10 ms·

It's not always the most efficient for every car, but there are lots of charts showing this tradeoff. Some cars are more efficient at 65 or 70 than 55 (97 Celi

by exue 12y ago

It's not always the most efficient for every car, but there are lots of charts showing this tradeoff. Some cars are more efficient at 65 or 70 than 55 (97 Celica), but in modern gas cars, 40-50mph is where you hit a peak, and you start getting worse from there. 40mph on the highway is painfully slow though, so people generally pick an optimal point higher than that. (One inconsistency I've found is how the Motor Trend test below sees a very high peak at about 40MPH and a decline after, while other sources show a flatter peak). And 55MPH seems more like a magic number the author picked, but not a bad one if we don't know the model. Generally cars are geared so the engine begins entering its optimal / most efficient RPM range where it can produce more torque in top gear around 40-50mph, and rolling resistance and wind drag (which is a cube function of speed) contribute to make mileage worse after that. Usually the speed at which you first shift into top gear and cruise comfortably is close to optimal. Efficiency doesn't start dropping for a while because of the cubic nature of drag, and the engine sometimes is more efficient at higher RPMs for a bit (see nols' post on manufacturer optimization). Fun fact: At top speed (254mph), a Bugatti Veyron will use its 26 gallon tank in 12 minutes. An older chart: http://upload.wikimedia.org/wikipedia/commons/a/a9/Fuel_economy_vs_speed_1997.png http://upload.wikimedia.org/wikipedia/commons/a/a9/Fuel_econ... A newer chart: http://blog.automatic.com/wp-content/uploads/2013/12/mpg-vs-speed-all.png http://blog.automatic.com/wp-content/uploads/2013/12/mpg-vs-... Motor Trend chart: http://image.motortrend.com/f/roadtests/sedans/1208_40_mpg_compact_sedan_comparison/38489659/40-MPG-comparison-constant-speed-chart.jpg http://image.motortrend.com/f/roadtests/sedans/1208_40_mpg_c... http://www.motortrend.com/roadtests/sedans/1208_40_mpg_compact_sedan_comparison/ http://www.motortrend.com/roadtests/sedans/1208_40_mpg_compa... (One thing that bothered me is how the Civics in the last two charts were so different)

taeric 12y ago
As I stated in a sibling, this still boils down to "this is how current cars are built." I would imagine if you looked at the average vehicle built in, say, the 1940s, you would see a vastly different "optimal" number. Which is all to say, do we expect this to stagnate forever at 55? Is this a hard physical limit? I understand drag gets higher there. Are there no tricks left to us? And again, I fully concede that this is likely not the most pressing fact around. Just one I am curious on.
- exue 12y ago
  OK, now I understand where you're coming from. The physical limit is that at higher speed, we must expend more energy per unit of distance, to overcome drag (relatively little is lost in braking or rolling resistance of tires.) [1] So on any planet with an atmosphere you will have to contend with this. The 'ideal' speed isn't really even 55MPH, but lower if you had all variables at play to get the maximum MPG at any speed (probably 30-40MPH), but manufacturers expect people to cruise on the highway faster so they adjust the gearing. If you're asking how to push out the curve so that going faster than 75MPH doesn't offer a huge loss of speed, lower drag coefficients are the trick. Or switching out of the 4-wheels-on-ground automobile. [1] http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Energy_flows_in_car.svg/555px-Energy_flows_in_car.svg.png http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Ene... In this generic diagram, the ratio of drag (air)/rolling (ground) resistance is 11-to-7. As you get to higher speeds, the ratio tips even more in favor of drag. Fun fact: The Bugatti Veyron gets 2.15MPG at its top speed of 250MPH.
  - taeric 12y ago
    This was exactly the angle I was looking for. I am still not entirely satisfied with this, as it does not flat out state why better gearing couldn't achieve some increase. That is, my naive understanding is aligned with what a sibling post said. That there is an "ideal" torque rating of my engine. Seems that if my cruising speed isn't stuck at that number, than some gearing changes could be made to put me there. Why does that not work? (Adding a 7th gear, for example, seems to be a natural idea.) I'm curious to hear that the ideal speed would actually be below 55. Do you have good references on that? Also, I'd assume the "switching out of the 4-wheels-on-ground" refers to such as trains and friends?
    - dragonwriter 12y ago
      > That is, my naive understanding is aligned with what a sibling post said. That there is an "ideal" torque rating of my engine. Seems that if my cruising speed isn't stuck at that number, than some gearing changes could be made to put me there. Why does that not work? (Adding a 7th gear, for example, seems to be a natural idea.) You could add more gears -- or you could just use a CVT.
userbinator 12y ago
When taking into account wind resistance, the optimal speed for efficiency varies greatly with the current weather conditions too: with a tailwind, the optimal speed goes up, while the opposite is true with a headwind. Usually the speed at which you first shift into top gear and cruise comfortably is close to optimal Although automatic transmissions and lack of tachometers are probably responsible for this, I've noticed that a lot of drivers remaining at the upper RPM range for a certain gear while cruising, when they could've sped up 2-3MPH and upshifted. I don't know if there's a term for this, but it's certainly recognisable as a passenger: the engine is much louder than it should be, often with accompanied higher levels of vibration and discomfort.
- vlasev 12y ago
  Over the long term you expect the weather to average out and cancel itself out of the calculation. The question is does the headwind lower your efficiency more than the equivalent tailwind raise your efficiency?
  - PeterisP 12y ago
    Yes, in general getting 50/50 up+down hills, as tail+head winds decrease your total efficiency, as if you drive both directions (getting both the advantage and disadvantage), then you tend to drive the disadvantaged half slower (but not using less power, so no efficiency benefits from that) = it affects you longer = it has more effect on efficiency.

