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What Is The Best Gear Ratio For A Semi Truck? (Solved!)

Gear ratios can feel a little overwhelming at first, but at the core, it is simply a term used to refer to the number of turns of the gears of the driveshaft in relation to the turn of the axle. 

The turn of the axle is important in determining whether your truck is power-oriented or speed-oriented. 

Here is the answer to what is the Best Gear Ratio for a Semi Truck:

The best gear ratio for a semi truck to haul a heavy load is 4.11:1. However, this gear ratio is not good for fuel economy. A gear ratio of 3.25:1 is good for long-distance traveling without heavy hauling. A gear ratio of 3.55:1 is considered the best median level, for both good speed and torque.

What do Gear Ratios mean in a Semi Truck?

Gear ratios refer to the number of turns produced by the axle in turn in relation to the work done by the driveshaft (or output shaft).

Think of the ratio as the turn relation, such as 4.11:1, which is a common setting for heavy loads. 

What does that mean? 

This translates to the output or driveshaft turning a total of 4.11 times for one turn of the axle one time. But, it says more than just that. It also says that it requires more work to generate power than a gear ratio of 3.08. 

Why would you wish to have a higher gear ratio or a lower ratio? 

Semi trucks operate through horsepower and torque. Horsepower is the amount of power the truck maintains over a given distance, and torque is the amount of weight it is capable of hauling over distance.

Both of these are related to the gear ratio of your truck.

What Gear Ratio do Semi Trucks have?

First, it should be clarified that the gear ratio is sometimes referred to as the axle ratio. This is because they are directly related. The turn of the gear relates to the turn of the axle. 

Most class 8 semi trucks have a gear ratio of between 2.26 and 3.36:1. These ratios are determined by the number of teeth on the pinions, which impact the number of turns the axle makes. 

These are not the only ratios, however. A semi truck using a gear ratio of 2.26 would have good fuel economy and sufficient power for small tasks, however, it would lack a great deal of power. 

This would not be ideal for highway driving because if you need to traverse some difficult spaces such as hills, it can be extremely difficult. 

Likewise, a common gear ratio is 4.11, which is powerful for hauling, but really bad for fuel economy. You would want to have this setting if you needed to haul heavy loads in a lot of city traffic since it has decent pick-me-up strength. 

How to Calculate the Gear Ratio on a Semi Truck?

The calculation to determine your engine’s gear ratio is found by determining the number of teeth you have on the gear ring and dividing it by the number of teeth on the pinion. 

For example, if you have 37 teeth on your gear ring and 9 teeth on your pinon you would have 37/9=4.11 (rounded to the nearest hundredths place). Likewise, if you have 39 teeth on your gear ring and 11 on your pinion you would have 39/11=3.55 (rounded again). 

This website has a calculator which may make this more simple: https://www.tremec.com/calculadora.php

How to find Gear Ratio on a Semi Truck?

The calculation is similar to what was discussed above but is expanded upon here.

To find the gear ratio you can turn the wheel and see the number of teeth on the gear shaft and see how many times it takes to turn for one turn of the axle wheel. 

There are a couple of ways to determine your semi truck’s gear ratio.

The most straightforward way to see what it is is to check your truck. It is sometimes mentioned in the manual, however, most trucks have it listed on the outside of the truck by the tire. 

If you do not have this, you can take apart the differential on your truck and count the number of teeth on the gear. Divide this by the number of teeth on the pinion. This should give you your truck’s gear ratio.

The more turns, the more power is generated. More weight can be hauled with more power generated and makes pulling uphill possible with large loads. 

Although, one thing to keep in mind is the number of turns given corresponds to more fuel consumed. Large semi trucks have more power than medium-sized, but they also burn more fuel overall. 

How to change Gear Ratio on Semi Trucks?

You can change the gear ratio on your semi truck, but it can be a bit costly. There are several reasons why people may wish to have their gear ratios altered. 

The primary reasons include more fuel efficiency and/or more strength to travel uphill or pull more weight. 

On average though, you can end up dishing out up to $5,000 to have the alterations completed for both the front and back ends of your truck. 

What do these Gear Ratios mean and what are they Used for?

Each gear ratio is used to help determine the amount of power and endurance your truck has. Each has a different use and purpose. Let’s explore this.

Semi Truck 3.55 Gear Ratio

A gear ratio of 3.55 means that the number of teeth on the ring gear is divided by the number of teeth on the pinion to give the ratio 3.55. A gear ratio of 3.55 is on the higher end of the spectrum (although not the highest).

A gear ratio of 3.55 means your transmission is seeing a good output of energy. It can scale hills relatively easily, and pull a good sized load.

Semi Truck 3.42 Gear Ratio

A gear ratio of 3.42 is the relation of the teeth on the ring gear compared to the teeth on the pinion. 3.42 is in lower gear, and has more pulling power (although the distinction between 3.42 and 3.55 is not significant). 

This will give you some more strength for hauling. However, keep in mind it has a lower top speed than higher gear ratios. 

Semi Truck 3.36 Gear Ratio

Once you hit the lower end of the spectrum, you are looking at greater immediate speeds and lower overall speeds. Additionally, they do not have the power of the higher gear ratios.

This gear is best for long highway driving, without many stops, and fairly flat.

Semi Truck 3.25 Gear Ratio

A gear ratio of 3.25 is lower, and creates a decent amount of RPMs, without the heavy amounts of force. This is an ideal ratio for someone looking to save some money on their gas when traveling far.

You want to be sure the roadways aren’t very difficult, similar to the ratio above. If you can maintain a fair speed of around 55-60 MPH, you should be able to maintain good fuel economy.

Semi Truck 3.08 Gear Ratio

A gear ratio of 3.08 is not very popular amongst truck drivers. This ratio does not have much strength, and the fuel economy savings do not make up for it.

This ratio may be suited for flat surfaces with few stops, but keep in mind that the fuel savings can be had at another slightly higher number. 

What is the best Gear Ratio for heavy haul?

If you are looking to drive heavy loads through city traffic or towns, then a gear ratio of around 4.11 is ideal. A gear ratio of 4.11 is good for acceleration and for hauling. However, it is best loved for the strength it offers.  

The higher number gear ratios have more “giddy-up” power, or more force when accelerating after stopping. This is really useful when you are in stop-and-go traffic. 

What is the best Gear Ratio for driving at Speed?

The best gear speed for driving is around with higher speeds (such as highway driving) then a gear ratio of around 3.55 is good.

This helps to give a balance between force and speed for when you are trying to put in a lot of miles while hauling a decent sized load.

You can go lower to 3.42 or below to get more immediate speed. Always keep in mind where you usually drive, however, because the lower the number for the gear ratio, the lower your overall speed. This is because the RPMs will be lower.

Best Gear End Gear Ratio for Semi trucks?

Rear gears are usually referred to as either tall or short, which refers to the height of the gear in the back. The gears can be tall to create fewer RPMs, or short to create more RPMs. 

If you’re looking for greater immediate speed (but slower overall) then it is best to have your gears short. Likewise, if you want higher top speeds over time, you would want them shorter. 

The ratio you need will depend a lot on the type of driving you do, however, 4.7 is a relative median ratio that suits many needs. 

What is the best Gear Ratio for Speed and Torque?

To get both speed and torque you want to be careful not to stare too far from the middle ground when you look at ratios. 4.11 is a lot of power but also can be excessive since it burns a lot of fuel. 

3.42 is too much immediate power without good long-term force or speed. Therefore, you want to look somewhere between 3.6-3.8.

Many truck drivers like 3.7 because it gives them strength and speed without burning off all their fuel.

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