Mileage questions

[WyrmDL]

So when you're driving, is fuel consumption based on RPM? Say if you're going at 2000 RPM in 2nd gear, versus 2000 RPM in 4th gear. What's the difference? Does it work that way?

And when you're using the air conditioning, does it suck up more fuel/power when you're using it on high rather than on low, or if it's on a cooler setting rather than a warmer setting?

I'm a newbie when it comes to cars, and I've always been wondering. Thanks!

 

[Munjibunga]

Fuel consumption is measured in miles per gallon (mpg). If you drive a certain distance in 2nd gear, your mpg will be lower than if you drive the same distance in 4th gear. Yes, running the air conditioning does increase fuel consumption; the lower the temperature, the worse the mileage. Mythbusters did a show on whether running the air conditioning burns more fuel than driving with the windows open. I forget the results.

 

[RosieB]

I burn substantially more fuel, like astronomically more, in my Astro van while using the ac than I do with the windows open.

I would think you'd use more fuel at lower gears while getting up to speed, then less while cruising (just maintaining) in a higher gear once you're there? (Yes, more if you start in a higher gear vs a lower gear?)

 

[tplyons]

Mythbusters did a show on whether running the air conditioning burns more fuel than driving with the windows open. I forget the results.

I believe the cutoff was ~45 mph. Under 45 mph, having the windows down is more efficient, whereas over 45 the increase in drag on the car makes AC more efficient.

As far as 2000 rpm's, the engine is doing the same amount of work in either gear, would be burning the same amount of gasoline.

 

[WyrmDL]

As far as 2000 rpm's, the engine is doing the same amount of work in either gear, would be burning the same amount of gasoline.

Alright, that's the main thing I was wondering about.

With the AC though, there's the factor on the fan setting. Does it suck up fuel/power when you have it on high fan, rather than low fan?

 

[RosieB]

Alright, that's the main thing I was wondering about.

With the AC though, there's the factor on the fan setting. Does it suck up fuel/power when you have it on high fan, rather than low fan?

It's all bad for me. I like to pretend that low is better, but I don't know for sure. I do know I get less of a drag on the engine on low than on high, but not much, and turning the ac off is what makes the biggest difference. I turn it off when starting from a stop or driving uphill, then turn it back on. It's a hassle, but it makes a major difference in my gas consumption.

 

[elgecko]

Alright, that's the main thing I was wondering about.

With the AC though, there's the factor on the fan setting. Does it suck up fuel/power when you have it on high fan, rather than low fan?

The fan runs on electricity. Crank it to 11!

Two big factors in maximizing fuel economy are how you accelerate (easy does it) and keeping your max speeds down (e.g. 65 mpg).

 

[WyrmDL]

So it really doesn't matter if I'm running the fan on low or high does it? Either the AC is on or off, right? I always tried to keep it on low, because I thought it would save me gas...

 

[Sleepwalker42]

To answer briefly, as far as I know:
- RPM counts in fuel consumption, the lower the better
- AC warmer saves you milage
- fan up doesn't make any difference.

 

[escaraBAJO]

AFAIK, although RPM are the same, the higher the gear, the less fuel you burn.
Don't know why though.
Just look at the specs from new cars: driving at 90 km/hr (around 50-55 mph) in 5th gear gives you the more economical figures.

 

[Quickie]

Fuel consumption is measured in miles per gallon (mpg). If you drive a certain distance in 2nd gear, your mpg will be lower than if you drive the same distance in 4th gear. Yes, running the air conditioning does increase fuel consumption; the lower the temperature, the worse the mileage. Mythbusters did a show on whether running the air conditioning burns more fuel than driving with the windows open. I forget the results.

FYI


When it gets warm outside, your air conditioner becomes your best friend. This is most definitely true in your car, where temperatures can sky rocket to 170 degrees under direct sunlight. But most people these days are under the impression that the more you run your air conditioner the more gas your car will guzzle. Now that gas prices are higher than the cost to eat at McDonald's, people are starting to take precautions and do what they can to help with their cars fuel economy. The assumption that running your air conditioner burns way more gas is somewhat untrue. Leaving your air conditioner off makes barely any difference in the amount of gas you burn. With the advent of modern, more fuel efficient cars, this myth is busted!

The air conditioner draws its power from the engine which uses some gas, but with today's cars, this use of gas is minute. The air conditioner can decrease the fuel economy of your car up to 20% in some vehicles, but it depends on the type. Driving with the air conditioner is more fuel efficient than driving with the windows down. Driving with the windows down creates drag which means that your engine has to work harder to keep your car at the current speed; this burns more gas than if you just ran your air conditioner.

