Assuming 235/80/16 tires are 30.8" diameter (that is the spec I found online), 2600 rpm should yield 61 mph with a 3.90 rear gear, not 55 mph. The tach and/or the speedo is off. Try it with a GPS speedo function on a smartphone to determine which.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
| 
