Why Your E-Bike Isn’t Reaching Its Top Speed (and How to Fix It)

If your e-bike isn’t reaching its top speed, it’s almost always because of one of four things: a built-in speed limit (class cap), a battery that can’t deliver full power (low charge/aging/voltage sag), extra drag from tires or brakes, or a setting/sensor issue telling the bike to hold back. 

The fix is usually simple: confirm your bike’s speed limit, fully charge and check battery health, remove mechanical drag, then verify your display and speed sensor setup.

This guide will walk you through exactly how to diagnose the cause and get your speed back.

Understanding E-Bike Top Speed and Limits

Before diving into fixes, it helps to understand what “top speed” really means for an e-bike. First, make sure you know what top speed to expect from your specific model.

Legal Class Limits

E-bikes are categorized by classes with set speed caps. In the U.S., Class 1 and Class 2 e-bikes stop assisting at 20 mph, while Class 3 e-bikes assist up to 28 mph. (In Europe, the limit for assist is typically 25 km/h, about 15.5 mph.)

Even if your bike’s motor can go faster, the controller will cut power at these limits to comply with the law. That means if you have a Class 1 or 2 bike, it’s normal that it won’t go above 20 mph under motor power. You can pedal faster downhill, but the motor won’t help beyond its cap.

Manufacturer Specs vs. Reality

Sometimes an e-bike is advertised with a high “up to” speed, but that might require ideal conditions or active pedaling. For example, a bike might only assist to 20 mph, but marketing claims “28 mph with hard pedaling” – meaning you’d have to pedal vigorously to reach 28 mph on your own power.

Understand where the motor stops helping. If your e-bike top speed is too slow compared to what you expected, double-check the specs: the assisted top speed might be lower than the headline speed in fine print.

Built-in Limiters

Many e-bikes have software or hardware limiters set by the manufacturer. These ensure you don’t exceed legal or safe speeds. In some models, you might even have the option to adjust the limiter (for example, switching between 20 mph and 28 mph modes) via settings or by unlocking a higher class mode.

Always do this legally – removing a speed limiter for road use can be illegal and unsafe. For instance, tampering to go beyond 28 mph on public roads could result in fines or your bike being classified as a motor vehicle. So, know your bike’s intended limit before assuming something’s wrong.

Environmental Factors

Remember that advertised top speeds assume flat ground, no wind, and a reasonably light load. If you’re riding uphill or against a strong headwind, or if you’re a heavier rider carrying cargo, your e-bike may not reach its maximum speed. That’s normal physics – more load or drag slows any bike down. We’ll discuss ways to mitigate these effects later (like using the right gears and keeping tires pumped up).

Common Reasons Your E-Bike Isn’t Reaching Top Speed (and Fixes)

Here are the most common reasons an e-bike runs slower than it should—and the exact fixes, from quick beginner checks to advanced troubleshooting.

1. Battery and Power Issues

Why it happens: The battery is your e-bike’s fuel tank. If it’s weak or nearly empty, you’ll experience a drop in performance. An ebike not reaching top speed is often caused by insufficient battery power. Here’s why:

Low Charge

E-bike batteries deliver higher voltage when fully charged. As the charge drops, voltage and current output drop too. That means a 50% charged battery can’t push the motor as hard as a full battery, reducing your peak speed. You might notice your bike hits top speed in the first part of your ride, then struggles to later – that’s the low battery effect.

Aging Battery

Over time (typically after a few years or hundreds of charge cycles), lithium-ion batteries hold less charge and their voltage under load sags more. For example, after ~500 full charge cycles, a battery might only deliver ~80% of its original capacity. An older battery can cause your electric bike not reaching top speed because it simply can’t supply the necessary power.

Voltage Sag and Power Limit

When you demand high speed (which requires high power), an older or smaller battery might experience voltage sag – the voltage drops momentarily under heavy load. If the voltage falls below what the motor/controller needs for top speed, the bike will plateau at a lower speed. This is more pronounced if your battery is near empty or not designed for high discharge.

Loose or Weak Connections

Sometimes the issue isn’t the battery cells themselves but the delivery of power. Loose battery connectors, corroded terminals, or a worn wiring harness can restrict current flow. The result is similar to a weak battery – the motor can’t draw enough power to hit top speed.

How to fix it:

Charge Up

The simplest fix – make sure you start your ride with a full charge. If you need max speed, avoid running the battery down to the last bar. Lithium batteries perform best above ~20% charge. If you notice speed dropping as the battery empties, plan to recharge sooner. Carrying a spare battery is an option for longer rides where performance is critical.

Check Battery Health

If your e-bike is a couple of years old and suddenly feels slower, test the battery. Many e-bike display systems can show voltage or have a diagnostic. You can also have the battery’s capacity tested. If it’s degraded, consider a replacement pack. A fresh battery can restore that zippy top speed you remember.

