There are a lot of uncertainties surrounding the truck industry when it comes to the best gear ratio for speed, pulling power, and fuel consumption. To select the ideal gear for your semi-truck, you must be familiar with the peak RPM of your engine and cruising RPM.
This guide is a compilation of all this information. You will also get to learn what a gear ratio is, how it is calculated, and the various types out there for your truck.
What Do Gear Ratios Mean In A Semi Truck?
Gear ratios simply mean the number of turns an axle produces to the work done by the output shaft or driveshaft.
A common example (a popular ratio for heavy loads) is 4.11:1. What does this ratio imply? 4.11:1 signifies that the driveshaft has to turn 4.12 times in exchange for one turn of the axle.
There is more, we can also conclude that more work is required to generate power in this gear ratio than in a 3.08 ratio.
| Manufacturer | Product | Type | Ratio Available |
| Meritor | MT-14X | Tandem | 2.28-7.17 |
| Dana | Spicer AdvanTEK 40 | Tandem | 2.26-3.91 |
| MACK | 538/40 Series | Tandem | 3.86-8.37 |
| Detroit | Detroit Tandem Rear Axles | Tandem | 2.16-4.778 |
How To Calculate The Gear Ratio On A Semi-Truck?
To calculate the gear ratio of a semi-truck, you have to divide the number of teeth on the gear by the number of teeth on the pinion.
For context, let’s say you have 37 teeth on your gear ring and 9 teeth on your pinion. To know your truck’s gear ratio, all you need to do is:
Gear ratio (G.R) = No of teeth on gear ring/No of teeth on the pinion
G.R = 37/9 = 4.11
This method may require you to take out your differential and count the number of teeth on the gear ring and pinion if you don’t have your truck’s manual or cannot find the information there.
Common Gear Ratios And What They Offer
The gear ratio of a truck determines the power and endurance enclosed within it. Now that you know how gear ratio is calculated, let’s take a look at the major features of common ratios:
Semi Truck 3.55 Gear Ratio
This ratio guarantees good output energy and allows trucks with loads to ply hills seamlessly. In the ratio spectrum, the 3.55 gear is on the higher end.
Semi Truck 3.42 Gear Ratio
This gear has more pulling power and belongs in the lower end of the gear spectrum. Trucks with this ratio possess hauling strength and it is not so different from the 3.55 ratio.
Semi Truck 3.36 Gear Ratio
When you move down to the lower end of the spectrum, you will find the 3.36 gear ratio. This ratio offers great speed and reduced overall speed. In terms of power, they do not offer much as they are best suited for long highway travels with little or no stops.
Semi Truck 3.25 Gear Ratio
Still moving down in the spectrum is the 3.25 gear ratio. This ratio produces decent RPMs if there is not much load attached to the truck. Trucks with this ratio are great for highways with fewer stops and guarantee good fuel economy if you can maintain a speed around 56-60 MPH.
Semi Truck 3.08 Gear Ratio
Truck drivers do not rate this ratio because it offers less strength and fuel economy. The ratio is best for flat roads with few stops.
Why Would You Prefer A Higher Ratio In Your Truck?
Semi-truck operations depend majorly on horsepower and torque. Horsepower signifies the amount of power a truck maintains over a particular distance.
Torque, on the other hand, is the amount of weight the truck can haul over a given distance. A higher gear ratio for your truck simply means more horsepower and torque. Vice versa.
Can You Change The Gear Ratio On Semi Trucks?
It is possible to change the gear ratio on your semi-truck. However, this procedure can be a bit expensive. People modify gear ratios primarily to improve the fuel efficiency or power of the truck.
Changing the head ratio of both the front and back ends of your truck should cost you about $5,000. It may be more or less.
Things To Consider When Deciding On A Semi Truck Axle Ratio
When deciding the best gear ratio for your truck, Speed, you have to know what is primary and secondary for your truck. Things to consider include:
1. Speed
Trucks that spend the most time on the highway with decent load should have an axle that is perfect for speed and towing prowess. Fuel economy is a secondary feature to consider here.
2. Gradeability
Gradeability simply means the ability of a truck to navigate slopes at a specific speed. Let’s say your truck’s gradeability is rated at 10% at 80mph, this means it can maintain that speed on rough terrains. The steeper the slope, the higher the gear ratio your truck requires.
3. Weight And Towing Capacity
Trucks mostly maintain a relatively constant load weight throughout a journey. Hence, you need to have an idea of the average weight (of load) your truck can transport to provide the perfect ratio.
Towing capacity implies how long the truck will be traveling at its gross vehicle weight rating (GVWR). If most times, is close to its maximum GVWR, you might want to sacrifice some road speed to favor towing capacity. This means a higher ratio.
4. Tire Size
Trucks with small tires enjoy more speed while those with big tires do not. This means a low gear ratio belongs to a smaller tire circumference. Tires with bigger circumferences require a high gear ratio.
Conclusion
To enjoy a blend of speed and torque, the 3.55 gear ratio is ideal for semi-trucks. Heavy-duty trucks, on the other hand, should comfortably handle anything between 3.6 to 3.8.