and Environmental Protection Department inspectors fine offenders. In Belgium the rule reads that idling is forbidden unless there is a specific reason, and that article was due to be replaced by a new one from September 2026.

In the UK, Rule 123 of the Highway Code says a driver must not leave a parked vehicle unattended with the engine running, nor leave the engine running unnecessarily while stationary on a public road. It advises that a driver likely to be stopped for more than a couple of minutes should apply the parking brake and switch the engine off to reduce emissions and noise pollution. The engine may be left running when the car is stuck in traffic or when a fault is being diagnosed. Stationary idling is also an official offence under Regulation 98 of the 1986 Construction and Use Regulations and Section 42 of the Road Traffic Act 1988.

In the United States, the Department of Energy and the Environmental Protection Agency both run programmes. The Department of Energy funds research on alternative vehicles, including quantitative data on medium-duty trucks and a CoolCab project on curtains for semi-trucks. The EPA's work includes the SmartWay Transport Partnership, which offers freight incentives, the Model State Idling Law for diesel and Clean School Bus USA. Colorado offers a tax credit for alternative fuel and idle reduction technology and a Green Truck Grant Program reimbursing up to 25% of costs for owners of commercial trucks in interstate commerce. Many local ordinances limit the minutes per hour a vehicle may idle, and Denver's Engines Off! campaign relies on voluntary change in behaviour.

How fuel efficiency is measured

Fuel efficiency, or fuel economy, is a form of thermal efficiency: the ratio of result to effort when a fuel's chemical energy is turned into kinetic energy or work. In transport, it is the distance a vehicle covers per unit of fuel burned, and it depends on engine efficiency, transmission design and tyre design among other things.

Most countries that use the metric system state it as fuel consumption in litres per 100 kilometres (L/100 km) or as kilometres per litre. Others, including the US and usually the UK, use miles per gallon (mpg). That causes confusion, because the imperial gallon is 20% larger than the US gallon, so mpg values from the two systems cannot be compared directly. Norway and Sweden once used litres per mil, but both have lined up with the EU standard of L/100 km.

Fuel consumption in litres per 100 km is the more accurate measure of performance, because it has a linear relationship with fuel used, whereas fuel economy figures distort improvements in efficiency. Freight can be rated by weight-specific efficiency, and passenger vehicles by passenger-specific efficiency.

How to save fuel: what a car's efficiency depends on

A vehicle's fuel efficiency depends on many parameters: the engine, aerodynamic drag, weight, air conditioning use, the fuel and rolling resistance. Advances have been made in all of these in recent decades, and careful maintenance and driving habits improve it further. Air conditioning ties back to idling too, since the idle speed is raised whenever it runs.

Hybrids attack the problem from the other end. They combine a small combustion engine with electric motors, and recapture braking energy, which would otherwise be lost as heat, as electrical power. Fleet-wide, though, technological gains can be cancelled out if buyers shift to heavier, less efficient vehicles.

The fuel itself matters. In 2004 a group of carmakers, BMW, General Motors, Honda, Toyota and Volkswagen/Audi, came up with the Top Tier Detergent Gasoline Standard for brands in the US and Canada, setting minimum detergent content and banning metallic additives. Top Tier petrol has more detergent to prevent deposits, typically on the fuel injectors and intake valves, that reduce fuel economy and engine performance.

How much energy is in a litre of fuel?

The energy content of a fuel is the heat obtained by burning a given quantity, and there are two values for the same batch. The high, or gross, value counts the water in the exhaust as liquid. The low, or net, value counts it as vapour. Because vapour gives up heat when it condenses, the high figure is larger, by about 8 or 9%, and that gap accounts for most of the apparent discrepancy in published numbers for petrol. The US has traditionally used the high values, while many other countries use the low ones.

Neither figure says how much useful work a fuel can do. The theoretical maximum, given by the change in Gibbs free energy, is around 45.7 MJ per kilogram for petrol. In practice, the work obtained depends on the engine: 17.6 MJ/kg is possible with a petrol engine and 19.1 MJ/kg with a diesel.

Electric motors are the most efficient machines for turning energy into rotary motion, but electricity is not a primary energy source, so the efficiency of generating it has to be counted too, and the pollution is emitted at a distant power station instead of on the road. Hydrogen is a similar story. Toyota has been test-marketing fuel-cell vehicles in southern California, and such cars promise near-zero tailpipe pollution if the hydrogen is made from sources such as solar, wind, hydroelectric or nuclear power. Commercial hydrogen production today uses fossil fuels and produces more carbon dioxide than hydrogen, so zero pollution describes only the stage where the car turns stored energy into movement.

Where this goes next

Some rules are scheduled to change. Belgium's idling article was due to be replaced from September 2026, and Toronto's health department plans to seek a one-minute limit with no exemptions. Some conventional vehicles now include start-stop systems, and hybrids are already designed to avoid prolonged idling. Hydrogen cars, which Toyota is test-marketing in southern California, may become commercially available in future.