All 3 of our cars have automatic transmissions and tachometers. When traveling (say) 55 mph and I see a light turn red up ahead I immediately take my foot off the gas and start to coast. I notice that the rpm’s continue to hover around 2000 even tho’ my foots off the gas. Am I using the same amount of gas decelerating at 2000 rpms as when maintaining speed at 2000 rpms?
You’re saving gas. Many cars shut of the fuel in the coasting situation you describe.
No, You closed the throttle to minimum air which reduces the fuel use. The engine is often “locked” in overdrive until your speed drops below 35. Then normal down shifts will drop you to idle speed as you come to a stop.
When the car is pulling the engine at an above idle speed with your foot off the throttle, modern cars shut the fuel off, so you are using no gas at all until the engine drops to a speed at which the injectors start firing again.
I’m glad you asked! A lot of people write in with boneheaded “hypermiling” schemes like coasting in neutral or with the engine shut off, both of which are bad ideas.
One of the best ways to economize is to get off the gas when closing on a stop situation. Yes, get that last eighth of a mile cheaply.
You are doing the right thing. The throttle is closed regardless of engine speed. Most modern cars shut off the fuel injectors when coasting, so you’re using ZERO gas.
If you do this far enough in advance, you can coast through the cycle time on the light. If you don’t have to stop, you don’t have as much speed to gain when you pass through the light. That saves fuel, too.
Thank you! Good to know.