TM 9-2320-209-10-1 FRAME 11 1. Trucks covered in this manual should be operated within the following rpm limits, as shown on tachometer (l): (a) 1200. Lowest engine rpm when operating under load. (b) 1400-1800. Engine rpm recommended for downshifting. (c) 1400-2200. Engine rpm recommended for cruising. (d) 2600. Highest engine rpm recommended when pulling a very heavy load and when going up a very steep hill. 4-54