Rear Axle Differential Ratio Comparisons for Gas MPGby Skip Shelton
A vehicle's rear axle ratio is numerically expressed. A higher number denotes a lower ratio. For example, if the axle ratios were listed as 3.31:1 and 3.42:1 and 3.55:1 and 4.10:1, the lowest ratio of the four gears is 4.10:1. The lower the gear ratio, the more rpm required to maintain a given speed with equivalent tire sizes.
While the gear ratio will have an effect on the actual fuel economy of your vehicle, your driving habits will offset any gains or losses. The actual difference between a 4.10 gear ratio and a 3.73 gear ratio at highway speeds is less than 200 rpm. At less than 60 miles per hour, fuel economy is primarily dependent on driver habits, rather than gear ratio. To lower your fuel consumption, drive at a consistent and moderate speed.
Gear Ratio Effect
At a given speed, and if all else is equivalent, a higher gear ratio axle -- again, a lower number -- will have a positive effect on fuel economy. The effect is generally minimal and will account for less than a single mile per gallon, and typically the gains are generally seen at highway speeds. Additionally, the fuel economy gains or losses from an axle ratio change may be directly offset by the transmission selected. The transmission gearing directly affects the fuel economy at a given speed. The engine's rpm and fuel consumption are generally lower in higher gears. For example, a transmission with four standard speeds operating at 55 mph may see a 200 rpm difference between a high and low axle ratio, whereas a transmission equipped with an overdrive may only see a 160 rpm difference between the same two axle ratios.
Gear Ratio and Tire Size
The gear ratio is not only affected by the axle, but also the tire size. If your vehicle has a low gear ratio, you can offset this by installing slightly larger diameter tires. For example, a vehicle with a 4.10 axle with 30.2 inch diameter tires will have a new gear ratio of 3.97 with 31.2 inch diameter tires installed. See the Axle Ratio Calculator in the References section below for specific calculations.
- Cog Wheels image by Alis from Fotolia.com