Accellerate/Coast method more fuel efficient than cruise control?
June 14, 2007 9:39 AM   Subscribe

Question about cruise control vs. a special acceleration/coasting technique in terms of fuel efficiency.

If you've got say, a speed limit of 45, and while driving on flat clean road, no wind aside from your own forward movement, etc, you accellerate to 47, coast until you're down to 40, then accellerate to 47, coast back down -- could this period of brief gassing up and long coasting by momentum, use less gas than having the pedal down the entire time and keeping at 45 (which I would assume cruise control would do)?

Would there be an ideal range (instead of up to 47 and down to 40, and back up/down) that would achieve greater efficiency? Would it be better to quickly accellerate to 47, or slowly (as the transmission gear may affect gas consumption)?
posted by Quarter Pincher to Travel & Transportation (11 answers total) 1 user marked this as a favorite
Best answer: I have no citation but constant speed will be as efficient as any other method. If you are happy to coast down to 40 as a minimum speed then cruising at 40 would be the most efficient.

Top Gear in the UK did a test of how best to drive if you are short of fuel. The options they tried were a) getting to the filling station as fast as possible b) accelerate and coast as you describe and c) minimum practical constant speed. Option c) won.
posted by SpacemanRed at 9:45 AM on June 14, 2007

Always constant speed is the most efficient. The juice you burn up accelerating will not be equally compensated with what you save coasting, ever! Short Answer - Basically engines are not 100% efficient.
posted by jannw at 10:07 AM on June 14, 2007

Edmunds recently did a study on gas mileage.

Two notes:

1. They note that cruise control is usually a good choice, but that it should not be used on hills.

2. Though I didn't find it in the article I linked to, when I heard about the study on the radio, the Edmund's guy noted that it wasn't necessarily the lowest speed possible that was the most efficient, but that each car engine had a different "sweet spot" where it was most efficient (though this is usually lower than what we drive at.

So, SpacemanRed is mostly right, it's just that minimum practical speed, depending on your engine, may not be the most efficient. It's definitely not the accelerating and coasting, though.
posted by kingjoeshmoe at 10:23 AM on June 14, 2007

"quickly accelerate" is always the antithesis to fuel economy. The gentler you accelerate, the better.
posted by Rhomboid at 10:24 AM on June 14, 2007

The simplest way to think of economy is that the fewer times you accelerate at all the better. If you are going for economy you should always be thinking 'how can I avoid having to slow down and speed up again' look ahead at traffic situations and intersections and make the choice that means not accerating back up to speed.
posted by Cosine at 11:10 AM on June 14, 2007

I agree with Cosine and others: constant speed is most always the most efficient.

I have a personal "rule" when driving on the highway or freeway: the less braking or hard accelerating I do, the better I'm driving. Besides the fuel efficiency, this also results in my leaving more space between myself and other vehicles, and being more aware of what's going on ahead of me.
posted by Artful Codger at 1:32 PM on June 14, 2007

nthing constant speed, and what Artful said about the 'rule' - this is a big part of what makes driving fun for me, actually. I get a big kick out of seeing someone zoom past me in a rush to get to a red light... while I maintain my even 30mph and arrive at the light when it's green, and easily pass them again without ever accelerating.

Rule of thumb: keep your derivatives minimized. (Less acceleration and less jerk.)
posted by dmd at 1:59 PM on June 14, 2007

A mediocre analogy: Think of walking up a hill with a constant slight grade (constant expenditure) vs. a series of plateaus, where you walk flat for a while (coast), but then have to scramble up a 3-foot ledge every so often. I'd get sweatier doing the latter.
posted by svenx at 3:02 PM on June 14, 2007

Check out this article on a guy who gets 59 mpg out of a regular old Accord. He has some... interesting driving techniques.
posted by exogenous at 3:04 PM on June 14, 2007

It's called "pulse and glide". The writer of the linked article claims to have measured 8% better efficiency using this technique, in a non-hybrid without turning the engine off. No doubt its effect varies widely depending on the engine characteristics and aerodynamics of the car, the speed and timing of the bursts of engine power, etc.
posted by sfenders at 4:29 PM on June 14, 2007

Given that most newer cars (even small ones) rely on power steering and power brakes, "pulse and glide" sounds like a recipe for disaster.
posted by randomstriker at 5:26 PM on June 14, 2007

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