10 Things To Think About When Spec’ing A Medium Duty Truck Drive-Axle Ratio
So you’re in the market for a new medium duty truck. One of the specs you’ll need to consider is the drive-axle ratio. There’s a lot to think about here, especially considering you may have a dozen or so drive-axle ratios to choose from, depending on your truck’s class, make, model and rear-axle capacity.
Medium duty trucks with a gross vehicle weight rating (GWVR) between 19,500 lb. and 26,000 lb. can operate with a drive-axle ratio anywhere from 2.69:1 to 7.17:1. Your choice will affect your truck’s top speed and fuel economy as well as its load-pulling ability. And it’s something of a trade-off:
- With a higher ratio, you get more power. But you’ll get reduced fuel economy and top-end speed.
- With a lower ratio, you get the reverse.
So for obvious reasons, your goal is to choose the ratio that best matches your medium-duty truck’s intended use and workload. Here are 10 things you should keep in mind to determine the optimal drive-axle ratio for your next vehicle:
- Since drive-axle ratio directly affects this, what average speed do you anticipate? What RPM in top gear?
- Over-the-road terrain. Grade affects speed and power needed to maintain speed. So where will your truck be working – on the flats or in the mountains?
- Off-road terrain. If unpaved roads are the norm for your truck, what is the surface like? Rough terrain will point you toward a different drive-axle ratio than a smoother surface such as crushed rock or packed earth, but you’ll also need to consider how fast your truck needs to travel.
- Will your truck carry a consistent weight load, or will it increase or decrease during the day? With a constant, heavy load you should focus on start-up power and fuel economy rather than top-end speed. If the load will get lighter as the day goes on, you can choose a faster gear.
- Trailer size, length and weight (loaded and unloaded) all come into play. Again, what is the most likely scenario for your medium-duty truck? Experts recommend a slightly lower ratio if you’re towing 20% of the time or more, to boost load capacity even though you’ll give up some speed.
- Tire size and RPMs per mile affect the drive-axle ratio. For instance, a smaller tire circumference behaves much like a lower drive-axle ratio.
- Every fleet owner these days is acutely aware of the effects of wind drag on larger trucks. It takes more power to push through the air, which requires a deeper axle ratio. On the other hand, it could be more cost-effective to improve your truck’s aerodynamics and outfit it with a higher drive-axle ratio.
- An automatic transmission with a torque converter can allow you to use a lower axle ratio. You’ll have the juice you need at start-up without sacrificing so much top-end speed.
- Let’s face it. Unless you’re going to drive your medium duty truck into the ground, you’ll want to sell it someday. You need to make spec choices that will facilitate resale. Choosing a drive-axle ratio that’s too far to either end of the available range will take your truck out of the mainstream. You’ll have a smaller pool or potential buyers, and the dollar value of your truck may be smaller, too.
- We noted earlier that (Class 6 and 7 medium-duty trucks) in the 19,500 lb. to 26,000 lb. GWVR range) can handle many different drive-axle ratios. When you drop below that range, however, you may find just two or three choices.
So which drive-axle ratio is the best for your new truck? If you consider all these factors, you’ll pick the right number, whatever it turns out to be.