A stock engine (no "hot" cam) in good shape will probably have around 22" going downhill with your foot off the gas, 18" at idle, near 8" at a 30 MPH cruise on flat ground and less as you add more load. Uphill at 50 mph? Practically zero. 12-15 inches should do well,
'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.
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