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Opinion: Why it’s time to raise the speed limit in Australia to 130km/h
(www.drive.com.au)
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this post was submitted on 17 Aug 2023
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You don't use exponentially more fuel, but cubically. Exponentially is not just a word for "quickly", but a function.
And anyways, that isn't only the case for speeds higher than 80, but for every higher speed. So it's not like there is an objective sweet spot.
The 80 kph rule of thumb is actually part of the design parameters of most regular cars. They are built to be most fuel efficient at 80 (or probably more accurately aerodynamic designed for 80).
I was using exponential colloquially (and fair cop given its usage during Covid), but I think you're just using cubic as a rough guide also due to air resistance. I'd note there are no extra gears at the higher speeds, so you're probably less efficient on the tyres etc.
I didn't know about that. Thanks, that's interesting! Sorry if I sounded a little condescending, I didn't intend to.
And cubic just means an exponent of 3, which is one kind of exponential relation.
No, exponential is N^x^, polynomial growth takes the form x^N^, and in the case of a cubic, N is 3. Exponential functions tend to grow much, much faster than polynomial ones.
Only if you're talking about air resistance exclusively. I don't know what number it is precisely, but at lower speeds other forces dominate (like efficiency in the gears), and at a certain point air resistance becomes the more dominant force, growing with the cube of velocity. It's certainly possible that the number is 80 km/h.
In any car built in the last ~20 years, you can monitor your real time consumption either on the dashboard or else by hooking up your phone into the mechanic's diagnostics port (there are cheap bluetooth dongles).
In general, fuel consumption is infinite when the engine is running while you're not moving. At very low speeds economy is terrible and as you increase speed fuel efficiency improves until the sweet spot which is usually at about 60km/h. That sweet spot is fairly wide - up to around 80km/h in most cars and then it starts getting bad again.
It's different for every car - but as a rule of thumb if your car uses X amount of fuel at 60-80km/h, then it probably uses about twice that much fuel at 20km/h and 130km/h.
HOWEVER that 130km/h number assumes the car hasn't been modified. If you've installed a roof rack for example then it could be more like triple the consumption you had at 80km/h! Low speed would be less affected by modifications.
Ultimately the only number that matters is the number for your car, so why not measure it? Modern cars use a computer to calculate the fuel injection speed and it's possible to monitor that number.