Lesson 5: Coasting
As easy as it is for you to keep your car moving at a constant speed with just the lightest touch of the gas, it is still possible to use even less fuel than that while moving forward, and in some cases no fuel at all! Once again, Isaac Newton’s law of inertia indicates that an object in motion will remain in motion unless an outside force acts upon it. What does that mean to you? How about a nearly free ride?
You may have heard about how hybrid cars recharge their batteries when coasting to a stop. The motors that drive the wheels under accelerating and cruising conditions become generators under coasting conditions. They take the energy that you’ve built up accelerating and maintained while cruising and recover some of it when you’re decelerating. That’s really clever and it saves hybrid drivers money, but you don’t need to actively generate energy while decelerating to save money. The whole point of your car is to move you forward and what better way is there to do that than to capitalize on your car’s momentum by gliding forward without using your accelerator at all?
A typical sedan could potentially achieve fuel economy ratings of 2L/100km (118mpg) while coasting and, in some cases, even 0L/100km but we’ll get to that later.
In previous lessons, I’ve emphasized how important it is to avoid accelerating. Naturally, that also means that you must avoid decelerating because each time you slow down you must speed up again, unless you’ve come to the end of your trip. In practical driving, though, it’s impossible to avoid stopping. Stop signs, red lights, cross walks, school zones, etc. help us all share the road but they also cost drivers money by creating a stop-and-go driving environment that is not conducive to fuel economy. Fortunately, ecomiling can help you improve your fuel economy and limit the amount of stopping you do in spite of everything out there on the road.
Instead of stopping you should be coasting. You want to minimize the time you spend idling because when your car is idling your fuel economy is as bad as it’s going to get (infinite L/100km or 0mpg), but coasting is much more than that because it doesn’t just minimize idling, it uses your existing momentum to move you forward for free, or at least for money you’ve already spent during acceleration. By combining what you’ve learned in the previous lessons with coasting, you can achieve exceptional fuel economy. The key to coasting, as with cruising, is to maintain a gap between your car and the one ahead of you and to keep on top of traffic and signals.
Say you’re cruising and up ahead you see a red light. Release the gas pedal and coast into the red. In a best-case scenario, that red light will turn green while you casually coast in and you will be able to accelerate back up to cruising speed having preserved a tremendous amount of momentum. In a worst-case scenario, you will be forced to stop but at least you will have coasted into the red light consuming very little fuel in the process and minimizing your idle time. How often do you see drivers stay on the accelerator until they’re 50 feet away from the light and then brake rapidly? Do you do this? You end up at the same light anyway, so why not coast in and do it for free?
With proper practice, you will be able to recognize situations in which you can use coasting to reduce your speed or come to a stop. Pay attention to traffic lights in particular. Is the green light ahead stale (ready to turn yellow – pay attention to walk signals)? If so, start coasting. Is the car ahead of you indicating that it plans to turn right? Start coasting. Are you transitioning from a road bearing a higher speed limit to a reduced one? Start coasting. When it appears that you are going to have to stop, you should coast for as long as you can and avoid the use of your brakes as much as possible.
Coasting in particular tends to offend some drivers who feel that you are simply not moving fast enough. Do not let them influence you into wasting gas. Such aggressive drivers will often change lanes around you and fill the gap between your car and the one in front of you. Plan for this reduced space and maximize your coasting to minimize your acceleration and idling.
Ideally, if you employ good coasting technique, you won’t have to stop at all! It’s important to emphasize that the goal of coasting is not to come to a stop economically but to prevent stopping and preserve as much momentum as possible for as long as possible so that your obstruction can clear and you can resume cruising with a minimum of acceleration. Stopping is the worst case scenario and coasting to a stop only makes the best of a bad situation. That means you need to pay attention to the road and plan your driving with fuel economy in mind. Practice makes perfect and, as is the case with cruising, practicing will start to save you money immediately so get on it!
Coasting in a modern car has some pretty interesting benefits as well, even if the car isn’t a hybrid. Most modern fuel injected engines enter what’s called deceleration fuel cutoff (DFCO) under certain circumstances in which fuel to the engine is cut off and the vehicle’s momentum is transferred to the engine through the gearbox, causing the engine to continue to turn without consuming any fuel. That’s 0L/100km (infinite mpg). Whether your vehicle has this mode and under what circumstances it kicks in will vary from car to car and it’s hard to know exactly whether you’re in DFCO mode without using a ScanGaugeII. In my 2.3L Mazda3 hatch, for example, DFCO is active when I am in gear, coasting and my engine is turning in excess of approximately 1000RPM.
It really pays to make an effort to maximize the amount of time you spend coasting in DFCO mode. If you have a ScanGaugeII and a manual or manumatic transmission, you might even want to experiment with downshifting while coasting in order to keep your RPM level above the DFCO mode threshold and ensure that you keep using absolutely no fuel as you coast. I often downshift into third and then even second if I am coming off a hill or slowing down from freeway speeds and entering a red light. I can use my momentum and gearing to force my engine to remain in DFCO mode almost until I stop.
Some people will recommend coasting in neutral and/or even turning off the vehicle’s engine when coasting to a stop but these maneuvers can be dangerous. You never know when you might need to suddenly accelerate in order to avoid an emergency so it is recommended that you never follow these practices. You’re better off attempting to safely and economically maximize the scenarios under which you are in DFCO, as indicated above, and then just using the minor amount of fuel that your vehicle consumes while coasting to a stop without DFCO.
As you practice, you will undoubtedly come upon circumstances in which you must brake quickly, either to avoid accidents or because a light has changed suddenly or for any number of similar reasons. In such circumstances, the ideal course of action is to brake as hard as you safely can without stopping. You want to scrub as much speed as you can and then coast for as long as you can before you stop. Coasting at slow speed is better than idling and, with any luck, the reason you had to stop will clear out as you crawl forward, leaving you able to salvage at least some momentum and resume cruising. In other cases, you will at least have eked out minor savings by minimizing the time you spend idling.
Takeaways from Lesson 5:
- Coasting consumes very little fuel, and in some cases none at all.
- Your vehicle’s fuel economy is as bad as it can possibly be while idling – coast to minimize idling.
- As much as possible avoid the use of your brakes for stopping – coast instead.
- Leave a large gap between your car and the one in front of you – this gives you room to cruise and coast.
- Use your awareness of the road in combination with coasting to time traffic and traffic lights – you want to avoid stopping as much as possible.
- Do not be pressured into hitting the accelerator during a coast or reducing the duration of a coast by aggressive drivers – allow them to pass and waste all the gas they want.
- Find out whether your vehicle has a DFCO mode and make an effort to safely maximize the time you spend in it for zero fuel consumption.
Coasting, cruising and accelerating are the three fundamental lessons of ecomiling. They are the three pillars on which ecomiling is built and by implementing and practicing the techniques of the previous three lessons you can save yourself a bundle on gas. But there’s still more to learn! Move on to Lesson 6.
Lesson Quicklinks
Lesson 1: Get Started by Doing the ObviousDisclaimer: It is of critical importance that you always place safety first and fuel economy a very distant second. Although ecomiling may be an engaging, exciting and money-saving challenge, never place fuel economy above your safety or that of other road users. Obey all local traffic laws.