A Guide To Fuel Efficient Driving – Part Two

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Published on January 4th, 2021 | by Barry A.F.

January 4th, 2021 by  

While electric vehicles (EVs) are very slowly taking over the world, most people still drive gasoline and diesel powered vehicles, known as internal combustion engine (ICE) vehicles. While we cannot bring them to carbon neutrality via greater efficiency, we can reduce their carbon footprint with how we drive.

Driving Behaviors

A cold engine is far less efficient than an engine at operating temperature, and your vehicle uses extra fuel specifically to warm itself up. The colder the outdoor temperature, the more fuel it takes to fully warm the engine to operating temperature. Hence your vehicle uses fuel to warm the engine as well as to get you to your destination. And when you first start the vehicle it is using a richer fuel to air mixture to keep from stalling until it’s up to operating temperature. This varies by vehicle, but expect about 0.2-1 liters (0.05-0.25 gallons) of fuel just to warm the engine, irrespective of the fuel used to get you to your destination.

You can substantially reduce this repeated penalty by combining trips, as restarting a warm engine reduces this penalty substantially. Thus if you were going to get groceries today and pick up your prescription from the pharmacy tomorrow you will save fuel by doing both consecutively. Also the less time the car is not running between errands the less the engine cools down and the smaller the re-warm up penalty. In general in spring/summer/fall an hour or two between warm starts won’t cause a huge penalty, but in winter a few hours will likely return the engine to almost ambient temperature. Also idling to warm the engine takes longer and wastes fuel as you are getting 0.00 mpg. And because the engine is only about 10-25% efficient in moving the vehicle and the rest is converted to heat, starting the vehicle, giving it 15-30 seconds, and then beginning your trip helps warm it faster and reduces the warm up penalty slightly (or more accurately combines it with propulsive fuel consumed).

For trips under a kilometer (1/2 mile) consider alternative transportation forms since taking the car with a cold engine will provide less than half its rated fuel economy. Walking, bicycling, e-bike, scooter, hoverboard and more are possible alternatives.

Most ICE vehicles get better mileage on the highway than in city driving (hybrids and EVs are an exception to this rule). This is due to lack of frequent acceleration/braking (excepting rush hour traffic). However, highway driving has much higher wind resistance which reduces mileage. The faster one travels, the more fuel is needed to overcome aerodynamic wind resistance. And wind resistance increases as a squared value, 70km/h (45 mph) using half the energy to maintain vs 100km/h (62 mph). It takes almost triple the energy to maintain 120km/h (75 mph) as it does 70km/h (45 mph). All these values to travel the exact same distance. Hence lower speeds reduce aerodynamic resistance and uses less fuel to overcome. During the oil crises in the 1970s, highway speeds were reduced in many countries to save fuel, which was being rationed. This has long since been forgotten and today speed limits are more likely to increase rather than decrease. And traveling more slowly on the highway is often not practical unless you want to meet the bumper of the person behind you. Do your best on this one.

Sometimes regional roads are parallel or lead to the same destinations and have lower speed limits and not too many stops or traffic lights. Consider these when practical.

There are a few more wrinkles to the speed vs efficiency calculus. All engines have an efficiency curve, and test data on this is hard to find. Hence even though 70km/h (45 mph) has half the aerodynamic resistance of traveling at 100km/h (62 mph) the fuel used is not double as more torque in the same gear is more efficient (to a point) in an ICE engine. This disappears in an EV which has less inefficiency to exploit.

For most vehicles, the most efficient speed is at the slowest speed that allows it to maintain the top gear. Modern 8-10 speed automatic transmissions can interfere with this calculus. Hence aim to stay as close to 80km/h (50 mph) as practicable to reduce fuel consumption.

Wind will affect your mileage in city driving somewhat and greatly on the highway. However, there is not much you can do about this, as it’s down to luck whether or not you are traveling on a calm day or have a tailwind or headwind. That said, in a headwind your savings from slowing down are multiplied, as are savings from a tailwind. Crosswinds lower your efficiency in any direction. You can check out current and future anticipated wind conditions at Windy.com. Hills affect your mileage greatly as well, however, you typically regain the loss from fuel used going up when you go down the other side later unless you use that as an opportunity to travel faster than you were before, feeding fuel into higher squared wind resistance. If you drive an EV and early in your journey is a significant downhill, you can charge to less than 100% and bank some power for later in your trip.