In the long run, using your air conditioner is far better than any other means of cooling yourself off in the car, unless you're driving in traffic. When you drive in traffic you aren't going very fast so the drag on the car is minimal. In these cases, you burn more fuel running the air conditioner than having the windows down. That makes sense. When you drive on the highway you can use either the air conditioner or the "window down" method to cool yourself, because, honestly, there is no noticeable difference in gas usage at those speeds.

 

[tastybasslines]

Mileage info for trucks: If you are traveling long distances, it is better to keep your tailgate UP.

 

[Munjibunga]

I believe the cutoff was ~45 mph. Under 45 mph, having the windows down is more efficient, whereas over 45 the increase in drag on the car makes AC more efficient.

As far as 2000 rpm's, the engine is doing the same amount of work in either gear, would be burning the same amount of gasoline.

Yes, but you'd be traveling further at the same RPM in the higher gear, hence your mpg would be higher (better) in the higher gear.

 

[hartke20g]

FYI


When it gets warm outside, your air conditioner becomes your best friend. This is most definitely true in your car, where temperatures can sky rocket to 170 degrees under direct sunlight. But most people these days are under the impression that the more you run your air conditioner the more gas your car will guzzle. Now that gas prices are higher than the cost to eat at McDonald's, people are starting to take precautions and do what they can to help with their cars fuel economy. The assumption that running your air conditioner burns way more gas is somewhat untrue. Leaving your air conditioner off makes barely any difference in the amount of gas you burn. With the advent of modern, more fuel efficient cars, this myth is busted!

The air conditioner draws its power from the engine which uses some gas, but with today's cars, this use of gas is minute. The air conditioner can decrease the fuel economy of your car up to 20% in some vehicles, but it depends on the type. Driving with the air conditioner is more fuel efficient than driving with the windows down. Driving with the windows down creates drag which means that your engine has to work harder to keep your car at the current speed; this burns more gas than if you just ran your air conditioner.

In the long run, using your air conditioner is far better than any other means of cooling yourself off in the car, unless you're driving in traffic. When you drive in traffic you aren't going very fast so the drag on the car is minimal. In these cases, you burn more fuel running the air conditioner than having the windows down. That makes sense. When you drive on the highway you can use either the air conditioner or the "window down" method to cool yourself, because, honestly, there is no noticeable difference in gas usage at those speeds.

i don't particularly like that statement, so let's try to clear this up a bit.

at high speed, having your windows open typically does create enough drag to make it more economical to use AC.
in situations where you're typically at low speeds (i.e. start-stop city traffic) the amount of fuel you use just getting up to speed from stopping/slowing down is significantly more than simply maintaining speed, so since you'll be getting crummy mileage anyway, just go ahead and use the AC (windows open however, is the more fuel efficient way to keep cool). want to save the most fuel? windows up, AC off. sucks, don't it?

now, when you're on the highway, we've established that "windows open" creates too much drag. so run the AC.
but wait- doesn't the AC use power from the engine?
yes. but you have to understand how vehicle AC systems work, which is *essentially* the same as a refrigerator. the compressor (driven by the belt [and thus also the crankshaft]) has an electric clutch. when the compressor is disengaged, the pulley on the compressor is allowed to spin freely. when the compressor 'turns on', the clutch on its pulley engages and uses the belt to run the AC compressor. [that is the "click" you can hear periodically outside your vehicle; also electric cooling fans if you have them]
just like a refrigerator, the compressor only "kicks on" when the temperature sensor feels that the ambient temperature inside is too high for the current setting. so because the compressor only runs intermittently, it uses less energy than the amount of energy needed to overcome the drag of having your windows open. (obviously not in every case; there's always the possibility of extenuating circumstances)


i'm a bit long-winded; i oversimplified some things; i under explained other things. i'm sure someone will chime in and do a better job than i did.
hope this helps you in your quest to understand why.

 

[tplyons]

Yes, but you'd be traveling further at the same RPM in the higher gear, hence your mpg would be higher (better) in the higher gear.

Correct, mileage per gallon would be better as you would be traveling faster over the same period of time, leading to longer distance with the same amount of gasoline.

