Because my earlier post on fuel-efficiency (on a Straits Times article) has sparked up some discussion and heated debate on the topic (just kidding), here comes a follow-up. Oh, and long post ahead!
As I mentioned, I’ve read a few times that the most efficient speed for most cars is between 60 to 80 km/h. I assumed so, and even applied it to my driving (well, at times).
Why at 60-80 km/h instead of at other speeds, or in particular, at lower speeds? Well, I came to think of trying to drive a car efficiently like riding a bicycle: at lower gears, you’d have to pedal hard but slowly to get the bike moving at a certain speed; but at higher gears, even though it feels soft to pedal, you’d have to pedal quickly to achieve the same speed (as using lower gears)… thus in both cases, you’d burn out your legs pretty quickly, and so the solution would be somewhere in the middle where it’s not too energy-draining.
That ‘theory’ of mine might have been confusing, and I know a car, compared to a simple bicycle, is A LOT more complex. My point is that, in terms of gears in particular (you know that you need to start off with the first gear and so on), driving at the lower gears is really inefficient, and driving at the higher gears isn’t any different, so the most efficient range of speed to aim for would be somewhere in the middle, i.e. 60 to 80 km/h.
Kahar made a great point when he answered to my question, that: perhaps, the actual ‘most efficient range of speeds’ was lower than 60-80 km/h, but putting practicality into the equation, the most practical range of speed people would actually want to drive at would be around 60 to 80 km/h.
You can skip the next three parts.
1. I Wiki-ed, and as always, good information:
Fuel economy at steady speeds with selected vehicles was studied in 1973, 1984, and 1997. The most recent study indicates greater fuel efficiency at higher speeds than earlier studies; for example, some vehicles achieve better mileage at 65 than at 45 mph (72 km/h), although not their best economy, such as the 1994 Oldsmobile Cutlass, which has its best economy at 55 mph (29.1 mpg), and gets 2 mpg better economy at 65 than at 45 (25 vs 23 mpg). All cars demonstrated decreasing fuel economy beyond 65 mph (105 km/h), with wind resistance the dominant factor, and may save up to 25% by slowing from 70 mph (110 km/h) to 55 mph (89 km/h).
2. The physics part:
The power to overcome air resistance increases roughly with the cube of the speed, and thus the energy required per unit distance is roughly proportional to the square of speed. Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h), results in about one-third the power to overcome wind resistance, or about one half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy by 1.9 times, and decreases fuel economy. In real world vehicles the change in fuel economy is less than the values quoted above due to complicating factors.
The power needed to overcome the rolling resistance (which itself is broadly proportional to speed) is also a factor, particularly at lower speeds. At very low speeds the dominant losses are internal friction.
3. And the many number of ways energy can be lost:
- Engine efficiency
- Aerodynamic drag force, which increases roughly by the square of the car's speed, but note that drag power goes by the cube of the car's speed.
- Rolling friction
- Braking
- Losses in the transmission (manual transmissions can be up to 94% efficient whereas older automatic transmissions may be as low as 70% efficient)
- Air conditioning, though only when in use. The extra mass of the air conditioning system will cause a slight increase in fuel consumption
- Electrical systems, as the energy to power devices causes increased load on the alternator
The relevance of all this? Maybe none, but here’s another good article on fuel-efficient driving, with NUMBERS:
1. Use the highway whenever possible. The more you stop and start, the more fuel you'll consume.
2. Be smooth and steady in the city. You'll pay for sudden accelerations and hard braking with fuel dollars -- and you won't be saving as much time as you think.
3. Don't be an idling fool. Sure, it's bad for the environment, but idling can also be bad for your car since it can contaminate the oil and damage components.
4. Keep your tires inflated to recommended psi (pounds per square inch).
5. Practice smart seasonal driving. In the summer, rely on air vents to cool the interior rather than air conditioning and you'll save on fuel. In the winter, many drivers idle their car to heat up the engine.
Finally, I’m sorry if this long post wasn’t worthy of your time, but if it taught you new things about driving, then GOOD for you and for the environment. :p
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