"I'm driving everywhere twice as fast so I use less gas"

[Jonathan]

Was anyone else watching the UK Live Earth as amused by Brit comedian Jimmy Carr's above comment as I was?

Today though, I'm very tired, and not quite thinking straight, and a question popped into my head: is it really so stupid? Sure, going faster uses more gas, as more engine power is required to accelerate to higher speeds. But once at those speeds, one can go into a higher gear and cruise, using less engine power, thus less gas.

So: does driving faster actually help the environment? :-)

Or at least use marginally less gas in the long-run. Any physicists / mechanics wanna help out?

[Gautam]

Most car-companies recommend keeping your car between 40-60 Kmph and in the 4th gear as much as possible, this is where most cars are at their economic peak.

Also a good trick one of my friends suggested was to keep off the accelerator when on slopes or when you have enough momentum to travel a certain distance at a decent speed. He actually upped his car's mileage from 15 Kmpl to 18 Kmpl with this method. Although, a lot of other factors play a major role.

[Tuumble]

Something just occurred to me.

If you drive fast you are driving for less time because you get to your destination quicker. That's less time for harmful gases to be emitted from your car. Is that better for the environment?

Of course, I'm not saying you should drive everywhere on two wheels...unless you want to save wear and tear on your tyres.

[Ak47]

^i do believe i read somewhere that for the majority of cars the most gas efficient speed for non-stop travel is on average 55mph

i'd expect that avg to rise slowly over time as well, manufacturers are constantly trying to make cars go faster and use less gas these days...

[silicon toad2000]

the faster you go, the more wind resistance you have. I beleive this is an exponential relationship as is the realtionship between accerleation, force and mass.
if you want to drive in the sweet spot, get a dyno run done at partial throttle so you can see where the engine is at its most efficient and select the gear that keeps the engine in that range.
there area lot of tips on the web about saving fuel, keep your tyre pressures up, keep the car well maintained. keep the weight of the car down, dont carry unneccesary items. Check the difference in MPG between regular and premium then factor in the cost difference and work out which is better (if you have an old car with a distributor, advance the timing half a degree on premium and see how it goes)

[Chinese Dragon]

Yup, silicontoad got it. Driving faster simply MUST consume more gas because you're going the same distance, but you have to cover it in a smaller time. That extra speed takes energy because of wind resistance. It's like walking vs. running. Running a mile is healther than walking a mile, AND it's quicker.

[Mayo]

three words: AIR POWERED CAR...

The MDI Air Car - The World´s Cleanest Car.

vid:
YouTube - The Air Car

[Matthew Shea]

Originally Posted by Chinese Dragon

Yup, silicontoad got it. Driving faster simply MUST consume more gas because you're going the same distance, but you have to cover it in a smaller time. That extra speed takes energy because of wind resistance. It's like walking vs. running. Running a mile is healther than walking a mile, AND it's quicker.

Wind resistance isn't the only reason why it takes more gas to travel at a higher speed. Even in a vacuum, as in outer space, the speed you're moving at is proportional to the square of the energy used. In other words, if you're traveling twice as fast, it requires four times the amount of energy to move a given mass. Wind resistance is then added to that.

There are a bunch of other factors, too, such as powertrain efficiency at varying throttle positions, engine RPM, etc, but suffice it to say that you'll use a LOT more energy driving twice as fast to get somewhere, even if it takes half the time.

[silicon toad2000]

Originally Posted by Matthew Shea

Even in a vacuum, as in outer space, the speed you're moving at is proportional to the square of the energy used. In other words, if you're traveling twice as fast, it requires four times the amount of energy to move a given mass.

In fact I buggered it up. a=f/m, the relationship between acceleration and force or energy is linear, not exponential.

To accelerate to twice the speed will take twice the energy, but in a vacuum maintaining that velocity requires no energy (discounting the micro impacts of space dust and debris)

Now take wind resistance into the equation
p=1/2*pv(cubed)AC
power is half the density of the fluid by velocity cubed by area by drag coefficient.

to drive twice as fast will use 8 times as much fuel to push the car through that air (ignoring engine and gearing inefficiencies) so it will be 4 times as much fuel per distance travelled.

The efficiencies in the gearing, engine and drivetrain of a car seem to dictate the fuel efficiency at speed. I guess a car is at its most efficient in top gear at revs where there is the most power at partial throttle.

[Gautam]

Anyone have any thoughts on the idea i suggested? The one where you don't accelerate when going downhill or when you have enough momentum to go forward without any acceleration. There's a slope in my neighbourhood where by the time you reach the end of it, the car's automatically running on 60kmph..which is enough to do the subsequent uphill climb without any acceleration..

[Matthew Shea]

You're absolutely right Silicon Toad, it's really the acceleration that kills you, not the speed, per se, except that wind resistance becomes much higher at faster speeds.

Gautam, wind resistance and friction will not allow you to reap the full benefits of the extra downhill momentum when going up the next one, but your idea is arguably the most efficient way to drive on hilly terrain. Even so, the best efficiency comes at a constant speed on flat ground. Hills will always reduce efficiency because, from a physics point of view, you have to accelerate against the force of gravity, even if your speed is constant, when going up a hill.

[Dan.Linehan]

Originally Posted by Mayo

three words: AIR POWERED CAR...

The MDI Air Car - The World´s Cleanest Car.

vid:
YouTube - The Air Car

That is the fricken' coolest thing I have ever seen.

[Carl Carlson IV]

That air car sure is interesting...