What device limits speed on a car?

The device that limits a car’s top speed is a speed limiter—also called a speed governor—usually implemented in the engine control unit (ECU). It monitors vehicle speed and, when a preset threshold is reached, reduces power (by closing the throttle or cutting fuel/spark) to prevent the car from going faster.

What a speed limiter (governor) is and why it matters

In modern vehicles, the speed limiter is primarily a software function inside the ECU that enforces a maximum speed chosen by the manufacturer, fleet manager, or in some cases the driver. Its purpose ranges from protecting tires and driveline components to meeting regulatory or company policy requirements. Older or specialized vehicles may use mechanical governors, but most contemporary cars use electronic control.

How it works

Electronic control in modern cars

The ECU reads data from the vehicle speed sensor, compares it with a stored limit, and intervenes when the threshold is reached. Typical interventions include closing the electronic throttle, cutting fuel injection, retarding or skipping ignition events, or requesting an upshift in an automatic transmission. In hybrids and EVs, the powertrain controller may limit motor torque at higher road speeds. The intervention can feel like a firm “ceiling,” distinct from gentle cruise-control corrections.

Mechanical and aftermarket systems

Mechanical governors—once more common on carbureted engines—used linkages or vacuum-actuated devices to restrict throttle at high speed. Today, aftermarket and fleet systems more often integrate with the ECU or use telematics modules that can enforce limits by GPS, geofencing, or policy profiles set by fleet managers.

Types of speed limiting you may encounter

Different technologies and policies can limit how fast a car can travel, either as a hard cap or a soft, overridable constraint. Here are the most common forms drivers and owners might see in real-world use.

• ECU-based top-speed limiter (Vmax): A hard cap set by the manufacturer or tuner that prevents exceeding a specific speed (for example, 155 mph/250 km/h on many performance cars).
• Rev limiter: Protects the engine by limiting maximum RPM; in lower gears this can indirectly cap speed, but it’s not the same as a top-speed limiter.
• Intelligent Speed Assistance (ISA): Uses camera/GPS to detect posted limits and warn or gently restrict acceleration; typically driver-overridable and not a fixed top-speed cap.
• Fleet/telematics limiters: Company-set caps (often 65–70 mph/105–113 km/h for trucks) and geofenced controls managed via connected modules.
• Valet/teen modes and user-configurable limits: Features like Ford MyKey or similar systems that let owners set max speed and audio volume for specific keys or profiles.
• Limp-home protection: A temporary, strict speed/torque limit triggered by faults to protect the powertrain until service.

While these systems can feel similar from behind the wheel, only the ECU top-speed limiter and some fleet systems act as a true hard ceiling on maximum vehicle speed under normal conditions.

Why manufacturers set speed limiters

Automakers cap top speed for safety, liability, and hardware protection. Tire speed ratings, brake and driveline capabilities, and aerodynamic stability all inform the limit. Market norms also play a role: many German brands historically capped cars at 250 km/h (155 mph), while Volvo now limits all models globally to 180 km/h (112 mph). Some performance packages raise or remove the cap when supported by upgraded tires and brakes.

Rules and common settings by region

Policies and norms vary by market, affecting how speed limits are implemented on new vehicles and commercial fleets. Below are notable examples that influence how speed limiters appear in everyday driving and purchasing decisions.

• European Union: Intelligent Speed Assistance (ISA) has been mandatory for new vehicle types since July 2022 and for all new vehicles sold from July 2024. ISA provides warnings and can gently limit acceleration but is designed to be overridable and is not a fixed top-speed cap.
• United Kingdom and Europe: Many performance cars are software-limited to 155 mph (250 km/h). Some brands (e.g., Volvo) cap at 112 mph (180 km/h) across the lineup.
• United States: No federal requirement for passenger-car speed limiters. Many heavy trucks are governed by their engine control modules; fleets commonly set 65–70 mph. Federal proposals for mandatory heavy-truck speed limiters remain under consideration.
• Canada: Provinces such as Ontario and Quebec require heavy trucks to have speed limiters set to 105 km/h (65 mph).

These differences mean two otherwise similar vehicles can have different limiter behaviors depending on where they’re sold or how they’re configured for fleet use.

Can it be changed or removed?

Manufacturers can alter the limiter via approved software updates or performance packages that also upgrade tires and brakes. Aftermarket tuners can raise or remove the cap by reprogramming the ECU, but this can void warranties, violate insurance terms, and be illegal in some jurisdictions. It can also exceed the safe operating limits of tires and components. Fleet vehicles often log tampering attempts and may impose severe penalties for changes.

How to tell if your car has a limiter

If you want to know whether your car’s top speed is electronically capped, several practical checks can help you confirm the presence and type of limiter without taking unnecessary risks.

