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I made a spreadsheet years ago that calculates rpm tables based on tire size or actual diameter, axle ratios, and transmission gear ratios. It also factors in transfer case, range box, or overdrive unit ratios into the RPM tables. I setup a table at the bottom that calculates speedometer gears and ratio adapters too if needed.The archive contains two spreadsheets *-an .XLS file for Microsoft Excel 1997/2000 or newer.*-an .ODS file for the free Open Office/Libre Office suites calculated applet. http://www.mediafire.com/download/zk4j8ymtctekx5r/RPM_Gear_Axle_Transmission_TransferCase.zip

I use the factory RPO codes in the glove box