Typically when specifying out medium-duty trucks and vans for drive-axle ratios, it is easy to figure out what ratio to use. This is true especially since there is only two to three depending on the vehicle. However, for those vehicles weighing in 19,500lbs to 26,000lbs on the GVWR (Gross Vehicle Weight Rating) scale, the process becomes more complicated. There are more than a dozen options to choose from. Some drive-axle ratios range from 2.69:1 to 7.17:1 with ten or two different ratios between them. It all depends on the truck’s class, make and model, and the rear-axle capacity.
Drive-Axle Ratios & Fuel Economy
The ratio determines how the performance, speed, and fuel economy will be. For example, if the drive-axle ratio is higher more power is gained but lowers the top-end speed and fuel economy. Whereas the lower ratios result in less pulling power but overall improved top-end speed and fuel economy. Here are somethings that to b account for when selecting ratios.
- For the speed for each truck, calculate the maximum road speed and then use those calculations. This ensures the torque and horsepower curves are at the right spot. The truck operates in optimal operating condition.
- The gradeability is the ability to maintain a certain speed on a particular percentage grade, or incline. Think about where the truck is operating – such as mountains, plains, etc.
- Consider load size. Is the truck’s cargo weight always constant or always changing? If it’s diminishing, allow for a faster gear in the truck. Iif it is constant, then it’s always near the maximum, Itt is necessary to back off on maximum road speed.
- The truck’s towing power. Determine trailer and load weight and the percentage of time the truck will be towing. Pulling a trailer 20% of the time, that’s enough to perhaps go a bit lower with the gear in order for the truck to handle the load better.
- calculated the terrain the truck goes through everyday for off-road or on-road use. Look at the percentage of time the truck will be operating or towing on crushed rock, hard clay, dirt, or pavement. At what speeds does the truck need to be traveling on these different terrains in terms of RPMs and what does that look like for drive-axle ratios?
- Any time the truck is over 55 mph, the wind drag is huge and it need to think about either dropping deeper in terms of axle ratio or offsetting the wind drag with aerodynamic enhancements.
- The circumference of the tires along with the revolutions per mile affect axle ratio of the truck.
- With an automatic transmission, the truck has the gains of a torque converter and ‘gets away’ with a bit lower drive-axle ratio.
Consider the resale value. If the ratio has gone too far on either end of the drive-axle spectrum, this could drop the resale value of the truck by $50 to $500.
For further help understanding how to select the drive-axle ratio on a vehicle, give Truck-N-Trailer a call at 405-912-5800.