The aerodynamics of an AD truck is similar to a billboard. Fighting wind resistance requires engine torque. The torque of the 235 is shown on a curve of a GM RPM/torque/HP graph. The sweet spot or power band will be between RPM points below and above the maximum torque. If you look at the graph, we can agree (to disagree) that at 1200 the engine is "getting it's legs" and after 2800 torque takes a big nosedive. The problem is that the engine(s), that provided data points for the graph, was (were) brand new. The cylinder compression is the best that it's ever going to get. The Rochester B was new and didn't have leaking problems. Fuel to air ratio was perfect for the needs of the engine testing. So, the torque or power band of today's used 235 will be less than the GM graph.
If at 65 MPH we want to speed up, do we want to press down the accelerator pedal or do we want to downshift? I for one would like some torque in reserve so I don't have to shift. This means I want to be somewhere inside the power band. Using 52Carl's setup as an example, I plugged the numbers into the calculator (
http://www.wallaceracing.com/gear-speed.php). I also added 3.42 and 3.73 ratio rear axles for comparison.
I made a rough graph (questionable accuracy) for explanation purposes. The yellow is the power band. The red, blue and green represents the slope of the respective rear axle ratios. I like to look at the center of each slope, within the power band, to find the MPH below. The idea being you pick the rear axle for the highway cruising speed you want.
