How to drive to be the most fuel efficient

Question

I saw a poster at a thruway rest stop once that said something along the lines of accelerating quickly is worse for fuel efficiency and carbon emission than idling for several hours (I don't remember the exact statistic). It made me wonder: what ways can I drive to be more fuel efficient? My car is 20 years old, automatic transmission, and I can't afford to buy a new car, or a hybrid car, so I have to make the best with what I have. I already drive predominantly on highways to avoid the gazillions of traffic lights in the Chicago suburbs.

Answer 1

Several factors come to mind

• Air-conditioning
  Probably one of the bigger factors for saving fuel. At low speeds, open the windows instead (if it's hot); close the windows & wear a jacket (when it's cold). For high speed, close the windows, even when it's hot. At higher speeds the drag created by open windows uses more fuel than the air-conditioning. (See comment by Jay Bazuzi & reference by walker below.)

• Air pressure of your tyres
  Not the biggest factor, but you decrease wear (long-term benefit) and friction, which saves fuel on the day, so to speak (short-term benefit).

• "Defensive" (anticipatory) driving
  ... meaning try to drive without having to step on the brakes. Keep your distance from the cars in front of you. Take your foot of the gas when the traffic light turned to red. (But remember, braking is more cost-efficient than having an accident. Safety comes first, also for sustainability.)

• Avoid traffic
  Like the point above, avoid traffic, because in traffic you'll have to brake (for other cars, cyclists, children, traffic lights), which is necessary, but not fuel efficient. Scheduling a longer route around town may well be more efficient (esp. if you are driving defensively and slowly) and doesn't drive you up the wall. (You said that already, I know.)

• Reduce weight
  No reason to drive around with unnecessary heavy objects in your trunk. Only take with you what you need.

• Drive slowly
  Driving twice as fast increases the air resistance 4-fold. Driving faster thus necessarily means you use more fuel for the same distance. (I mean, don't drive faster than everyone else, not drive slower than everyone else, forcing people to have to wait for you. That will result in aggression and risky overtaking manoeuvres of other people, using more net fuel...)

• Make your car aerodynamic
  Don't transport anything on the roof (if possible). The air resistance is very high. If you do, drive slowly. (See "Drive slowly")

• Don't accelerate too fast
  Give your cylinders a chance to use all the fuel you give to them efficiently. Accelerating too fast results in incomplete combustion and more fuel being used for nothing.

And a few obvious ones (for sake of completeness)

• Form alliances
  Take your neighbours to the shopping mall and they'll return the favour.

• Only drive when necessary
  You save the most fuel when you don't drive at all. Take the bus or train or hitch a ride with friends or neighbours.

Answer 2

Driving Strategies

I did some research and personal experimentation on the topic, and here are some findings (more information in attached links). To achieve better mileage I would advocate the appropriate driving. The term defensive driving relates to proactive risk management of on-road hazards, and can be done regardless of speed and acceleration.

Arbitrary “slower speed” does not guarantee better fuel economy, as different vehicles have different optimal RPM range for any given gear (with highest gear being most economical). However, at motorway speeds aerodynamic drag plays ever increasing role, and that optimal RPM range is usually allows for lower speed than most people indulge themselves on the autobahn.

As a general rule, smooth acceleration and aptly timed deceleration is key, but there are instances where it does not hold true. For example if one pulls out of side street during a rush hour, and the choice is to accelerate rapidly to safely merge into traffic, or be faced with the long line of cars and couple of minutes of idling, the winner is clearly the former (more on evils of idling later).

Proper deceleration is important, because fast braking (even if one “drives defensively” well under speed limit) does nothing but converting kinetic energy into waste heat, whereas there are more efficient ways to slow down. Releasing throttle well ahead of time not only reduces wear of brake pads and tyres, but allows the car to use natural losses of energy (inside the engine and drivetrain, aerodynamic drag etc.) to slow itself down. Importantly, it has to be done in gear (engine braking) which qualifies as an engine overrun condition (throttle is off and RPMs are higher than idle) at which point ECU on modern fuel-injected petrol motors shuts off fuel delivery entirely.

It is possible to greatly reduce need for acceleration by planning ahead and properly timing the traffic signals, slowing down well ahead of red lights in order to roll onto green, or accelerating when needed to make the green light, to avoid idling on red.

On Evils of Idling

The next big thing is to reduce idling. Besides the obvious “burning fuel with no miles” (0 MPG), there are more serious mechanical consideration against idling (even aforementioned fuel shut-off during overrun was primarily introduced to protect catalytic converter from incomplete combustion). Those are enumerated elsewhere on SE, but in short that incomplete combustion leads to reducing effectiveness of spark plugs (which, in turn, further reduces fuel economy), glazing of combustion chamber, contaminating of oil, and excessive amount of water in exhaust system which increases corrosion. Moreover, idling produces more pollution from the same amount of fuel. For those concerned about implication of frequent stopping and restarting the vehicle, there is some math in above link, and more in another.

