Which two methods can a driver use to know when to shift up through the gears?

49 CFR 383.111(a)(5); AAMVA CDL Manual Section 2.3.1, p.2-9

Shifting at the wrong rpm can lug the engine or over-rev it, both of which waste fuel and wear out the drivetrain faster. Learning to read either your tachometer or your speedometer gives you a reliable way to time every upshift correctly, even if one gauge is hard to read. The exam expects you to know both methods exist and that either one works.