Gear Ratio and Fuel Economy

Gear axle ratios play a huge role in the overall performance of your commercial vehicle.

ratio

A correct gear ratio will yeild optimum fuel economy at top speed.

Know what ratio works for you.

Do you need a low gear ratio that delivers more torque for super heavy loads?
Do you need a high gear ratio that can achieve a higher top speed but may compromise take off power?

Gear ratio is determined by the difference between the rotations of the drive shaft and the axle. Because the ratio is expressed as a number, it can get confusing, but a high number yeilds a low ratio. A ratio of 4.0 is lower than a 3.4 ratio.

differential

For slower speeds and rural driving, for many inclines on your route and several stops, you might find a low ratio (indicated by a high number) will deliver the torque you need start your pull and to keep your load moving on inclines. A low ratio (high number) provides more low speeed wheel torque and takes less throttle to get your load moving. It allows for quicker acceleration as well.

If you do alot of highway driving, keep the ratio number low for a higher gear ratio. The higher the gear ratio, the less rpm is required to maintain speed. This will have the best effect on fuel economy at high speeds.(Passing performance and speeds above 60mph is a matter of horsepower, not axle ratio.)

Tire size will also affect what gear ratio you have. Your tire size is actually the end of the equation when calculating your gear ratio. For instance a tire that is 22.5 might spin 400xs where a 19.5 tire may spin 600xs.

The key to understanding gear ratios is to remember that as the numerical ratio goes up, towing capacity increases, but fuel economy goes down. Therefore, a truck with optional 3.73 gears, for example, will tow a heavier trailer than one with 3.55 or 3.21. But it will also use more fuel in all situations because the engine will rev higher.