
Originally Posted by
ebbfolls
That depends on what you mean by efficiently. If you mean efficient as in the quickest way to accelerate, theory dictates that you shift up ONLY at the roadspeed that will cause your next higher gear to spin the engine at the rpm slightly lower than the rpm where it is making the most TORQUE. Fpr example, if your car is running at 40 mph and the engine is spinning at say 5,500 rpm in first gear, and that 2nd gear at 40 mph vehicle speed would spin the engine at 4,200 rpm, while peak torque is found at 4,500 rpm, then yes 5,500 rpm is your most efficient shift point when shifting from 1st to 2nd. If you upshift from 1st at 30 mph, the engine would only be spinning at 4,125 rpm and would then translate to 3,150 rpm at 2nd gear, which is way below the 4,500 rpm at which max torque occurs(depending on the torque curve characteristic of the engine, this may not be too bad--but if the engine is "peaky" it would really make a big difference).
On the other hande, if you mean efficient as in frugal, then it would be best to shift at the vehicle speed that would allow the engine to spin at an rpm enough for it to make the torque required to move the car in the next higher gear. The engine is operating at its leanest at the least amount of throttle opening for a given speed and load. So, in order to be fuel efficient, one needs only to push down on the accelerator gently and shift up soon enough as to allow the engine to pull the car without need for excessive accelerator pedal pressure(which may be accompanied by knocking).