Hello, I have a 96 f250 2wd 351w 5spd zf. I recently swapped the original speedo/cluster for one with a tach. The new one came from a 96 also but 4x4 and think it was a manual trans, no letters in the cluster for prnd21. I finally got to drive it for a good distance today and wanting some opinions. At 65mph the tach reads 1800rpm, is that about right ? Since new plugs,wires, cap, rotor, air filter and oil change I'm getting about 15 mpg on the highway, good,bad,average ? 186K miles. Thanks Rick

 

Got around to doing somemore research and found a formula for RPMs.
RPM= mph x gear ratio x 336 ( x .76 {OD}) / tire diameter.
65 x 3.55 x 336 x .76 / 33 = 1786

 

I'd say your fuel economy is at least average. That RPM at that speed sound about right to me. Have your reprogrammed the PSOM for the 33 inch tires? Your speedometer and odometer may be reading a little low with the tall tires.