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> We already have an energy-saving self-driving mechanism - ... (and obeying the speed limit.) Obeying the speed limit is irrelevant; the actual speed is. A g

> We already have an energy-saving self-driving mechanism - ... (and obeying the speed limit.) Obeying the speed limit is irrelevant; the actual speed is. A given car at a given speed consumes the same amount of fuel per unit distance regardless of what the speed limit is. All the data indicating that "speeding wastes fuel" is basically just proving that fuel economy is a function of engine power output.

Retric 11y ago
You're ignoring breaking. If you go significantly faster than the speed limit your far more likely to break at highway speeds. Further, IC cars often get better fuel efficiency by mixing slow acceleration periods at optimum RPM and coasting. (Car engines get ~10-30 efficiency based on usage.) You can do this manually, but it's hard. However, air resistance is going to be the dominate factor at high speeds.
- vonmoltke 11y ago
  > You're ignoring breaking. If you go significantly faster than the speed limit your far more likely to break at highway speeds. I'm not ignoring braking. Aggressive driving wastes a lot of fuel, but aggressive driving happens regardless of the posted speed limit. In fact, that rapid acceleration-brake cycle is more likely to happen when the flow of traffic is below the speed limit. > Further, IC cars often get better fuel efficiency by mixing slow acceleration periods at optimum RPM and coasting. (Car engines get ~10-30 efficiency based on usage.) You can do this manually, but it's hard. I don't think it's hard. It just requires paying attention to the traffic around you.
Houshalter 11y ago
Actually it's more to do with air resistance. It increases exponentially at higher speeds. You also don't burn less by driving really slow, since the engine burns some amount of fuel just by running at any speed. 40 mph is the sweet spot, supposedly. Driving at the speed limit will definitely save you more fuel than speeding.
- vonmoltke 11y ago
  > Actually it's more to do with air resistance. It increases exponentially at higher speeds. You also don't burn less by driving really slow, since the engine burns some amount of fuel just by running at any speed. 40 mph is the sweet spot, supposedly. Actually, the sweet spot is dependent on the design and condition of the car. I'm not sure where exactly it is, but I think you are right that it is usually around 40. > Driving at the speed limit will definitely save you more fuel than speeding. How does a number on a sign affect how much fuel I'm burning? If, as you indicated, the sweet spot for my car is 40 but the speed limit is 30, how am I saving fuel by going 30 instead of 40? What if the speed limit is 65? If they drop it to 60 am I suddenly using more fuel? If they increase it to 70 can I now go faster without using more? The speed limit is totally irrelevant to fuel consumption.
  - Houshalter 11y ago
    Of course it depends on the design of the car, but 40 is an average. >How does a number on a sign affect how much fuel I'm burning? I never claimed that it would. I said "Driving at the speed limit will definitely save you more fuel than speeding.", which is definitely true. If you speed you are going faster and using more fuel.
    - vonmoltke 11y ago
      > I never claimed that it would. I said "Driving at the speed limit will definitely save you more fuel than speeding.", which is definitely true. If you speed you are going faster and using more fuel. That statement has nothing to do with the speed limit, though. It may be trivially true, but adding the speed limit ties an irrelevant variable to it. Speed limit is a semi-arbitrary number that has nothing to do with how much fuel a car uses and how efficiently it uses it.
      - phpnode 11y ago
        The implication is that driving over the speed limit is likely, on average, to be less fuel efficient than obeying it. Due to the laws of human nature, very few people drive significantly below the speed limit, and very few people prioritise their fuel economy over everything else so this translates to a reasonable guideline.
sliverstorm 11y ago
I think you're being deliberately obtuse. For 99% of people, obeying the speed limit will result in slowing down, which is what your parent was alluding to.

Retric 11y ago

You're ignoring breaking. If you go significantly faster than the speed limit your far more likely to break at highway speeds. Further, IC cars often get better fuel efficiency by mixing slow acceleration periods at optimum RPM and coasting. (Car engines get ~10-30 efficiency based on usage.) You can do this manually, but it's hard. However, air resistance is going to be the dominate factor at high speeds.

vonmoltke 11y ago
> You're ignoring breaking. If you go significantly faster than the speed limit your far more likely to break at highway speeds. I'm not ignoring braking. Aggressive driving wastes a lot of fuel, but aggressive driving happens regardless of the posted speed limit. In fact, that rapid acceleration-brake cycle is more likely to happen when the flow of traffic is below the speed limit. > Further, IC cars often get better fuel efficiency by mixing slow acceleration periods at optimum RPM and coasting. (Car engines get ~10-30 efficiency based on usage.) You can do this manually, but it's hard. I don't think it's hard. It just requires paying attention to the traffic around you.

Houshalter 11y ago

Actually it's more to do with air resistance. It increases exponentially at higher speeds. You also don't burn less by driving really slow, since the engine burns some amount of fuel just by running at any speed. 40 mph is the sweet spot, supposedly. Driving at the speed limit will definitely save you more fuel than speeding.

vonmoltke 11y ago
> Actually it's more to do with air resistance. It increases exponentially at higher speeds. You also don't burn less by driving really slow, since the engine burns some amount of fuel just by running at any speed. 40 mph is the sweet spot, supposedly. Actually, the sweet spot is dependent on the design and condition of the car. I'm not sure where exactly it is, but I think you are right that it is usually around 40. > Driving at the speed limit will definitely save you more fuel than speeding. How does a number on a sign affect how much fuel I'm burning? If, as you indicated, the sweet spot for my car is 40 but the speed limit is 30, how am I saving fuel by going 30 instead of 40? What if the speed limit is 65? If they drop it to 60 am I suddenly using more fuel? If they increase it to 70 can I now go faster without using more? The speed limit is totally irrelevant to fuel consumption.
- Houshalter 11y ago
  Of course it depends on the design of the car, but 40 is an average. >How does a number on a sign affect how much fuel I'm burning? I never claimed that it would. I said "Driving at the speed limit will definitely save you more fuel than speeding.", which is definitely true. If you speed you are going faster and using more fuel.
  - vonmoltke 11y ago
    > I never claimed that it would. I said "Driving at the speed limit will definitely save you more fuel than speeding.", which is definitely true. If you speed you are going faster and using more fuel. That statement has nothing to do with the speed limit, though. It may be trivially true, but adding the speed limit ties an irrelevant variable to it. Speed limit is a semi-arbitrary number that has nothing to do with how much fuel a car uses and how efficiently it uses it.
    - phpnode 11y ago
      The implication is that driving over the speed limit is likely, on average, to be less fuel efficient than obeying it. Due to the laws of human nature, very few people drive significantly below the speed limit, and very few people prioritise their fuel economy over everything else so this translates to a reasonable guideline.

sliverstorm 11y ago

I think you're being deliberately obtuse. For 99% of people, obeying the speed limit will result in slowing down, which is what your parent was alluding to.