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?
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.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.
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.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.
Then, best mileage will be around the rpm's which give the highest torque with the highest gear. Does it make sense?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.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.
Almost correct - except you are interested in DISTANCE/gallon not TIME/gallon.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
If you went 2000rpm at 60 then you MUST be doing 2333 at 70. Which must give the same ratio. The gears are locked. For every rotation of the engine it does a certain rotation of the wheel.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.