Semi-trucks, also known as big rigs or 18-wheelers, are an iconic sight on highways across North America. With their massive size and numerous axles, semis are capable of hauling extremely heavy loads over long distances. But just how fast can these colossal trucks go when they really put the pedal to the metal? Here’s a look at semis’ top speeds and the factors that influence them.
Typical Top Speeds
The top speed of a typical semi truck is between 55 and 75 mph. However, their speed is often restricted by regulations to 60 or 65 mph when traveling on highways and interstates. Some key factors impacting a semi’s top speed include:
- Engine power – More powerful diesel engines allow semis to reach higher speeds.
- Number of axles – More axles distribute weight better for handling at high speeds.
- Aerodynamics – Sleeker front profiles and side skirts reduce drag.
- Gross vehicle weight – Heavier loads take longer to accelerate and restrict top speed.
- Mechanical governor – An electronic chip limits maximum speed as a safety measure.
- Legal speed limits – Interstate speed limits for large trucks are lower than for cars.
Despite their bulk, semis are surprisingly speedy. Loaded 18-wheelers can typically get up to highway speeds between 50-70 mph without much difficulty. But they take longer to achieve those speeds compared to smaller passenger vehicles.
Speed Limiters
One reason semis in the U.S. and Canada don’t go above 75 mph is because of federally mandated speed limiters. In the past, some trucks didn’t have limiters and could reach up to 90 mph. But to improve safety and prevent dangerous high-speed crashes, speed limiters are now required.
Speed limiters electronically control a semi’s top speed by limiting fuel delivery once a set speed is reached. They are typically set to around 65 mph, but can be set as high as 75 mph. Exceeding the limiter requires disabling it, which is against the law.
Downhill Speeds
While semis struggle to reach fast speeds on flat roads, they can gain momentum much quicker going downhill. Their massive weight allows them to accelerate rapidly without needing as much engine power. As a result, semis often reach their highest speeds when traveling down steep mountain grades.
Signs warning truck drivers to use lower gears and watch their speed when traveling downhill are common in mountainous regions. Special “escape” ramps are also provided on some steep slopes as a safety precaution. These ramps allow out-of-control trucks a place to slow down before collisions occur.
Land Speed Records
Although regulated trucks are limited to about 75 mph, modified semis have achieved much greater speeds. Specially designed for top speed racing, these trucks have set impressive land speed records over the years.
Some record-setting speeds include:
- 2017 – London, ON Canada – Big Bison truck – 103 mph
- 2016 – Austin, TX U.S. – Modified Peterbilt semi – 210 mph
- 2006 – United Kingdom – Modified Leyland DAF – 137 mph
These examples demonstrate the potential of semis when engine power and aerodynamics are maximized for straight-line speed. But such speeds are not practical or legal for highway trucking.
Speed vs. Fuel Efficiency
While speed can be exhilarating, trucking companies are very mindful of fuel efficiency. Diesel fuel is a major operating expense, so boosting mpg is a bigger priority than speed for most fleets. In fact, trucks often travel 5-10 mph below speed limits to optimize fuel efficiency.
Studies have shown 65 mph is the most fuel-efficient highway speed for semis. Traveling faster significantly increases wind resistance and fuel consumption. For example, going 75 mph rather than 65 mph boosts a semi’s fuel use by 20% or more.
Safety Concerns
High speeds in big rigs also raise serious safety issues. The faster a semi travels, the longer it takes to stop. Loaded tractor-trailers require nearly 300 feet on average to come to a complete stop from 65 mph. Higher speeds dramatically lengthen braking distances.
Rollover crashes are another major risk, as high speeds make tractors more prone to losing control and tipping over. In addition, the force of impact during collisions increases exponentially with speed. For these reasons, excessive speeding is especially dangerous for semis.
Conclusion
In summary, semi trucks are governed by mechanical and legal speed limits for safety, fuel efficiency, and compliance reasons. Typical top speeds fall in the 55-75 mph range. While modified semis can reach over 200 mph, such high velocity is not practical for highway trucking operations. For now, companies and drivers will continue balancing priorities like on-time delivery with safe and economical highway speeds.