Will my speedometer be off after installing the lift kit?

Only if you intend to install a different diameter tire than the vehicle originally had installed. If installing larger tires your speedometer will read lower than the actual speed you are traveling. How far it will be off depends on the gear ratios and difference in tire sizes. There are solutions, there are electronic programmers on the market able to calibrate for changes in tire size as well as mechanical fixes for older vehicles. There are also GPS speedo apps for your phone that can be used to check your speed and validate accuracy.