Okay. Speedometer doesn't work, tachometer exists and does work, properly. Right? All that crawling around under the truck sounds like dirty and unnecessary work unless you have good reason to believe that your ratio is inadequate, and you want to know what you have, so you can speculate on what you need.
Let's say, just for the sake of argument, that your engine is a '55 235 thriftmaster. Maximum torque is 200 pounds-feet at 2,000 rpm. Maximum horsepower is 112 at 3700 rpm. Therefore, your sustained-full-load operating range (outside of which you will probably hurt your engine, if you are in the habit of driving the bag off your truck) is 2,000 - 3,700 rpm. Above that range, your valves will begin to float (not fully close) and your engine will begin to hammer itself apart. Below that range, upper-cylinder temperatures will quickly rise and your engine will begin to burn inside.
These numbers are not set in stone, even for the '55 235. I'm just quoting them as guidelines, so as to make a point.
Let's say you're working your truck for a brick plant, doing deliveries. These trucks are so pretty, sometimes at this late date it is difficult to remember that they were designed to work like oxen. So, okay. You work your truck hard. You need to keep up with traffic as much as you can while you're loaded, but you also want it not to wear out for a long time. What you do, then, while you're running with your foot on the floor all the time is avoid overspeeding your engine by picking a higher gear, and avoid lugging (underspeeding) your engine by downshifting. Now, a lug condition exists when you are running with an open throttle, close to the bottom of your powerband, and the engine is not picking up speed.
If you are in the midrange of your power band, say, from 2400 rpm to 3300 rpm, you are okay.
If you are not at full throttle, you are okay.
To make your engine go further between overhauls, avoid the top end of your powerband except when you absolutely need to, as when lifting a full load of bricks from a traffic light up the steepest grade you can find around where I live, which is plenty steeper than anything around D/FW, unless you have acquired some of these;
[img]http://thumb11.webshots.net/t/55/455/1/17/97/2005117970096215731NlbdoZ_th.jpg[/img] of which I didn't see any the last time I was down there.
So to answer your question - at long last - for the kind of driving you'll most likely be doing - empty, on flat ground - since you'll rarely be using full throttle, drive for economy: Use the least throttle and the least engine speed to get you up to your cruising speed, then maintain a steady speed. From the traffic light up to 50, you can probably shift at 10 and 35 mph. Or, since your tach is working, shift up at 2500, and down at 1500. Use a light throttle, just enough to pick up speed. If you need more power, use more throttle and more engine speed. You won't hurt it: Mine likes being run hard when it is loaded.
The best fuel mileage and longest life that can be achieved by any engine is to be found near its peak torque speed, with a steady, light throttle.
