This ratio tool is broken down into Simpson, Ravenaux, overdrive and undersrive gear sets. The tool allows you to calculate gear ratios on a wide range of transmissions. The basis of the calculator is the three-speed Simpson or Ravinaux gear set. Many late-model transmissions use a combination of these gear sets and the overdrive (or underdrive) set to achieve the various ratios. Once you understand the combinations of gear sets being used you easily calculate all the ratios of the transmission. This tool comes in handy for performance applications where you're considering a change in gear sets and need to know the ratios for each range.