by exue 12y ago

taeric 12y ago

As I stated in a sibling, this still boils down to "this is how current cars are built." I would imagine if you looked at the average vehicle built in, say, the 1940s, you would see a vastly different "optimal" number. Which is all to say, do we expect this to stagnate forever at 55? Is this a hard physical limit? I understand drag gets higher there. Are there no tricks left to us? And again, I fully concede that this is likely not the most pressing fact around. Just one I am curious on.

exue 12y ago
OK, now I understand where you're coming from. The physical limit is that at higher speed, we must expend more energy per unit of distance, to overcome drag (relatively little is lost in braking or rolling resistance of tires.) [1] So on any planet with an atmosphere you will have to contend with this. The 'ideal' speed isn't really even 55MPH, but lower if you had all variables at play to get the maximum MPG at any speed (probably 30-40MPH), but manufacturers expect people to cruise on the highway faster so they adjust the gearing. If you're asking how to push out the curve so that going faster than 75MPH doesn't offer a huge loss of speed, lower drag coefficients are the trick. Or switching out of the 4-wheels-on-ground automobile. [1] http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Energy_flows_in_car.svg/555px-Energy_flows_in_car.svg.png http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Ene... In this generic diagram, the ratio of drag (air)/rolling (ground) resistance is 11-to-7. As you get to higher speeds, the ratio tips even more in favor of drag. Fun fact: The Bugatti Veyron gets 2.15MPG at its top speed of 250MPH.
- taeric 12y ago
  This was exactly the angle I was looking for. I am still not entirely satisfied with this, as it does not flat out state why better gearing couldn't achieve some increase. That is, my naive understanding is aligned with what a sibling post said. That there is an "ideal" torque rating of my engine. Seems that if my cruising speed isn't stuck at that number, than some gearing changes could be made to put me there. Why does that not work? (Adding a 7th gear, for example, seems to be a natural idea.) I'm curious to hear that the ideal speed would actually be below 55. Do you have good references on that? Also, I'd assume the "switching out of the 4-wheels-on-ground" refers to such as trains and friends?
  - dragonwriter 12y ago
    > That is, my naive understanding is aligned with what a sibling post said. That there is an "ideal" torque rating of my engine. Seems that if my cruising speed isn't stuck at that number, than some gearing changes could be made to put me there. Why does that not work? (Adding a 7th gear, for example, seems to be a natural idea.) You could add more gears -- or you could just use a CVT.

userbinator 12y ago

When taking into account wind resistance, the optimal speed for efficiency varies greatly with the current weather conditions too: with a tailwind, the optimal speed goes up, while the opposite is true with a headwind. Usually the speed at which you first shift into top gear and cruise comfortably is close to optimal Although automatic transmissions and lack of tachometers are probably responsible for this, I've noticed that a lot of drivers remaining at the upper RPM range for a certain gear while cruising, when they could've sped up 2-3MPH and upshifted. I don't know if there's a term for this, but it's certainly recognisable as a passenger: the engine is much louder than it should be, often with accompanied higher levels of vibration and discomfort.

vlasev 12y ago
Over the long term you expect the weather to average out and cancel itself out of the calculation. The question is does the headwind lower your efficiency more than the equivalent tailwind raise your efficiency?
- PeterisP 12y ago
  Yes, in general getting 50/50 up+down hills, as tail+head winds decrease your total efficiency, as if you drive both directions (getting both the advantage and disadvantage), then you tend to drive the disadvantaged half slower (but not using less power, so no efficiency benefits from that) = it affects you longer = it has more effect on efficiency.