Although you might think it’s easier on your car to let it sit and gently warm up, doing so is a bad idea for a number of reasons. Most importantly, it does indeed waste gas.

The vast majority of cars on the road today use electronic fuel injection. When your car’s engine is cold, the computer tells the fuel injectors to stay open longer, allowing more fuel into the engine to help it run cold. As the engine warms up, the injectors let in less fuel and everything returns to normal, so to speak. 

The problem is, letting your car sit and idle is the slowest way to bring it up to operating temperature because it’s generally sitting in your drive at just above idle speed. And this method to warm up also invites other problems. Remember that modern cars are equipped with a multitude of devices to help them run clean, including a catalytic converter (sometimes three of them), a device in the exhaust system that works to burn off unburned hydrocarbons in the exhaust stream. A cold engine emits a far higher percentage of unburned hydrocarbons than a warm engine. Unfortunately, the average catalytic converter can’t process 100 percent of unburned hydrocarbons even in the best of times. Importantly, the catalytic converter needs high exhaust temperatures to work properly. Throw in a cold engine emitting a high percentage of unburned hydrocarbons, repeat several hundred times, and you can end up with what’s called a “plugged” converter. In a nutshell, the converter becomes overwhelmed and literally ceases to function. This won’t happen all at once but over time, the end effect is the same: poor mileage and significantly dirtier exhaust.

The best bet? Even when it’s 10 F outside, start your car, let it run for 30 to 60 seconds to get all the fluids moving, then drive off gently. Your engine will warm up faster, your exhaust system will get up to temperature faster so the catalytic converter can do its thing, and you’ll use less fuel. Which is what you wanted all along anyhow, right?