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The two indicators that determine when you should upshift are

A Engine speed and road speed
Explaination
There are two indicators that determine when you need to shift into a higher gear. One indicator is the engine speed (rpm). You should read your vehicle's manual to learn its rpm range and shift up when you reach the top of that range. The other indicator is road speed (mph). You should learn which speeds each gear can accommodate, then keep an eye on the speedometer and shift up as needed.
B Other trucks and road conditions
C Timing and road position
D Oil pressure and air brake pressure