Gear AccordinglyHere's the math to show how Overdrive transmissions provide great acceleration and economy. A good all-around performance combination that will generally not kill mileage is a combined First gear ratio of 10.0:1 (First gear multiplied by the axle ratio) and the capability to drive at freeway speeds at low rpm with overdrive. In other words, if your current rear axle ratio is 3.31:1 and the overdrive transmission you'll be adding has a 3.06 First gear, your total combined gear ratio in First gear will be 10.13 (3.31 axle ratio x 3.06 First gear = 10.128:1). This combined First gear ratio is equal to having 4.56 axle gears and an M21 or M22 Muncie in First gear (2.20 low). We can illustrate this by dividing the final combined ratio of 10.128 by the Muncie M21 First gear ratio of 2.20:1 (10.128/2.20 = 4.60:1). Additionally, your effective High gear ratio with the 0.70:1 overdrive and 3.31:1 axle ratio will equal 2.32:1 (3.31 x 0.70 = 2.317:1). At 75 mph, this combination will put your freeway engine speed at 2,250 rpm.