Pittsburgh Power Gear Calculator

Instructions: Please click the yellow button below to open the Gear Calculator Excel document. If it opens in preview mode, please hit the download button. Each transmission has its own tab at the bottom of the page (18, 13, 10, and Meritor 10 Direct), please select the tab with your transmission. Input your rear end ratio and tire size in the two highlighted boxes located at the top of the page. Avoid editing anything outside those two boxes as it may lead to inaccurate results. 

This tool should help you see how changing tire size and rear end ratio will impact your RPM, which in turn will affect performance and fuel mileage. The X axis is the selected gear, the Y axis is speed in miles per hour, and the cells in the body are engine RPM. The green highlighted cells are your ideal RPM. Other factors will change results from truck to truck, but it should give you a good reference.