why do new cars turn off when stopped

[DiJoanna]

New cars turn off when stopped primarily to save fuel and reduce emissions. This feature, known as an auto start-stop system, automatically shuts down the engine when the vehicle comes to a complete halt (like at a red light) and restarts it instantly when you release the brake pedal. It's a direct response to stringent fuel economy and emissions regulations. While it can slightly improve your miles per gallon (MPG) in city driving, the frequent restarts can feel unusual at first.

The core benefit is fuel savings, especially in stop-and-go traffic. By eliminating idling, the system conserves gasoline that would otherwise be wasted. The fuel required to restart the engine is less than what is used during even a brief 10-second idle. This translates directly into lower carbon dioxide (CO2) emissions, helping manufacturers meet environmental standards.

The system relies on a robust starter motor, a reinforced battery (often an Absorbent Glass Mat or AGM battery), and a plethora of sensors. These sensors monitor parameters like battery voltage, engine temperature, and climate control settings. The system will typically not activate if the engine is cold, the cabin hasn't reached the desired temperature, or the battery charge is low, ensuring driver comfort and vehicle reliability aren't compromised.

You can almost always disable the auto start-stop system with a button on the dashboard. However, it resets to "on" each time you start the car. For those who find the sensation bothersome, aftermarket devices are available to permanently remember the last setting. It's a trade-off: a minor, momentary vibration for a measurable reduction in fuel costs and environmental impact.

Vehicle Segment	Typical Fuel Savings (City Driving)	Average Annual Fuel Cost Savings*	CO2 Reduction per Year
Compact Sedan	3-5%	$45 - $75	130 - 220 lbs
Midsize SUV	4-7%	$70 - $120	200 - 340 lbs
Full-size Truck	5-8%	$90 - $150	260 - 430 lbs
Luxury Sedan	3-6%	$60 - $110	170 - 310 lbs
Hybrid Vehicle	8-10%+	$100 - $150+	290 - 430 lbs+
*Based on 15,000 miles annually and an average gas price of $3.50/gallon.

[LeNadia]

Honestly, it freaked me out the first time my new car did it. I thought I’d stalled! But my mechanic buddy explained it’s just the computer saving gas. Now I’m used to it. You feel a little shudder when it kicks back on, but you learn to ease off the brake smoother. I notice I’m filling up the tank less often, especially with all the errand running I do around town. It’s a smart little feature, even if it feels weird initially.

[DamianDella]

From an engineering standpoint, it's a brilliant solution to a simple waste problem. Idling an engine at a light produces emissions and burns fuel for zero distance traveled. The start-stop system uses a heavy-duty starter designed for thousands of cycles, not just a few a day. It's seamlessly integrated with the vehicle's network. The slight delay on restart is a calculated trade-off for the collective benefit of reduced urban smog and better corporate average fuel economy (CAFE) numbers for the manufacturer.

[DylanRose]

Let's talk dollars and cents. That system saves you real money. If you do a lot of city driving, sitting in traffic or at lights, you could be idling for hours each month. That's gas literally evaporating. The system cuts that waste. While the savings per trip are small, over a year it adds up to a few free tanks of gas. It also puts less wear on your engine than idling for long periods. Think of it as a tiny, automated efficiency expert under your hood.

[DiMia]

I get the environmental reason, but I'm not a fan. I worry about the extra wear on the starter and battery, even if they say it's heavy-duty. The constant on-and-off is jarring, and sometimes there's a lag when I need to go quickly, like making a left turn. I just push the "off" button every time I get in the car. For me, the smoothness and immediate power are worth more than the little bit of gas I might save. It's a personal preference thing.