Fill in these blanks:
Truck speedometer reading = ______ (just for comparison to GPS)
GPS speedometer reading = ______ (at same time of above reading)
Tach RPM = ______ (at same time of above reading)
Tire diameter = ______ (because I'm too lazy to look up a 275/40/20
)
From that, can tell you your rear gear. Truck speedo will not be accurate with a tire change unless that tire just happens to be the same diameter as the original tires, which is not likely.