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How is hard braking less fuel efficient than gradual braking?

I understand the idea behind leisurely acceleration. But how can braking effect your fuel mileage? If two cars are both approaching a stop sign, and one slows down gradually and comes to a stop at the stop sign and the other one brakes late and comes to screeching halt, how can that possibly effect mileage. Both cars are going to have stop regardless. Let’s say both cars let off the gas at the same point approaching the stop sign, but one gently uses the brakes to come a soft stop. The other lets the car’s momentum car him/her to within 100 feet of the stop sign and then brakes firmly. I can’t see any reason why there would be a difference in fuel consumption



Oops, I meant to put this in the General Discussion section. Sorry about that.

CHANGED ANSWER.
Using your supposition that both car drivers let off the gas at the same time, and that both cars are fuel injected, and that fuel injection is cut off (or, nearly so) when the throttle plate goes to the idle position, neither car would use any gasoline (fuel) from that point until a complete stop. One car could brake early, and easily, up to the stop point. The other car would coast (throttle plate still at idle position) until closer to the stop point, and then brake heavily to the stop point.
This scenario sounds like a win-win; but, heavy brakers don’t coast. Observe real traffic, for yourself.

Late, hard braking is often accompanied by other poor driving habits like hard acceleration, excessive speed, etc. Also, if one is braking later, they are generally accelerating longer, ie, accelerating until the last minute up to the stop sign/light. Such driving habits will also result in overall higher operating and repair costs for the vehicle.

To answer the original question, in a perfectly controlled environment, there would probably be no difference in mpg as long as both cars started coasting at the same time and all other factors were identical between them. As I stated previously, bad driving habits often coexist, and most people who wait till the last second to start slowing down don’t coast up to it, but keep their foot on the gas till it goes firmly on the brake. That’s where the drop-off in mpg comes from.

“Let’s say both cars let off the gas at the same point”

After that the engine is out of the equation. Both vehicles start with the same amount of kinetic energy. All of it gets turned into heat in both cases.

The brakes by themselves make no difference in fuel consumption. Getting off the gas pedal sooner is what saves fuel. A really efficient vehicle like a train might coast for miles before it applies brakes to stop.

Braking gently means you have to stop less often, which means less fuel used to accelerate.

Imagine you are approaching a “stale” red light in a 45 MPH zone. You can either brake gently from far away or wait until you get close to the light. If you wait until you are close to the light, the light might turn green before you get to it, but the chances of that are small. If you gently slow to 30 MPH as soon as you see the stale red light, the chances of it turning green before you get to it go up. So slowing down gently increases the odds that it will turn green before you get to it.

There are far too many variables to give a single answer.

I will make one suggestion. The more work the brakes do, whether it is a lot over a short time or a little over a longer time, the more energy is lost and the lower overall efficiency and lower mileage.

I think the answer is TIME.

Lets say you plan on stopping at a light 1000’ away.

  1. Wait until the past 150’ and then brake hard.
  2. Let off the gas and start lightly braking at 600’.

Scenario #1 for that 450’ (difference between #2 and #1) you have the gas pedal down and using gas.
#2…for that same 450’ you’re NOT using any gas.

Do that 20-30 times a day and it can really add up.

Can’t say if there is a difference in fuel mileage but I do have the firm opinion that your brake pads/linings will last longer if you brake gently for a long time as opposed to braking strongly for a short time. Braking gently has to be a positive for tire wear too.

Assuming you accelerate to the same speed in both your examples and then just brake later I’d expect precious little difference in fuel economy. You’re right, both cars are going to stop regardless.

The difference comes in when you AREN’T required to stop. Say for example that’s a stop light rather than a stop sign. Zoom on up and you’ll have to come to a full stop. Hit the brakes and slow down half a block away and you may not have to stop at all since the light may change by the time you get to it. It’s starting from a dead stop that really kills fuel mileage. Every time you can avoid a full stop you’re improving economy.

If both cars are coming to a stop sign, not a light, and
both get off the gas at the same time, the hard braker has a time advantage.
The hard braker will reach the stop sign first. Through a series of stops the hard breaker will have a higher average speed with the same fuel consumption. The hard braker can then reduce his average speed by using less throttle.