You can use this Spreadsheet to change the Gear ratio and tire size on your Cummins powered Truck  You will see the typical peak power RPMS in green on the chart.               To use this tool, enter your Trucks values into the Yellow Highlighted Fields.             Tire Size is inputted into Field B5-D5 Truck type should be inputted to match the truck ( not useful yet) Options are "4wd Quad" "2wd Quad" "4wd Single" "2wd