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Re: Driving Above 60 MPH Uses More Gas?

Olinda Spider — Mon, 18 Apr 2011 21:48:47 GMT

limitgov:

My oringinal question was "uses more gas?" which implies total gas used.

Good point. I should have added "across the same distance", as in:

gas mileage" mean "miles per gallon of gas" and is comparable regardless of the time across the same distance.

Re: Driving Above 60 MPH Uses More Gas?

limitgov — Mon, 18 Apr 2011 21:42:08 GMT

""gas mileage" mean "miles per gallon of gas" and is comparable regardless of the time."

My oringinal question was "uses more gas?" which implies total gas used.

Re: Driving Above 60 MPH Uses More Gas?

Olinda Spider — Mon, 18 Apr 2011 19:20:07 GMT

"gas mileage" mean "miles per gallon of gas" and is comparable regardless of the time.

Re: Driving Above 60 MPH Uses More Gas?

limitgov — Mon, 18 Apr 2011 19:03:40 GMT

but even though you're gas mileage is lower at higher speeds, it will take you less time to get to your destination.

Does anyone take time into consideration when determining total gas used?

Re: Driving Above 60 MPH Uses More Gas?

Wibee — Mon, 18 Apr 2011 05:50:27 GMT

Whatever speed you reach right when you hit your max gear is the best MPG yu can get. Going faster then only increases the RPMs of the motor. The motor does more work, more gas is used.

Re: Driving Above 60 MPH Uses More Gas?

Sphairon — Mon, 18 Apr 2011 03:15:45 GMT

I drive on German autobahns a lot. I can tell you from experience that gas consumption does increase considerably once you pass the 55-60 mph mark.

For example, my car gets about 60 mpg at 55 mph. This drops to 50 mpg once I hit about 70. At 80, I'm lucky to get 40 mpg, and when I drive at max speed (~120 mph), I'm down to about 15-20 mpg. I have an instant MPG display, so this is not just guesswork on my part.

Still, it's fun to drive 120 mph in a French hatchback when the road in front of you is clear. Sucks to fill up again at the gas station, though.

Re: Driving Above 60 MPH Uses More Gas?

Broken Window — Mon, 18 Apr 2011 02:47:57 GMT

Shift into the highest gear available , then maintain the minimum speed just necessary to not lug the engine = most fuel-efficient speed (?)

Re: Driving Above 60 MPH Uses More Gas?

Raymond Keller — Sun, 17 Apr 2011 11:31:00 GMT

Speed has everything to do with gas mileage. Here is why:

The power which is required to overcome wind resistance is as follows:

P_d = 1/2pv³AC_d

Notice that the velocity of the object in the fluid is cubed. This creates a graph with characteristics like the one here http://en.wikipedia.org/wiki/Cubic_function.

As others have noted, the shape of the body does influence the miles per gallon. However, any specific body style is subject to this law, and any production body style will lose MPGs as velocity increases.

As for the braking, that is a little bit more self evident. The kinetic energy in the car takes power to produce. When you brake, the kinetic energy is turned into heat via the brakes, requiring you to input more energy to achieve more kinetic energy. This lowers efficiency.

The best way to see this in action is if you have a car that is all "high tech" with a digital instant read MPG display. Then find yourself a nice long stretch of freeway. In my particular car, at 65mph I get about 29mpg on the highway. Lower the speed to 55 and I get 34.5mpg.

It may also be interesting to note that back in the 1970s the federal government implemented the national speed limits at 55mph to save gas during the national oil crisis. It was not until 1995 that the national speed limit was raised.

To end, as many here will agree, it is your gas and your money that are used in the car. If you want to go 80mph and get less MPG's the government should not be the stopping you.

Re: Driving Above 60 MPH Uses More Gas?

Caley McKibbin — Sun, 17 Apr 2011 05:43:58 GMT

Having been in a vehicle with a device that gives a real-time mileage rating, I can't say that I've observed supposed bell curve at those speeds. My guess is that the optimum is based on a much lower speed at which the last gear can produce sufficient RPM to keep the motor running.

Re: Driving Above 60 MPH Uses More Gas?