• Owner’s manual and spec sheet: Look for Vmax, top speed, or feature notes about speed limiters or ISA.
• In-car menus: Some vehicles offer speed alerts and user-settable limits (e.g., teen/valet modes) accessible through infotainment settings.
• Instrument-cluster messages: Many cars display “Speed limited to X” when approaching the cap.
• Diagnostic scan: An OBD-II tool can reveal limiter parameters or coding (often labeled as Vmax) in the ECU.
• Fleet portal or telematics app: Company vehicles may show configured limits and geofence rules.

Avoid on-road “testing” at high speed; rely on documentation and diagnostic data for confirmation.

Summary

The device that limits a car’s speed is the speed limiter (governor), typically a software function within the ECU that reduces engine or motor power once a preset speed is reached. Other systems—rev limiters, ISA, fleet telematics, and protection modes—can also restrict speed in specific contexts. Limits vary by brand, model, region, and fleet policy. Modifying or removing a limiter carries safety, legal, and warranty risks and should not be attempted without full awareness of those implications and the vehicle’s hardware capabilities.

What is an electronic speed limiter?

The speed limiter (also called the governor) operates through electronic sensors and the engine control unit (ECU). The ECU is sometimes called the engine control module (ECM). The sensors detect how fast the car is travelling and communicates this information to the ECU.

Is it possible to put a speed limiter on a car?

Our speed limiters are very straightforward to install often taking less than 1 hour including a test run meaning you have very little downtime of the vehicle.

How do I restrict the speed on a car?

To set a speed limit in a modern car, find the “LIM” or speed limiter button on your steering wheel or indicator stalk to activate the feature. Once activated, use the control switch to set your desired maximum speed, which the vehicle will not exceed. For intelligent speed limiters, the car may read speed limit signs and automatically adjust the set speed, or prompt you to do so. 
Steps to Set a Manual Speed Limit (Manual Speed Limit Assist):

1. Locate the Speed Limiter Button: Look for a button labeled “LIM” on your steering wheel or an indicator stalk. 
2. Activate the System: Press the LIM button to turn on the speed limiter function. A display will show the set speed limit. 
3. Set the Limit: Use the speed control switch (often on the same stalk) to set the maximum speed. You can typically press the “SET” button to set the limit to your current speed. 
4. Adjust the Limit: Press the speed control switch to increase or decrease the maximum speed. 
5. Override the Limit: To exceed the set speed in an emergency, press the accelerator pedal fully, which will temporarily override the limiter. 
6. Resume the Limit: Pressing the “RES” (resume) button will re-engage the limiter at the previously set speed. 

Intelligent Speed Limit Assist (ISLA):

• Automatic Operation: This feature uses a camera to read speed limit signs. 
• Automatic Adjustments: If the detected speed limit differs from your set speed, the system may prompt you to adjust the speed or change it automatically. 
• Offset Options: Some systems allow you to set a speed offset, like 5 or 10 mph over or under the posted limit. 
• Potential Unreliability: This system can sometimes misread signs due to obstructions or poor weather, so it’s important to still pay attention to road signs. 

What device limits the speed of a car?

A device to limit car speed is called a speed limiter or governor and can be built-in (like in modern passenger cars and trucks) or added as an aftermarket device. It works by sending signals to the engine’s computer to restrict fuel and air, preventing the vehicle from exceeding a pre-set speed. Modern systems can also dynamically adjust based on GPS location and road speed limits, a feature known as Intelligent Speed Assistance (ISA). 
How Speed Limiters Work 

• Sensors: Speed limiters use sensors to detect the vehicle’s speed.
• Engine Control: This speed information is sent to the vehicle’s engine control unit (ECU).
• Fuel and Air Restriction: When the vehicle reaches the set speed limit, the ECU restricts the flow of fuel and air to the engine, which reduces power and prevents further acceleration.

Types of Speed Limiters

• Standard/Factory-Installed: Opens in new tabMany modern cars and all large trucks have built-in speed limiters, often set at a high speed by the manufacturer. 
• Aftermarket Devices: Opens in new tabThese devices are installed by a professional and can be used on a wide range of vehicles, especially fleets, to control top speeds for safety, fuel economy, and compliance with regulations like the European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR). 
• Intelligent Speed Assistance (ISA): Opens in new tabA more advanced system that uses GPS and speed limit data from cameras to determine the speed limit for the current road. It can warn the driver or gently limit acceleration to match the posted speed. 

Uses of Speed Limiters

• Fleet Management: Companies use speed limiters to ensure drivers adhere to safety regulations, improve fuel efficiency by keeping vehicles in an optimal range, and prevent speeding tickets. 
• Safety: Speed limiters prevent drivers from exceeding safe speeds on the road, reducing the risk of accidents. 
• Regulatory Compliance: Certain vehicles, like heavy-duty trucks carrying dangerous goods, are legally required to be equipped with a speed limiting device.