Inspect Connections

Ensure the battery is fitting snugly in its mount and the contacts are clean. Power off the bike and carefully unplug and replug the battery connectors if accessible, checking for any dirt or corrosion. Clean contacts with electrical contact cleaner if needed. A firm, clean connection helps deliver full power to the motor.

Upgrade Battery (Advanced)

If you’re a power user, you might consider upgrading to a higher-voltage battery and compatible controller/motor to increase speed. Warning: Only do this if you know what you’re doing, as it can damage the system or be unsafe if mismatched. For example, moving from a 36V to 48V system can raise the motor’s top RPM, but all components must handle it. This is an advanced modification to attempt only off-road or with compliance to local laws.

Tip for novices: Most of the time, simply keeping your battery well-charged and eventually replacing it when it’s worn out will solve battery-related speed issues. 

Tip for advanced riders: Monitor your battery’s voltage sag with a wattmeter or your bike’s app – if the voltage drops sharply under full throttle, it might be time for a new battery or a higher capacity one.

2. Speed Limiters and Software Settings

Why it happens: Modern e-bikes often come programmed with speed limits. If your bike consistently won’t go above a specific speed (like exactly 20 mph or 25 km/h), a limiter is likely in effect. This isn’t really a “problem” – it’s by design:

Class Restrictions

As discussed, if you have a Class 1 or 2 e-bike, the motor will cut off at 20 mph by law. That means no matter how hard you throttle or pedal (with assist), the motor won’t push past that. The bike isn’t underperforming; it’s obeying its programming. Riders sometimes aren’t aware of this and think something’s wrong.

Eco Modes or Speed Settings

Many e-bikes have multiple assist modes (Eco, Normal, Sport/Turbo). In lower modes, the bike may intentionally cap speed to conserve battery. For instance, Eco mode might only assist up to 15 mph even if the bike can do 20+ in higher mode. If you accidentally left your bike in a low-power mode, that could be why your e-bike top speed is too slow.

Similarly, some displays allow you to set a custom speed limit (for example, setting the max assist speed to 15 mph for a child or for safety). Double-check your settings.

Wheel Size or Unit Settings

Believe it or not, if your bike’s computer is set to the wrong wheel diameter, it could misread speed. This typically affects the speedometer accuracy and the limiter trigger. For example, if you have 27.5-inch wheels but the controller thinks you have 20-inch wheels, it will think you’re going much faster than you are and cut the motor early (because wheel rotations are counted differently).

Also ensure it’s set to the right unit (mph vs km/h) for your region’s limit. This is an uncommon issue but worth checking if you or someone tinkered with the settings.

Software Glitches

Occasionally, a firmware bug or error can cause the bike to go into a sort of “limp mode,” restricting speed. This might happen if a sensor fault is detected. Many e-bikes have error codes – if you see one on your display, look it up in your manual. A speed sensor fault, for instance, might cause the controller to cut power sporadically, making it hard to reach top speed.

How to fix it:

Adjust Settings/Mode

The quick fix if you suspect a limiter or mode issue is to check your display. Make sure you’re in the highest assist level when trying to hit top speed. Go into the settings menu (if your bike has one) and see if there’s a max speed parameter – set it to the highest allowed (typically the default for your class). If you deliberately need a slower setting (say for a beginner rider), remember to switch it back when you want full speed.

Remove or Raise the Limiter (Legally)

Some e-bike models allow you to “unlock” a higher speed mode (for example, converting a Class 2 into Class 3 for off-road use). Check your bike’s manual or manufacturer website – there may be a procedure or an official upgrade kit. Only do this in compliance with local laws. If you increase the top speed, use it on private property or areas where it’s legal.

Keep in mind that removing a speed limiter for road use can make your bike illegal and unsafe without proper upgrades (brakes, etc.). Always wear safety gear and understand the risks if you choose to derestrict the bike.

Recalibrate Sensors

If you suspect the speed sensor is misreading, inspect the sensor and magnet on your wheel or motor. Ensure the magnet hasn’t slipped out of place. Most sensors require the magnet to pass by within a few millimeters. Adjust its position if needed. Also verify that the correct wheel size is set in the bike’s settings (consult your manual for how to do this; some e-bikes let you change it, some auto-calibrate). Once things are calibrated, the controller will correctly allow assist up to the proper speed.

Reset or Update Firmware

For glitchy behavior, try turning the bike off and on, or doing a full system reset if your bike’s manual suggests a procedure for that. In some cases, updating the bike’s firmware (via companion app or USB) can resolve known issues. Check the manufacturer’s support to see if there are firmware updates that, for example, fix a misbehaving controller. If you’re not comfortable doing this, a local e-bike service shop can help.

