On a dirt oval track, more often than not, a two cycle will need a Jack shaft to achieve the optimal gear ratio. This ratio chart can find that optimal ratio. In the read squares marked “engine” and “primary”, enter your numbers there. In the vertical column marked secondary, and the horizontal columns marked axle, enter your preferred starting numbers in those red cells. After you do that, all the other locations will recalculate to reflect the ratios of the numbers you’ve entered.