How to Spec a Semi Truck Tractor

How to Spec a Semi Truck Tractor

Whether you choose to rent, lease, finance, or outright buy a used or new heavy-duty truck, then you will need to understand the factors that go into spec’ing and evaluating whether the truck is fit for your needs. This is often a detailed process so being familiar and knowledgeable about different truck components and metrics is vital to ensuring you choose a truck that will bring the most benefit for your business.

This article will mainly cover pre-purchase evaluation of semi/highway trucks used to haul trailers such as dry vans, reefers, and flatbeds.

In previous articles we have covered engines, transmissions, and drive axles in detail. This guide will provide brief information on the different factors to be considered when understanding how to spec a semi truck.

Engine Selection

The type of engine you select will have a direct effect on your bottom line especially when it comes to fuel consumption. Fleets are steadily increasing the use of LNG engines for their trucks. However, the vast majority of engines being used in semi-tractors, continues to be of the diesel variety. Diesel engines have come a long way in terms of fuel economy over the years, especially as government agencies such as the EPA have lobbied for stricter emission standards.

Some primary factors to consider when choosing a diesel engine for an over-the-road truck is to look at the horsepower. The horsepower directly reflects the power of an engine. Ideal horsepower range for heavy-duty diesel truck engines are between 400-600. Anywhere from 450-525 will provide excellent horsepower to haul less-than-truckload and most full-truck-load freight.

Torque range represented in lb-ft is a detriment of the rate at which the truck can pull a load from a stop. It factors into the tractor’s ability to carry heavy loads and assists in propelling the vehicle up steep hills (gradeability). Torque range generally starts at 1450 lb-ft and go up to about 2000 lb.-ft.

Engine displacement is another metric to assess because higher a displacement volume is suitable for heavier loads.
The miles per gallon is a metric that can vary depending upon route, payload weight etc. but looking at the average MPG for the model you’re interested in is a starting point. Also spec’ the different fuel efficiency features the company has included in their engine and compare those with other engines.

It is important to not over spec but to understand the overall performance capability of the engine. For example, for most over-the-road semi tractors horsepower will be more important than a high torque rating.


The transmission is another important part of the powertrain.  The first step is to choose the type of transmission suitable for your trucking operation. If the driver is highly skilled, a traditional manual transmission may be of interest. However, for the majority an automated manual transmission will work best.

Next, choose the number of speeds which will suit your needs. 10-speed, 13- speed, and 18 speed transmissions are some of the common configurations. The greater number of speeds means greater versatility. Aim to find a balance.

The different gear modes such as skip-shift and power-takeoff are important things to consider. Moreover, the transmission should have the option to switch between operating modes such as manual and automated manual.


The drive axle configuration is another feature to assess. Tandem axles are more common in semi-tractors. However, you will encounter configurations such as 6×2 and 6×4. The primary difference is that in 6×2 power is only supplied to one of the rear (tandem axles). While, this may help reduce fuel consumption there are drawbacks for long-haul commercial vehicles.

Axle ratio is a specification that will help you determine the towing abilities of a truck. A higher ratio means greater hauling ability but slower speeds while vice versa is true for lower ratios.

The Gross Combined Weight Rating will have an impact on your payloads. The axles should be able to withstand enough weight to haul the specific freight. To calculate this, add the weight ratings of the steer, rear, and trailer axles.

Exterior Shape and Aerodynamics

When Kenworth introduced the T600 model in 1985, it was estimated to improve fuel efficiency by 22% compared to the traditional long-hood design. The slope/slippery nose has proven to be the fuel saving alterative to the conventional truck shape commonly found in Kenworth, Peterbilt, and Western Star models. Therefore, consider choosing a truck with more aerodynamic exterior features such as fairings, side skirts, and low rolling resistance super single tires.

Gradeability is the steepness of a hill that a truck can travel up without losing speed in a particular gear. To calculate an accurate gradeability, use the steepest grade the truck will competently climb at the maximum allowed highway speed. Slope signs along your route will generally indicate the steepness percentage. The general rule of thumb is the truck should not require a downshift while at a 1% incline at peak torque in cruise gear for line-haul applications.

gradeability-semi-truck-tractorThe following has been mentioned in Kenworth’s January 2005 White Paper on Powertrain Spec’ing:

At peak torque rpm in top gear, Caterpillar recommends a minimum gradeability performance threshold of 1.8%. At cruise speed in top gear, 1.0% gradeability is ideal to achieve best performance.

Resale Value

Resale value is an essential point to consider and if you plan to upgrade your fleet every several years, many factors will affect its aftermarket price. Firstly, select specs for your truck that will yield a high resale value. For example, axle ratios skewed too far left or right, or an engine that has too high or low horsepower can decrease the number of interested buyers. Secondly, maintenance costs such as parts replacement, and the labor cost of knowledgeable technicians can add up. Therefore, consider equipment such as engines made by Cummins and transmissions made by Eaton that are more common and mechanic familiarly is higher which can result in lower maintenance costs. The miscellaneous factors can include the condition of the cabin, included amenities, and the sleeper size. Uncommon or extreme color combinations can also diminish a truck’s resale value.

Miles Operated

It is vital to select a truck with low enough mileage that it does not require an engine rebuild (in-frame) soon after you purchase it. Therefore, stick to trucks with under 500,000 miles (800,000 KMS) if you not willing to pay repair bills.


A wheelbase is the distance from the center of the front steer axle to the center-point of the tandem axles.  A longer wheelbase is capable of creating a smoother and more stable ride for higher speed, long-haul applications. And if the wheelbase is correctly optimized, it can decrease the drag on the headboard of the trailer.

Other implications that will affect your spec’ing of a truck will be how you wish to purchase it. There are several options to ‘owning’ a truck which includes financing, buying outright, leasing, and renting. Read our semi-tractor financing guide for detailed information.


In conclusion, spec’ing a truck is a complicated process but with the general overview of some of the primary factors to consider mentioned above, you should have a general idea of how to approach evaluating a semi-tractor. Seek the help of a professional consultant for guidance tailored to your specific business needs on how to spec a semi truck.

One Reply to “How to Spec a Semi Truck Tractor”

  1. I love K-9’S and Pete 379’s but reading about a 22% fuel difference is huge. I was hoping to find more info about short and tall gearing though. I would think an 18 speed would help stretch the usefulness of a given ratio – such as helping a fast truck climb, but I’m not sure about what would be the best ratio for a heavy truck that goes into the mountains, such as mine will.

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