A Guide To Fuel Efficient Driving — Part Three

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Published on January 10th, 2021 | by Barry A.F.

January 10th, 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.

Summer-specific advice

Air conditioning will increase your vehicle’s fuel usage. The difference is likely between 5–15%. Not nothing, but not enormous. According to various tests on the highway, it does use less fuel to close your windows and use the air conditioning. Though, this varies to some extent by vehicle model and the rate of solar gain, which is not constant — heatwave days will definitely use more, for example.

Some if not most recent vehicles will have infrared reflective glass to reduce AC load. Tinting can also help here if your vehicle does not have IR reflective glass, but it does reduce visibility (though, is legal in many locations up to a certain level of opacity).

Spring weather improves mileage over winter weather and summer weather increases it still. Fuel efficiency increases up to about 30°C, then AC use does reduce it.

Winter-specific advice

Winter means cold and snow/ice. Colder temperatures makes automotive fluids more viscous, which takes more energy (hence fuel) to overcome. Additionally, colder tires require more energy for the same flex (all tires flex in operation, partially absorbing bumps, supporting the weight of the vehicle and its occupants, and providing grip). In addition, there are several other factors, as talked about below.

Winter tires add road friction and reduce mileage but provide extra winter grip, which prevents accidents and saves lives. Install them when the average outdoor temperature gets below 7ºC, and in spring change them back to regular tires once the average temperature again gets above 7ºC. This is to reduce their wear rate, which accelerates rapidly at high temperatures. Also, changing them back to regular tires gets you back to maximum fuel economy. Finally, all-season tires are informally known as “do all seasons badly” tires. They are not a replacement for winter tires if you live in a location that gets more then a dusting of snow each winter.

Keep your car clear of snow and ice in winter to maximize aerodynamic efficiency. Foam snow brushes with rotating heads are amazing at removing snow from the roof and windows and hood/trunk, and they won’t scratch the paint.

Driving in rain and more so in the snow adds rolling resistance. It’s surprising how much extra energy is taken up displacing snow with tires when the road is not clear. This also applies to gravel roads or driving through mud/dirt or sand. This inefficiency is hard to mitigate except by trying to avoid trips when there is accumulated snow on the roads and waiting until the roads are clear (if practical, it’s not likely a workplace will accept this as a reason for not coming in).

Wind resistance, especially on the highway, is much higher in winter because colder air is denser than warmer air. And in many areas, winter has more wind than spring/summer/fall. Thus, headwinds cause extra gas penalty and tailwinds give less of a savings.

In an ICE vehicle, warming up the passenger cabin does not use much additional fuel, as the heat for the cabin comes from waste heat produced by the engine. The slight extra energy use is just for the fan to move the heat from the heater core into the passenger area. That said, some ICE vehicles have ceramic heaters so that you can get heat right away without waiting for the engine to warm up. In this case, you will save a couple percent by waiting until the engine is warm, if you can stand the cold that long.

Heated seats, on the other hand, use electricity produced by burning fuel — hence, they do lower economy a bit. Of course, this is a comfort vs. fuel consumed calculus on your part. If your vehicle has ceramic heaters for instant heat on a cold engine, then the heated seats are more efficient. If not, then using heated seats only until the engine provides enough heat is a reasonable way to get heat faster with reduced penalty.

In an EV, cabin heat does reduce range in winter because engine inefficiency is not available for exploitation. For these vehicles, heat makes a very measurable difference, but heat pumps are slowly becoming common and cut this penalty by 50% or more. Heated seats are more efficient in an EV vs. circulated heat.

With shorter days and darker nights, headlight/taillight use will be higher. This adds a few percent to fuel consumption. However, there is not much you can do to mitigate this besides attempting to do more of your driving in the daytime, which is safer anyway. LED headlight replacements will cut this use quite a bit, but most retrofits are not street legal in most countries because the beam pattern sucks compared to stock and often blinds other drivers — and/or the throw pattern does not match the OEM halogen design and you don’t get light where you need it even if the much higher claimed lumen counts are not fake. So, don’t bother with them. Headlight housings are designed to collect, collimate, and project light based on the beam pattern of the OEM light source (the bulb type chosen by the manufacturer) LED bulbs have a different throw pattern because of the underlying technology, hence are incompatible with housings designed for halogen bulb beam patterns. If the housing was designed for the LED throw pattern at the factory, then retrofit could be successful, but until this happens, stick with OEM bulb suggestions. Finally, you can get higher brightness halogen bulbs, but the technology has a wrinkle — higher brightness comes from a thinner filament, which means much shorter life. In some cases, the extra bright bulbs can last 6 months or fewer, versus many years for the regular filament bulbs. Also, there are no energy savings to be had here — the wattage is the same.

An integrated engine block heater will reduce the warmup penalty talked about in Part Two. By using a block heater to reduce warmup fuel, you are in a sense trading some carbon from fossil fuels for electricity, which is typically lower carbon intensity (and getting greener in most countries as time goes on). Use it whenever possible if you have a plug handy and your vehicle has a block heater. If it does not, see if one can be retrofitted by the manufacturer. The colder the outdoor temperature, the greater the positive impact from the block heater.

Miscellaneous information:

  • Some cars have an Eco mode that causes slower acceleration unless you mash the pedal. Some even have an Eco indicator that tells you when you are driving efficiently. Use both of these if your vehicle has them.
  • Ethanol and winter gas reduce mileage slightly. These are hard to avoid. E85 reduces mileage greatly but is rarely sold.
  • Avoid drafting (tailgating) other vehicles or transport trucks/lorries. This does improve your mileage, but the risk of injury (not to mention flying debris such as sand and rocks) is not worth the fuel savings.
  • People like to drive in convoys. We have an instinct to travel together. On city streets or highways that are not crowded, you will notice groups of vehicles traversing together. This does provide some unintentional fuel savings, as aerodynamic resistance is slightly shared between vehicles, but it also means a lot more accelerating and braking to maintain the convoy, which reduces efficiency, and potential for multi-vehicle accidents. In general, fight this natural inclination and travel away from other vehicles. This also gives you more reaction time and you can travel at a more fuel efficient speed.
  • Where you park your vehicle can also affect fuel efficiency. A carport or unheated garage will keep snow and ice from interfering with the aerodynamics, in cold weather a heated location will reduce warm up losses, during summer parking in the shade will reduce how much work the AC has to do to cool down the vehicle and keep it cool, and so forth.

In conclusion, these are the broad strokes to driving your vehicle for greater fuel efficiency. There are other niche tactics such as pulse and glide, but they get more complicated and can endanger the safety of yourself and other drivers and should be avoided. Finally, don’t forget about the possibilities of carpooling or working from home. The most efficient way to operate your vehicle is to leave it at home whenever possible.

In case you missed them: Part One and Part Two of this series.

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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