This calculator is for anyone looking to jump up in tire size, change their vehicle’s performance by swapping ring and pinion gears, or satisfying their curiosity about their current setup. The calculator determines your engine rpm at a selected vehicle speed. It uses gear ratio, tire height, and vehicle speed and deliver engine speed in the result boxes. Each box represents a transmission type and takes into consideration how gear reduction within the transmission will impact the final engine speed readings. The boxes are labelled Auto, AOD, Manual, and 5 Speed… simply refer to the box that best matches your transmission type.