City driving is very inefficient and has more ways to reduce consumption (to a point). You use fuel in accelerating, and maintaining a set speed, and you throw away the inertia you used fuel to create when you brake.

Acceleration uses more fuel than maintaining a set speed. Also the rate you accelerate at affects your rate of consumption. To save fuel accelerate slowly, as if you accelerate at a snails pace you won’t save as much fuel because you travel more distance in lower gears (the higher the gear you are in the better your mileage), and if you accelerate rapidly the engine uses a richer mixture to get you up to speed faster, which is less efficient. Hence you want to accelerate faster than a snail but typically slower than everyone else. But try not to be a nuisance and accelerate so slowly you annoy everyone behind you. Also for an EV the rate of acceleration does not affect efficiency that much, as the motor has a relatively flat efficiency band throughout its range and most have no gears, though at the top end, Ludicrous Speed acceleration (or other manufacturer equivalents) is probably less efficient.

Ideally you want to get your vehicle up to speed and maintain that speed. Aim for the speed limit if practical. Remember that above 70-80km/h you are losing efficiency. But don’t try going 70-80km/hr on a road with a lower speed limit, speeding tickets are not worth saving fuel and will cost you money in fines and probably higher insurance rates.

Braking destroys the inertia you built up, and hence reduces gas mileage. Gas gets you up to speed, and when you use the brakes you throw away that fuel expenditure. Hence gas-brake-gas-brake-gas-brake is incredibly inefficient. Of course never using your brakes is impractical. So you want to think ahead, keep a reasonable or better yet long distance between you and the car in front of you so you don’t have to brake every time they do. You can let your space buffer slow you down a bit by coasting when they brake or if they are turning they can complete their turn and you don’t have to brake at all. If you heading towards a red light, get off the gas early and coast towards the light. By the time you get there you will have lost some speed to friction and have less braking to do, hence you will have used less fuel to get to that light. And if you are lucky the light may have turned green in the interim and you can keep going without using your brakes at all. Balance this against being courteous to the people behind you. With practice you will figure out how soon to get off the gas to get to the light and how quickly your vehicle loses speed when you are off the gas (while still considering the people behind you). In an EV the aggressiveness of your regen will determine how quickly your vehicle loses speed when off the acceleration pedal, aggressive regen/one pedal driving is actually less efficient than coasting but you can try different settings (if your vehicle has them) and see what works for you.

Finally, you can learn to time traffic lights. We have an inborn ability to measure time even when we are not counting it with a stopwatch. If you take the same route on a regular basis you will find you can teach yourself to anticipate when lights will go green or red (also helpful are walk/don’t walk signs). Some traffic lights work on sensors built into the roads and change when traffic diminishes, but many if not most are on timers (and some change timing a few times a day, be prepared for this). Hence with practice you can learn to time your traffic lights and reduce your acceleration and braking. Of course if you have a hybrid, or better yet an EV, this won’t matter as much, for those vehicles simply use one pedal driving or apply brakes without mashing them at the last second so you get regenerative braking. The round trip regen to battery to re-acceleration is typically about 66-75% efficient compared to 0% regen efficiency in a conventional ICE vehicle.

All the city driving tips also apply to rush hour driving on the highway. Keep extra distance from the car ahead of you, accelerate slowly, and coast as much as possible and use the brakes as little as possible. It will feel unnatural until you get used to doing it. Also people will rush to fill in the gap you create, but you can still save 50% or more of the fuel by learning how to work the situation with the gas and brakes very methodically.

Stay tuned for Part Three – Summer- and Winter-Specific Advice

In case you missed it – Part One

The standard disclaimers apply. All advice is for informational purposes only — CleanTechnica is not responsible for any damages caused by inaccurate information, and the user assumes all risks of following any advice provided. 


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About the Author

I’ve had an interest in renewable energy and EVs since the days of deep cycle lead acid conversions and repurposed drive motors (and $10/watt solar panels). How things have changed. Also I have an interest in systems thinking (or first principles as some call it), digging into how things work from the ground up. Did you know that 97% of all Wikipedia articles link to Philosophy? A very small percentage link to Pragmatism.  
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