If one uses too high a gear at low speeds, there will be tendency for the engine to "snatch" or "jerk" because there is insufficient torque to pull the vehicle. If one always does this, there will be quite a lot
of stress on the driveline in particular and this will result in higher wear and faster failure. However if this happens occasionally, it is not a problem. Do not be fooled into thinking that if you use as little revs
as possible, you will save fuel. The most economical engine speed is when the engine is developing peak
torque, and in a petrol car this may be 3,500 rpm. However if one is driving on a flat road, the torque requirement will be very low, and so one can comfortably run at a lower rpm. without "snatching", and save
fuel but once an obstacle is reached, you will have to change to a lower gear to get more torque to get past it, whether it is a slower car or even a slope in the road.