Bit of a trick question since more than the engine is involved

 

[Quickie]

i don't particularly like that statement, so let's try to clear this up a bit.

at high speed, having your windows open typically does create enough drag to make it more economical to use AC.
in situations where you're typically at low speeds (i.e. start-stop city traffic) the amount of fuel you use just getting up to speed from stopping/slowing down is significantly more than simply maintaining speed, so since you'll be getting crummy mileage anyway, just go ahead and use the AC (windows open however, is the more fuel efficient way to keep cool). want to save the most fuel? windows up, AC off. sucks, don't it?

now, when you're on the highway, we've established that "windows open" creates too much drag. so run the AC.
but wait- doesn't the AC use power from the engine?
yes. but you have to understand how vehicle AC systems work, which is *essentially* the same as a refrigerator. the compressor (driven by the belt [and thus also the crankshaft]) has an electric clutch. when the compressor is disengaged, the pulley on the compressor is allowed to spin freely. when the compressor 'turns on', the clutch on its pulley engages and uses the belt to run the AC compressor. [that is the "click" you can hear periodically outside your vehicle; also electric cooling fans if you have them]
just like a refrigerator, the compressor only "kicks on" when the temperature sensor feels that the ambient temperature inside is too high for the current setting. so because the compressor only runs intermittently, it uses less energy than the amount of energy needed to overcome the drag of having your windows open. (obviously not in every case; there's always the possibility of extenuating circumstances)


i'm a bit long-winded; i oversimplified some things; i under explained other things. i'm sure someone will chime in and do a better job than i did.
hope this helps you in your quest to understand why.

You forgot that the computer kicks in a solenoid to compensate for the compressor drag.....you cant feel it but it raises the engine rpms to compensate for the additional drag.

But I agree with you mainly.

 

[1958Bassman]

So when you're driving, is fuel consumption based on RPM? Say if you're going at 2000 RPM in 2nd gear, versus 2000 RPM in 4th gear. What's the difference? Does it work that way?

And when you're using the air conditioning, does it suck up more fuel/power when you're using it on high rather than on low, or if it's on a cooler setting rather than a warmer setting?

I'm a newbie when it comes to cars, and I've always been wondering. Thanks!

It's based on a lot of things- fuel injection uses barometric pressure, air temperature, air flow rate, RPM and coolant temperature, among others. A cold engine isn't efficient, so it uses more fuel before it reaches normal operating temperature. Once there, hard acceleration, under-inflated tires, bad alignment, head wind, hills and additional loads will make the largest differences. Fuel with ethanol isn't as good as plain gas but it's not a huge difference when it's fresh. If it has been sitting for a long time, it's definitely a problem.

As far as electrical loads, there's no such thing as free energy, so an electrical motor that draws a lot of current at high speeds will cause a bit more load on the engine. A big stereo will cause a lot more load, though. The alternator resists the engine's rotation when it's under more load and that's why the RPM will drop when all of the accessories are on.

 

[Quickie]

It's based on a lot of things- fuel injection uses barometric pressure, air temperature, air flow rate, RPM and coolant temperature, among others. A cold engine isn't efficient, so it uses more fuel before it reaches normal operating temperature. Once there, hard acceleration, under-inflated tires, bad alignment, head wind, hills and additional loads will make the largest differences. Fuel with ethanol isn't as good as plain gas but it's not a huge difference when it's fresh. If it has been sitting for a long time, it's definitely a problem.

As far as electrical loads, there's no such thing as free energy, so an electrical motor that draws a lot of current at high speeds will cause a bit more load on the engine. A big stereo will cause a lot more load, though. The alternator resists the engine's rotation when it's under more load and that's why the RPM will drop when all of the accessories are on.

^ This. Great points.

 

[HugeInJapan]

Fuel efficiency is related to how hard you are working your engine, not RPMs. If you sit your car at idle at rev the engine up to 2500 rpm it will be consuming significantly less fuel than while you are travelling down the road (regardless of speed) with the engine RPMs at 2500. That being said you will be more efficient at 2500 RPMs in second gear than in fourth there is much less wind resistance at slower speeds and so your engine will work less and consume less fuel.

As for A/C.. I don't know which is more efficient A/C on or windows up. But in many cars temp setting is irrelevant. This is because instead of turning your A/C off to keep the car from getting too cold the heat will be turned on. This may seem counterintuititve but the reason behind it is to save the clutch on your A/C compressor which is far more expensive than losing a few 10th's of a mile per gallon.

 

[WyrmDL]

Fuel efficiency is related to how hard you are working your engine, not RPMs. If you sit your car at idle at rev the engine up to 2500 rpm it will be consuming significantly less fuel than while you are travelling down the road (regardless of speed) with the engine RPMs at 2500. That being said you will be more efficient at 2500 RPMs in second gear than in fourth there is much less wind resistance at slower speeds and so your engine will work less and consume less fuel.

Hmm. That's good to know. I always thought that how hard you worked you're engine was based on the RPMs.