John James — Sat, 16 Apr 2011 14:03:10 GMT

But the point is, there is a "sweet spot"...and for the vast majority of vehicles, it's under 60 MPH.

Re: Driving Above 60 MPH Uses More Gas?

Clayton — Sat, 16 Apr 2011 08:08:23 GMT

It all depends on the aerodynamics of the body and the fuel consumption characteristics of the engine. A car with low aerodynamic drag will reach its optimal gas efficiency at a higher speed than a car with higher aerodynamic drag and the exact same engine. For a given engine, the most fuel-efficient speed is reached at the point where the ratio of fuel consumption to power output is minimum. This is somewhere between idle and redline. Idle is very fuel-inefficient as is redline. Somewhere between these two extremes is the engine's "sweet spot". Driving at the speed that places exactly that load on the engine will extract the maximum number of miles per unit fuel consumed.

In other words, there is no one magic number, it just depends on the car you're driving.

Clayton -

Re: Driving Above 60 MPH Uses More Gas?

Scrooge McDuck — Sat, 16 Apr 2011 05:49:03 GMT

Incentive wise, wouldn't the government want you driving faster? More speeding tickets, more fuel used and thus more fuel taxes, etc?

Re: Driving Above 60 MPH Uses More Gas?

John James — Sat, 16 Apr 2011 05:33:09 GMT

It's not propaganda. It's just a fact. The gas milage of an internal combustion engine forms a bell curve (well, I'm not sure if it's a perfect bell, but it has a peak). The way the engine works in conjuction with the physical world (i.e. wind resistance) means there is actually an optimum speed at which your car is most fuel efficient (meaning optimum MPG).

This is basically exactly what I've been told by all the car guys I know. (And a few of them are actually engineers by trade...but they could build a car).

Roughly speaking, the lower the speed, the higher the fuel efficiency (at highway speeds). Drag increases with speed, and it is a losing battle (e.g. doubling speed more than doubles the power input). For the vast majority of cars, 55 is more efficient than 65 is more efficient than 75, etc. So go as slowly as you are willing. The rule of thumb does not extend all the way to zero MPH, so there is an optimum point. For many cars it is around 40-50MPH, but it depends heavily on type of car, load, road/driving conditions, etc. But 40-50 MPH is well below highway speed, so that is moot for this discussion.

If you're interested in the engineering and physics of it, check out here:

http://auto.howstuffworks.com/question477.htm

Oh here's another nice write up.

Re: Driving Above 60 MPH Uses More Gas?

Brutus — Sat, 16 Apr 2011 03:46:02 GMT

I noticed a difference between driving at 75-80 mph and driving at 60-65 mph. I didn't have to stop at much. Makes sense, I suppose, b/c the faster you're going, the more fuel you're using. Depends on the engine, I guess, and how it metabolizes fuel. idk much about cars, but...this brings up a topic I wish to enlighten everyone with.

I went to the doctor about 6 months ago, and I noticed the chart on the wall pertaining to blood alcohol level. Now, I don't pay much attention to that stuff, but being that I was bored for 20 minutes while waiting on the doctor to come in and say, "Don't worry, it's allergies," I read where the average adult male needs to consume 4 drinks in one hour to be over the legal limit to drive in my state.

Having talked with a number of people, they think that the number of drinks is grossly overestimated to hit the legal limit. I have no idea; all I know is that after 2 drinks in the period of an hour, I'm waiting around another hour to drive home. Many people are skeptical, and if college students, for instance, read the same chart and think, "I can have 8 drinks in 2 hours and be fine to drive," the state makes a hell of a lot more money, thus "justifying" its law enforcement methods that much more.

What say you?

Re: Driving Above 60 MPH Uses More Gas?

SirThinkALot — Sat, 16 Apr 2011 03:42:31 GMT

You will burn more gas the faster you go, due to the added wind resistance. Also constantly accelarating and decelarting will burn more gas(thats why driving on highways is actually more fuel efficent than driving in town, despite the higher speeds).

Although for some people the extra time spend commuting might not be worth the extra money saved on gas. So I really dont see why it matters.