4 ms·

Really depends, if I blitz along at 80, then yeah. If I stick to the speed limit, it’s not a bad estimate, on mostly slow local roads, we get substantially bett

by ericd 3y ago

Really depends, if I blitz along at 80, then yeah. If I stick to the speed limit, it’s not a bad estimate, on mostly slow local roads, we get substantially better. It’s a great demonstration of drag’s nonlinear nature (the cube of the speed?).

jstarfish 3y ago
> The National Maximum Speed Limit (NMSL) was a provision of the federal government of the United States 1974 Emergency Highway Energy Conservation Act that effectively prohibited speed limits higher than 55 miles per hour (89 km/h). It was drafted in response to oil price spikes and supply disruptions during the 1973 oil crisis and remained the law until 1995. I've always been under the impression 55 was the optimum speed for (ICE) fuel efficiency. Does this change at all for electrics (given regenerative braking)?
- olyjohn 3y ago
  No it's a tradeoff between speed and efficiency. If you went 45mph, you'd get even better gas mileage. But people already couldn't drive 55.
- mirrorlake 3y ago
  The best MPG you can get on the highway is just the slowest you can safely go without confusing other drivers, making people angry, or getting pulled over by the police. Drag is proportional to velocity squared, so every increase in mi/hr wastes more energy pushing air out of the way of the car. To answer your regenerative braking question: braking is inefficient by default, so just avoid tapping the brakes in situations where you could coast. With regenerative braking, you'll never get back 100% the energy because of thermodynamics. There's always wasted energy any time energy is moved, transformed, stored, etc.
  - tzs 3y ago
    Go too slow though and mileage goes down. At 60 mph my car gets around 25 mpg. At 1 mph it would get around 3 mpg. That's because my car never consumes less than around 0.3-0.4 gallons per hour when the engine is running.
- ht85 3y ago
  Modern cars with variable valve timing and other goodies can achieve very good combustion under low load. Looking around, it seems ~30km/h would be the sweet spot. It would be slightly higher if you run the AC on high, increasing total consumption by ~10% and moving the dial a bit.

by ericd 3y ago

jstarfish 3y ago

> The National Maximum Speed Limit (NMSL) was a provision of the federal government of the United States 1974 Emergency Highway Energy Conservation Act that effectively prohibited speed limits higher than 55 miles per hour (89 km/h). It was drafted in response to oil price spikes and supply disruptions during the 1973 oil crisis and remained the law until 1995. I've always been under the impression 55 was the optimum speed for (ICE) fuel efficiency. Does this change at all for electrics (given regenerative braking)?

olyjohn 3y ago
No it's a tradeoff between speed and efficiency. If you went 45mph, you'd get even better gas mileage. But people already couldn't drive 55.
mirrorlake 3y ago
The best MPG you can get on the highway is just the slowest you can safely go without confusing other drivers, making people angry, or getting pulled over by the police. Drag is proportional to velocity squared, so every increase in mi/hr wastes more energy pushing air out of the way of the car. To answer your regenerative braking question: braking is inefficient by default, so just avoid tapping the brakes in situations where you could coast. With regenerative braking, you'll never get back 100% the energy because of thermodynamics. There's always wasted energy any time energy is moved, transformed, stored, etc.
- tzs 3y ago
  Go too slow though and mileage goes down. At 60 mph my car gets around 25 mpg. At 1 mph it would get around 3 mpg. That's because my car never consumes less than around 0.3-0.4 gallons per hour when the engine is running.
ht85 3y ago
Modern cars with variable valve timing and other goodies can achieve very good combustion under low load. Looking around, it seems ~30km/h would be the sweet spot. It would be slightly higher if you run the AC on high, increasing total consumption by ~10% and moving the dial a bit.