vivesdn
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I imagine that this magnitude is dependent on a lot of factors, but does anybody know a good source to get the relationship betwen speed and mpg for at leas families of cars?
russ_watters said:The optimum speed (with no air conditioner running) is generally the lowest speed that you can easily maintain in your highest gear.
I remember this same belief from the 1970's, but it wasn't true. For cars back then the optimum speed was around 45mph. This is because gasoline engines are very inefficient at producing small amounts of power, and running at higher speeds, with more of a power load, results in better milage, because the increase in rate of fuel consumed is less than the increase in the rate of speed.The optimum speed is generally the lowest speed that you can easily maintain in your highest gear.
Jeff Reid said:..snip...
So other than a 1300cc VW bug from the 1970's, or a hybrid, the optimum speed for most cars will be between 45mph and 55mph.
That's where the "easily" part comes in.rbj said:what exactly do you mean by "maintain", Russ? i don't think "lugging an engine" is the most efficient use of it. even on perfectly flat and smooth pavement.
I drive stick, but don't most automatics only have 4 gears? Also, gear ratios vary a lot by engine and type of car. Ie, an SUV will be geared a lot lower (or at least be able to handle a lower rpm). So I think that's too narrow a range - maybe 35-55.Jeff Reid said:Modern cars have fairly tall gearing in the form of overdrive, and better aerodynamics than cars of the 1970's, so they should be more efficient at higher speeds. Considering that EPA measures highway milage at 55mph (this has just changed), the car designers may have designed their cars to get the best milage at 55mph.
So other than a 1300cc VW bug from the 1970's, or a hybrid, the optimum speed for most cars will be between 45mph and 55mph.
russ_watters said:I drive stick, but don't most automatics only have 4 gears?
Jeff Reid said:This is because gasoline engines are very inefficient at producing small amounts of power, and running at higher speeds, with more of a power load, results in better milage, because the increase in rate of fuel consumed is less than the increase in the rate of speed.
The amount of power generated for the amount of fuel consumed is best, but because of aerodynamic drag, it's consuming more fuel per mile traveled.vivesdn said:I imagine that regime for highest torque is likely to be the most efficient (power vs fuel), and using the highest gear will produce the lowest number of explosions for a given mileage.
The formula is such that 2x velocity will result in 4x wind resistance.Cameronblmr said:wind resistance at 60 mph is not 2x the resistance at 30mph
Almost correct - except you are interested in DISTANCE/gallon not TIME/gallon.kharding said:Since RPMs have a pretty close correlation to fuel consumed, don't we want the maximum speed per RPM? So if 2000 RPMs get me 60mph that is .030mph per RPM. At 70mph, my tach reads 2500 so that's .028mph per RPM
mgb_phys said:Almost correct - except you are interested in DISTANCE/gallon not TIME/gallon.
So at 60 you use a certain amount of fuel per min and at 70 you use 2500/2000 = 125% as much fuel but only go 70/60 = 116% as far per min so you are loosing.
But if you went at 2000RPM at 60 and 2300RPM at 70 you would be doing better at 70.
Since you would only be using 2300/2000 = 115% as much fuel to go 116% as far.
DaveC426913 said:This chart was done a few years ago on my 1993 Chrysler Intrepid:
http://www.davesbrain.ca/miscpix/mileage.gif
What I find odd about it is:
- the lack of curvature. I would have thought it would level out to the left.
- the minimal effect of the AC. It has a smaller effect than changing highway speed by 20kph.
mgb_phys said:Basically on a freeway you drive as slowly as possible in the highest gear.
Have you thought of converting it to just burn dollar bills - might be cheaper!Danger said:or if I put my foot in it on the highway, it was about 4 or 5 mpg.
mgb_phys said:Have you thought of converting it to just burn dollar bills - might be cheaper!
MechSoup said:I have always thought that fuel mileage was based on RPM. A lower, average, RPM would result in a higher fuel mileage.
Adrock1795 said:peak torque which depends on the power band of your camshafts
Adrock1795 said:From what i can tell, on a gasoline engine when you hit the throttle it opens the butterfly valve on the throttlebody which increases the air intake which is then mixed with fuel to create a constant air-to-fuel ratio maintained by the ECU. The mixture of fuel and air is then is then released into the cylinder chamber when the camshaft hits the valve lifter.
Camshaft speed is dependent to RPM speed, so the faster your camshaft is rotating the more fuel/air goes into the cylinder per minute.
So to use the less possible fuel you would need to be at the lowest possible RPM without stalling the engine.
But to get the most efficient power from your fuel, you would need to drive at the lowest RPM where you hit peak torque which depends on the power band of your camshafts
Adrock1795 said:From what i can tell, on a gasoline engine when you hit the throttle it opens the butterfly valve on the throttlebody which increases the air intake which is then mixed with fuel to create a constant air-to-fuel ratio maintained by the ECU. The mixture of fuel and air is then is then released into the cylinder chamber when the camshaft hits the valve lifter.
Camshaft speed is dependent to RPM speed, so the faster your camshaft is rotating the more fuel/air goes into the cylinder per minute.
So to use the less possible fuel you would need to be at the lowest possible RPM without stalling the engine.
But to get the most efficient power from your fuel, you would need to drive at the lowest RPM where you hit peak torque which depends on the power band of your camshafts
MechSoup said:I have always thought that fuel mileage was based on RPM. A lower, average, RPM would result in a higher fuel mileage.
DaveC426913 said:adult driving?