In practicality, this means turning off engine at long stop lights, railroad crossings, at the corner shop, and so forth, and reducing time spent warming up. Modern engines warm up better when they are put under moderate load rather than idling. And in the winter, one should protect the windscreen from defrosting with covered parking or putting blanket on it overnight, and to wear weather appropriate clothing, and not waiting for the car interior to turn into sauna.

More Better

Other suggestions are also true: proper tyre pressure (also, safety issue, as under-inflated tyres are at risk of overheating failure), low rolling resistance tyres (some concerns addressed), diligent maintenance (plugs, wires, oil and filters affect performance in little ways that can add up), and removing unnecessary weight or aerodynamic impediments (if you do not have skis or a bicycle on your roof rack, it has to go).

Outside of the mechanicals, alternative transportation (walking, biking, public transport, motorcycling) and carpooling can make an impact. Public transport generally takes longer time, but this time can be used more productively (reading, homework, work), one avoids traffic stress, parking costs, reduces wear, tear, and statistical possibility of an accident in own car. Also helps avoiding rush hour when possible, and combining trips (engine is more efficient at proper operating temperature, and many separate small trips do not allow motor to reach it).

Keeping track of the fuel usage (I use Fuelly, personally) helps not only for monitoring effects of any particular approach, but can also point to problems if certain negative trends develop.

The above is very sensible practises that many can readily apply today in improving their driving habits without entering arcane realm of hardcore hypermiling.

Answer 3

The existing answer is excellent but before you do any of that, measure your fuel usage. If you don't measure what you're doing, you don't know if you're even making an improvement.

Some cars have a fuel efficiency gauge on the dash but I don't recommend watching this as you drive. The times at which the greatest gains can be made (i.e. accelerating) are exactly the same times when the most concentration is required to ensure you don't crash. Crashing is not fuel efficient. If you have an efficiency gauge in your car, check it at the end of each journey, not while you are driving.

If you don't have one or even if you do, the tried and true method of writing it down when you fill up on fuel is my recommendation. For years when I owned a car I kept a notepad in the glove box that contained pre-drawn columns:

_______________________________________________________________________
|Date    |Odometer    |Fuel quantity    |Fuel cost    |Notes           |
|        |            |                 |             |                |
|        |            |                 |             |                |
|        |            |                 |             |                |

From these values, you can calculate how far you traveled since the last fill-up and how much fuel you used to travel that distance and from that, what your fuel efficiency was. The cost isn't necessary for calculating efficiency but I found that information interesting too.

Make sure to fill up consistently the same way each time. This is usually achieved by simply holding the fuel pump trigger until it clicks off automatically.

You can also buy car travel logs with similar columns (they usually have both Start odometer and End odometer columns). In this modern age of iPhones, Andriods and 3G, you can enter your travel details straight into websites designed to calculate your fuel efficiency such as Fuelly and MPGTune.

Once you have been recording your actual fuel usage for a while, start changing one of the above suggestions at a time and don't change anything else for several fill-ups.

---

I'd like to add a few notes from personal experience from when I went through improving my fuel efficiency. I was driving a large 4WD so the fuel costs hurt.

The first two ideas I tried were what are called above defensive driving and don't accelerate too fast. Leaving a large gap in front of me meant that often the traffic lights would go green before I even needed to brake, meaning I didn't need to accelerate again and waste fuel. Accelerating slowly simply uses less fuel to reach the same speed (but takes longer) however in small city traffic, I would usually catch the people who accelerated off ahead of me at the next set of red lights so I didn't end up being late due to my changed driving style.

With these two techniques, I improved my measured fuel efficiency from 15L/100Km to 11L/100Km. (That's roughly 15mpg up to 21mpg).

I didn't get to try any further ideas as I moved to London, sold my car and bought a bike. The fuel efficiency of a bike is generally given to be 53 miles per burrito.

Answer 4

One of the core practices most hypermilers use that hasn't been mentioned yet is DWB - Driving Without Brakes.

At least with accelerating, you are increasing your kinetic energy so can use it. Every time you hit the brakes you dissipate energy as heat and cannot recover it (unless you have a kinetic energy recovery system...)

From hypermiler.co.uk (and these guys really know what they are talking about), the top 5 tips are:

• Don’t drive
• Drive defensively, anticipating the road ahead.
• Drive without brakes (DWB)
• Slow down
• Coasting