Note: If you’re a novice rider and not very techy, focus on the basics: use the correct assist mode and don’t worry about firmware.

If you’re an advanced user, you might explore custom controller software or devices that trick the speed sensor for off-road high-speed riding – just be very cautious with such mods.

3. Motor or Controller Limitations

Why it happens: Your e-bike’s motor and controller are the heart of its performance. Sometimes, the reason your bike isn’t reaching top speed comes down to the capabilities or condition of these components:

Motor Power and Design

Each motor has a maximum RPM it can spin at given a certain voltage – this is a built-in physical limit. For example, a typical 250W hub motor on a 36V system might inherently top out around 20 mph on flat ground. Pushing beyond that might just be outside its design. In technical terms, as the motor spins faster, it generates a counter-voltage (back EMF) that eventually balances the battery voltage, so the motor can’t go faster without a higher voltage.

In short, if everything is working right and you’re still stuck at, say, 20 mph, it could be that the motor is at its max speed for the current setup. The fix for this would be upgrading motor or battery (not trivial), which we’ll touch on in the advanced tips.

Controller Current Limit

The controller (the electronic brain) governs how much current goes to the motor. If it’s programmed to limit current (amperage) to a certain level, that caps the motor’s torque and power. While current limit affects acceleration more than top flat-ground speed, if your bike needs a little extra push to overcome wind resistance at high speed, a low current limit might keep it from accelerating further.

Some enthusiast riders use programmable controllers to raise the amperage (within what the motor and battery can handle) for more power and a bit more speed under load.

Overheating or Thermal Limits

If you ride hard for a while (especially in hot weather or up hills), the motor or controller can heat up. Many e-bikes have thermal protection that will roll back power to prevent damage. A hot controller might temporarily restrict power, making it hard to reach top speed until it cools down.

If you notice the bike is fast at first but after a long climb it won’t hit top speed, it could be thermal throttling. The motor may also get hot; hub motors in particular can overheat on hills and then you’ll feel performance sag.

Sensor Issues

E-bikes rely on various sensors (speed sensor, pedal cadence or torque sensor, motor hall sensors). If any of these malfunction, the controller may limit output. For instance, if the cadence sensor isn’t detecting pedaling properly, the motor might cut in and out, never giving full assist to reach top speed. Or if a hall sensor in a motor is failing, the controller might go into a safe mode with reduced power. These issues often come with jerky performance or error codes rather than just a smooth low top speed, but it’s worth mentioning.

Firmware/Settings Limits

Beyond just speed, some e-bike controllers have settings for things like “soft start,” acceleration ramp, etc. If someone tinkered with a programming cable, they might have set conservative limits. Similarly, if you installed a new controller or motor kit, it might be configured for a different wheel size or speed limit by default.

How to fix it:

Respect the Specs

First, determine if your e-bike is actually performing within normal specs. Check the manual or product page for its rated top assisted speed. If it says 20 mph and you’re getting ~19-20 mph, that might be as good as it gets without modifications. However, if it used to go faster and now doesn’t, or the spec is higher than what you get, continue with troubleshooting below.

Cooling and Riding Style

If overheating is an issue, give the bike (and yourself) a break. Allow it a minute to cool down and see if speed returns. Long term, if you frequently push the bike to its limits, consider a motor with higher power or better cooling. Some riders add fins or heat sinks to hub motors to dissipate heat.

Also, try not to ride at full throttle for extended periods; easing off occasionally can help components last. Ensure the motor vents (if any) aren’t clogged with dirt.

Check Error Codes

Many controllers flash error codes via the display or LED when something’s wrong (like “ERR21” for motor hall sensor failure, etc., depending on brand). If your top speed issue is accompanied by any weird behavior, plug into those clues. The fix might be as simple as adjusting a sensor alignment or as complex as replacing a controller. If unsure, a technician can run diagnostics.

Controller Tuning (Advanced)

If you’re comfortable with e-bike electronics, some controllers (especially aftermarket or open-source ones) allow tuning via software. You could slightly increase the speed limit or current limit. Do your research first! Small changes can have big effects on reliability.

For example, upping a controller’s max current from 15A to 18A might give a little more oomph, but also more heat. And raising a speed limit beyond what the motor can handle might do nothing or risk damage. Proceed with caution and only within the safe range for your motor and battery.

Motor Upgrade (Advanced)

In cases where the motor is just too small or slow for your liking, you might consider an upgrade. This could mean swapping to a higher KV motor (in a kit build) or even a whole new wheel motor that’s geared for speed.

Another trick some use is increasing wheel diameter (which effectively increases speed if the motor has the torque, but it can strain it). Again, these are advanced modifications – if you’re a newer rider, it’s usually better to buy an e-bike that meets your speed needs rather than hot-rodding a low-speed model.

