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10 Drive-Axle Ratio Factors to Consider For Medium-Duty Trucks

With a wide range of drive-axle ratios for medium-duty trucks available, fleet managers must weigh/factor carefully which one they choose for their fleet. Their decision will affect a truck’s top-end speed, fuel economy and load pulling ability.

For light-duty service trucks, the process for selection is pretty simple because there are only two to three ratios from which to choose. A good starting point is understanding what the drive-axle ratio means and how it affects truck performance. To do this,  Ken Gilles, truck operations manager for GE Capital Fleet Services, has listed the following 10 factors to consider:

  1. Speed – for each truck, calculate the maximum road speed and then use those calculations to ensure the torque and horsepower curves are at the right spot so the truck operates in optimal operating condition.
  2. Gradeability – the ability to maintain a specific speed on a particular percentage grade, or incline. Think about where the truck is operating – mountains, plains, etc.  
  3. Load – is the truck’s load constant or changing? If it’s diminishing, allow for a slightly faster gear in the truck, if it’s constant, (operating a tow-truck with a crane and tools), then it’s always near the maximum and you should back off on maximum road speed availability.
  4. Towing – Determine trailer and load weight and the percentage of time the truck will be towing. They advise if you’re 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.
  5. Terrain – To calculate for off-road use, look at the percentage of time the truck will be operating or towing on crushed rock, hard clay, dirt, etc. At what speeds will the truck be traveling on these different terrains in terms of RPMs and what does that look like for the drive-axle ratio?
  6. Aerodynamics – Any time the truck is over 55 mph, the wind drag is huge and you need to think about either dropping deeper in terms of axle ratio or offsetting the wind drag with aerodynamic enhancements.
  7. Tires – the circumference of the tires  along with the revolutions per mile affects axle ratio.
  8. Transmission – with an automatic transmission, the truck has the gains of a torque converter and you can ‘get away’ with a bit lower axle ratio.
  9. Resale value – they warn that if you’ve gone too far on either end of the drive-axle spectrum, this could drop the resale value of your truck by $50 to $500.
  10. GVWR – There are fewer choices with the lower GVWR trucks.

Truck Performance Bottom Line

Getting the drive-axle ratio spec right is important because it directly affects truck performance, fuel efficiency and resale value.

If you want more information, please see the entire story at the Work Truck Online site.

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