Gear Ratios vs. Tire Size Whenever you change tire size from the original size or from one size to another, this will have a direct effect on power and driveability. This chart will help you to select the optimum gear ratio to bring the power and torque back to where you want them. First determine what your original gear ratio and tire size are. Then, using the chart, follow the lines to your approximate RPM at 65 mph. Next, find the new tire size on the left of the chart and look across to find the RPM closest to your original, the ratio listed at the top of that coulmn will be your first choice for gear ratio. If you are interesed in a change that will give more power or better fuel economy - Adjust your ratio using the shaded coding as a guide. This chart is based on 65 mph and a 1:1 gear ratio, on a manual transmission. Please note: If your vehicle is equipped with an automatic transmission the true RPM will be higher than shown on the chart, because some slippage will occur in the transmission and torque converter. With an overdrive transmission, your RPM in overdrive will be 15% to 30% less than indicated, depending on your vehicle.