When we want to compare two quantities a and b, but both are measured quantities such that neither can be considered to be an exact value, we use a "percent difference." percent difference is calculated by taking the absolute value of the difference of the values divided by their average multiplied by 100, or a-b % difference = 100% for example, in a measurement where no calibrated speedometer is present, we measure two values of speed, ua-10.0 m/s ± 1.1 m/s and = 12.0 m/s ± 1.3 m/s. calculate the percent difference of the two speeds above.