Abstract: Tires can be accurately and quickly measured by using a tire caliper that outputs data in digital form. A measuring instrument determines the diameter of automotive tires, preferably automotive race tires, and displays the circumference or the diameter at the users option, The tool of this invention has a digital display including a relatively elongate caliper body formed with a measuring part adapted to make measuring engagement with the tire to be measured. A member freely slidable in a longitudinal direction of the caliper body and formed with a measuring part disposed opposite to the measuring part of the caliper. In one preferred embodiment, the measuring instrument is marked with a transparent linear scale fixedly disposed on the caliper body with its axis extending in the longitudinal direction of the caliper body and a transparent index scale is mounted on the slider with its calibrated face disposed opposite to the calibrated face of the linear scale and slightly spaced apart relationship.