Originally Posted by
DanDombrowski
This however does not give the absolute best efficiency. That comes from fixing the engine at its best efficiency point (say X,XXX rpm and XX%load) and gearing to give the lowest drag force. The problem with that is is that if you change the gearing to give you a different speed and drag force, your engine load changes. If you try to figure that one out in your head (ie, which diff should I put in?), you’ll really get wrapped around the axle (pun intended) in a “which came first, the chicken or the egg?” type problem. The only way to solve it is to fix the gearing (low), run it through the whole range of RPMs, find the BEP of that system. Increase gearing, go through the whole process again. Increase gearing again, and again through the whole range of RPMs. Go through all that data and find the most efficient out of all that. That’s what the engineers do at car companies to determine the best gearing for your car, its not just “gear low for highway, gear high for city”.
Now, if the engineer that designed the car said “75% of the time, the customers using the car are going 55mph”, so I’ll use the first method and design for that speed. Guess what? 55mph is the most efficient speed, period. If the engineer uses the second method, it could be anything, but I guarantee you that the car was not designed to be most efficient at 75 mph (at least the US models) because we’re not technically allowed to go that fast. Why on earth would you design for a BEP at a speed you can’t attain? If you’re building a total highway cruiser (and they’re not) the best you can hope for is to design for efficiency at 65, 70 tops. In reality, engines, transmissions, and drag have not changed drastically since the 70s. Engine efficiency is based upon completeness of combustion, intake temperatures and exhaust temperatures. While engines may be able to rev higher now than in the 70s, that doesn’t make the general operation of them any different, and they will still have similar efficiency curves based on RPM and load, albeit somewhat better, and therefore the you will still get your best gas mileage at lower rpms in high gear, usually near 55mph.