4. Mechanical Drag and Environmental Factors

Why it happens: Sometimes the problem isn’t electronic at all – it’s basic physics or bike mechanics holding you back. E-bikes are still bikes, and standard bicycle issues can make your ride slower:

Tire Pressure and Rolling Resistance

Underinflated tires create more rolling resistance, which can significantly slow you down. Fat tire e-bikes or mountain e-bikes especially suffer if tire pressure is low – the wide tires will drag. Even on a road e-bike, if your tires are supposed to be at 50 PSI and they’re at 20 PSI, you’ll feel a huge drop in speed.

In fact, research shows that a 10% drop in tire pressure can cause around a 5–7% decrease in speed under optimal conditions. That’s a big hit to performance just from soft tires!

Brake Drag

Disc brakes or rim brakes that are slightly rubbing on the wheel will constantly slow you down. It might not be super obvious if it’s mild, but it can easily eat a couple of mph off your top speed because the motor has to fight the brake. Lift each wheel and spin it by hand: it should spin freely. If it stops quickly or you hear rubbing, you need to adjust your brakes.

Drive Train Resistance

A dry or rusty chain, misaligned derailleur, or grinding bearings in your bottom bracket, pedals, or wheels can introduce friction. Each bit of friction means the motor’s power is wasted overcoming it instead of propelling you forward. Most noticeable would be something like a seized bearing or extremely rusty chain – your bike would feel sluggish. Regular maintenance prevents this.

Weight Load

The more weight the motor has to move, the harder it is to reach top speed. This includes the rider plus any cargo. If you’ve added heavy accessories (like a loaded rear rack, child seat, etc.), don’t be surprised if the bike feels slower. The top speed on flat ground might not drop drastically just from weight (since wind resistance is the bigger factor at higher speeds), but getting up to speed will take longer, and on inclines or wind, a heavier load definitely keeps you slower.

Hills and Wind

While these don’t “limit” your top speed on a perfectly flat road, they effectively do in real-world riding. Going uphill, gravity fights the motor, often preventing you from hitting the usual max speed. A strong headwind can have a similar effect on flat ground, making your e-bike struggle to hit its normal max. This isn’t a fault with the bike – it’s just conditions.

Conversely, a tailwind or downhill will let you exceed the normal top speed (just remember the motor may cut out once you exceed the assist limit, and you’re on your own beyond that).

How to fix it:

Pump Up Your Tires

This is one of the easiest and most impactful fixes. Check your tire sidewall for the recommended PSI range. For road and hybrid e-bikes, it might be 50–70 PSI; for fat-tire e-bikes, maybe 15–30 PSI (depending on if on road or off-road). Keep them toward the higher end of the range for maximum speed and efficiency on smooth surfaces. The ride might be a tad bumpier with firm tires, but your bike will roll much faster.

Tip: Get a good tire gauge or pump with a gauge and check at least weekly, as tires naturally lose pressure over time.

Brake Adjustment

If you find your brakes are rubbing, adjust them. For disc brakes, this might mean re-centering the caliper (loosen mounting bolts, squeeze brake, tighten bolts), or adjusting the pad clearance if your system allows. For rim brakes, turn the adjustment barrel or recentralize the caliper so it’s not constantly touching the rim. Make sure wheels are true (straight) as well; a warped wheel can rub brakes periodically.

After adjustments, do a spin test again – the wheel should spin freely with maybe a slight sound of light contact at one spot at most.

Lubricate and Tune the Drive Train

A well-lubed chain can make a noticeable difference in pedal effort. Clean your chain and apply bicycle chain lube regularly (e.g., every 100-200 miles or after rain). Ensure your gears shift smoothly; index the derailleur if needed so the chain isn’t trying to climb to another gear (which can create drag).

If you suspect a bearing issue (gritty feeling when spinning wheels or pedals), consider servicing or replacing those components. Smooth, freely moving parts let more of the motor’s power translate to speed.

Lighten the Load (if possible)

This might be as simple as removing unnecessary items from your e-bike when you want to go fast. Do you really need that heavy lock or pannier during this ride? Of course, you can’t change your body weight easily, but you can make small choices like not carrying extra water or tools unless needed.

If you often carry cargo or a passenger, you might look into a more powerful e-bike designed for that load (some cargo e-bikes have stronger motors to compensate).

Adapt to Conditions

For hills, use the appropriate gear if you have a multi-speed e-bike. If you’re in too high a gear on a climb, you’ll bog down and the motor will struggle – shift to a lower gear to keep the motor RPM up and help it out by pedaling. You might not reach top speed uphill (most e-bikes won’t), but you’ll climb faster with the right gear.

For headwinds, tucking a bit (reducing your upright profile) and maintaining a steady pedal cadence can help. If high speed is crucial, plan routes on calmer days or with more sheltered roads. These are more riding tips than “fixes,” but they can improve your average speed significantly.