The short answer
The US Department of Energy’s fueleconomy.gov says warming up the car before a trip lowers your fuel economy, and that most manufacturers recommend you drive off gently after about 30 seconds. The Utah Division of Air Quality gives the same figure: about 30 seconds of idling is enough to get oil moving through the engine. After that, driving warms the engine faster than idling, so the heater starts working sooner too.
Why idling does not warm the car
Idling only warms the engine, and slowly. The rest of the car does not warm up at all while you stand still: wheel bearings, tires and suspension only reach working temperature once the car moves. That is why the advice is to drive gently for the first few miles, without hard acceleration or high revs, rather than to idle longer.
What idling costs
- An idling car gets 0 miles per gallon.
- Idling uses about a quarter to half a gallon of fuel per hour, depending on the engine and what is switched on, according to the Department of Energy as cited by Kelley Blue Book.
- The Utah Division of Air Quality says two minutes of idling uses about as much fuel as driving one mile, and that idling for more than 10 seconds can use more fuel than switching the engine off and restarting it.
- Restarting does little harm to a modern starter and battery: the same agency puts the extra wear at about $10 a year.
Ten minutes of idling every winter morning adds up to real money over a season. Cold weather already cuts fuel economy on short trips; our tip on why mpg drops in winter shows by how much.
Better ways to start warm
- Park in a garage if you have one. fueleconomy.gov notes that a warmer place raises the starting temperature of both engine and cabin.
- Use a block heater in very cold places. The Utah agency says an engine block heater improves fuel efficiency and cuts emissions in cold climates.
- Clear all the snow and ice from the windows, mirrors, lights and roof while the engine runs its first 30 seconds. Seeing out matters more than a warm cabin.
- Electric car? Preheat the cabin while the car is still plugged in, and use the seat heaters instead of the cabin heater. fueleconomy.gov says cabin heating accounts for about two thirds of the extra energy an EV uses in the cold.
The safety rule: never in a closed garage
Carbon monoxide kills. The CDC says never run a car or truck inside a garage attached to your house, even with the garage door open. Back the car out right away instead of letting it idle inside. If you are ever stuck in snow, NHTSA says to clear snow from the exhaust pipe and run the engine only now and then, just long enough to stay warm.
When a cold start is a warning
NHTSA notes that cold weather reduces battery power while the engine needs more power to start, so a slow crank on a frosty morning deserves attention. It recommends having the battery and charging system checked before winter. Our guide on car batteries in cold weather explains what to look for. Badger’s fuel log measures your real fuel economy between full tanks, so you can see how much a winter of short trips really costs.
