6 ms·

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 (re

• 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.
    • taeric 12y ago
      Apologies for missing this. I have limited understanding of CVT. I would include ideas like CVT in my question, though.
  • toast0 12y ago
    For an automobile, at cruising speed, the force of the drivetrain pushing the car forward, and the forces of drag and friction are balanced (If the net force is zero, the acceleration is zero). In the engine and drivetrain, force per unit of fuel is dependent on velocity (this is probably a pretty complex dependency), but you can probably tune the system to have peak efficiency at whatever speed you want. For aerodynamic drag, it's a function of the square of velocity. You can certainly work to lower the drag coefficients, but whatever the force is at 39 mph, at 55 mph it will be about double, and about 78 mph will be double that. Realistically, it makes sense to put the peak of engine and drivetrain efficiency in the range people are going to be driving the most; so this is why many vehicles will be most efficient around the 55-70 range. If you're willing to radically change behavior, you would likely have a much more efficient vehicle if you tuned for 40 mph, and people drove it at 40 mph. At lower speeds, other frictional forces become more significant as well, so maybe tuning for 5 mph isn't a great idea.
  • exue 12y ago
    Longer gearing will improve the high-end efficiency where it wasn't yet optimal - to help 90MPH you could target that with your super 7th gear at "ideal" torque/RPM. However the overwhelming aerodynamic drag means the peak efficiency won't shift to the right much, and it'll probably stay around 40-45mph (the power required is proportional to the cube of speed and 11x stronger at 90MPH than 40MPH) [4]. In many cases that super 7th gear wouldn't help 40mph (peak) at all if 5th/6th had already optimized 40mph. I think the 55-70 range is pretty optimized on production cars today, including those with 5 or 6 speed transmissions - but you may be able to make more gains with longer gears/a CVT at speeds above that. In an earlier age of 3-4 speed autos (and sometimes a national 55mph limit), manufacturers had fewer gears to work with so a super long gear for 80MPH efficiency would trade off midrange efficiency and acceleration, and the EPA didn't test that anyway until 2006 w/ higher highway speeds [4] My reference on the ~40MPH ideal speed (which I'm now more convinced about is the ideal for cars today) is the Motor Trend [1] test (all economy sedans peaked 35-40ish), several Hypermiler/car specific forums [2] and some personal cruise control tests with some rental/Zipcars with a digital gauge. Other ways to test include Scangauge/the Torque Android app. (The reason the ~40mph peak isn't even lower, is due to gas engines especially larger ones, being less efficient if they produce too little power - there is a minimum RPM at idle and always friction). So my earlier statement of "optimal peak would have been 30-40" - it's already around 40, and it doesn't have to achieved in top gear. I put "ideal" in quotes earlier since the torque peak is one of many factors for the engine - the ideal cruise RPM is almost always lower due to less engine friction and lower pumping losses when you aren't asking for full power. If you ask for more power, your optimal RPM goes up closer to the torque peak. A generic motor from a friend's auto engineering class [3] (If you say had a Honda S2000 with a torque peak at 7500rpm, cruising there would kill your mileage) Yeah, though by 4-wheels-on-ground I was mostly referring to flying/maglev type vehicles, or perhaps a vastly different design that had very little aerodynamic drag [1] http://image.motortrend.com/f/roadtests/sedans/1208_40_mpg_compact_sedan_comparison/38489659+w968/40-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... [2] http://www.metrompg.com/posts/speed-vs-mpg.htm http://www.metrompg.com/posts/speed-vs-mpg.htm http://www.metrompg.com/posts/photos/florida-speed_vs_mileage-z.gif http://www.metrompg.com/posts/photos/florida-speed_vs_mileag... http://www.metrompg.com/posts/rpm-mpg.htm http://www.metrompg.com/posts/rpm-mpg.htm [3] http://i.imgur.com/5MEEDnx.jpg http://i.imgur.com/5MEEDnx.jpg - the asterisk would be near the torque peak