How to Get the Most Speed from Your Honda Grom or Monkey

Why Rider Weight and Aerodynamics Matter More Than You Think!

Let’s face it, chasing top speed on a Honda Grom or Monkey is part of the fun. Whether you're dialing in a custom ECU flash, changing sprockets, or swapping parts to squeeze out every last MPH, it's an addictive process.

But what many riders overlook is that adding power is only half the equation. Aerodynamics, rider setup, and reducing drag can often yield more noticeable improvements than engine mods alone—especially on small displacement bikes like the Grom and Monkey, where every watt counts.

At CJR, we’ve tested dozens of Groms and Monkeys in controlled conditions, and the results are clear: parasitic drag and rider aerodynamics are just as important as a proper tune when it comes to top speed. Let’s break it down.

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1. Rider Weight and Body Position: Your Biggest Airbrake

▸ Weight Affects Acceleration, But Drag Limits Top Speed

While a heavier rider can hurt low-end acceleration, top speed is governed by aerodynamics more than weight. Once you’re in top gear and nearing the rev limiter, the limiting factor is how much drag your bike and body are generating.

▸ Tuck Position = Free MPH

We’ve seen firsthand how much speed riders leave on the table by sitting upright. On a stock Grom, tucking into a proper "race crouch" position can add 5–10 MPH over a fully upright position (Depending rider size). There are a few different types of tuck - 

• The simple tuck - Generally good for 3-5mph. Head slightly down, out of some of the air but still a lot of wind drag, head is above the bars and arms tight.
• The full tuck - Almost completely out of the air and good for 5-10mph depending wind conditions. Your head is all the way down, almost touching the tank or bar pad, with your butt all the way back on the seat. Knees and arms locked tight to the bike.
  

Even better? Slim-fitting gear. That oversized parka in winter creates a parachute behind you. In fact...

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2. Cold Weather = Slower Top Speed Runs: The Clothing Factor

▸ Summer Speed Gains Aren’t Just Warmer Air

During warmer months, we see many riders post better top speed runs because they’re wearing less bulky riding gear. As a back to back test, one day I went out with just gym shorts and a t-shirt. (Grom - Intake/Exhaust/CJR Flash), I was able to net 70-73mph very consistently with my weight coming in at 160lbs. I quickly swapped to a sweatshirt and jeans. With the same road and conditions, I was struggling to get to 65mph.  

• Winter jackets and loose pants increase frontal area and trap air.
• Heavy gloves and layered boots reduce rider mobility and tuck tightness.
• More clothes - More weight which decreases acceleration rates and maintaining top speed.
• Reduced tire pressure - Often overlooked, is the tire pressure during cold months often drop off slightly.  
  

All of this adds drag—dramatically. We’ve observed up to 5-10mph AVERAGE losses just from the switch in rider apparel alone.

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3. Parasitic Drag: The Silent Speed Killer

If aerodynamics is one side of the coin, parasitic drag is the other. These hidden losses rob you of precious top-end power. During dyno testing for example, we have seen a 12hp bike with 30psi tire pressure, drop all the way down to 10.2hp at 15psi tire pressure. A 15% loss of power! Here’s what to look out for:

▸ Tire Pressure

Low PSI = higher rolling resistance.
For top speed runs:

• Run the maximum recommended pressure listed on the tire sidewall, generally 30psi for the grom/monkey/dax/cub/ct125.
  

▸ Wheel Spacers and Seal Drag

The OEM wheel spacers on Groms and Monkeys tend to press into the wheel seals too tightly, creating friction at speed. Upgrading to aftermarket low-drag spacers can free up rotational resistance and net 2-3mph AVERAGE in real-world gains.

▸ Brake Drag

Check that your calipers and pads aren’t lightly dragging after a ride. Spin the wheel—if it doesn’t coast freely, that’s lost speed. Some aftermarket rotors and caliper brackets reduce drag while maintaining stopping power. Race bikes often remove their caliper springs, which is designed to keep light pressure on the pads for instance brake feedback through the pedal/lever. Not recommended to do this on the street as a delay in brake response may occur. 

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4. Chain Health and Gearing for Your Setup

▸ A Clean, Lubed Chain = Free Power

On the dyno, just like tire pressure, we have seen upwards of 12-15% loss of power from an old rusty chain, compared to a brand new one on back to back testing. 

Grimy, stretched chains sap horsepower. Always lube your chain right after a ride while the chain is still warm. This allows the lubrcation to soak into the pivots and provide the best longevity for you chain. Since these bikes do not use o-ring chains from the factory, they often sling off all the lubrication on the first speed run. O-ring chains do provide better lubrication, less maintence, however slightly increased weight. Clean and lube your chain with a lightweight lube, and ensure it’s properly tensioned—not too loose or tight.

▸ Gearing Matters

You need to match your final drive ratio to the engine’s power curve. If your Grom/Monkey makes peak power at high RPM, consider taller gearing (e.g., 14/34) to let it stretch its legs. But be careful: too tall, and you’ll never reach redline in top gear. If you bike makes a lot more mid range power, for example from a big bore kit or other hard parts upgrade, consider taller gearing as it will be able to power through the midrange into the higher RPM must better.
CJR recommends buying a -1 and +1 front sprocket as they are generally only $5-15 each. Test each and see which you like best, from there you can further fine tune your gearing with the rear sprockets. Each build configuration, and even rider preference, rider weight, aero, etc. can all alter a proper gearing ratio to suit the riding style needed. 

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5. Fuel, Oil, and Fluids: Don’t Overlook the Basics

▸ Use the Correct Fuel

Run the octane your tune was built for. For stock piston confirmation, generally it is low compression which would prefer a FASTER burning fuel with LOWER octane like 87-91. When running a high compression application like a high comp piston or big bore kit, using lower octane than recommended may cause ignition knock, also known as detonation, which hurts performance. High compression will always want a slower burn/higher octane. Which is why a lot of customers opt for E85, as it burns slower and much cooler with high compression applications, which makes for more power.

▸ Oil Level and Viscosity

• Too much oil = Drag from crankshaft spinning through oil and reducing ramp up speed
• Too little oil = Risking oil starvation which can damage the engine internals. 
• Viscosity matters—lighter oils (e.g., 0W-10 or 10W-30) reduce internal drag slightly compared to thicker oils (10w-40 or 20w-50). Just ensure it’s still safe for your environment and riding style.

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Summary: Tuning is Only Part of the Picture

Your top speed potential isn’t just in the ECU—it’s in the details.
Combining a solid flash with rider optimization, reduced drag, and mechanical efficiency will give you the best chance at posting high numbers on GPS.

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Want to Go Faster?

Our ECU Flashing services are designed with all of this in mind. We build maps not just for peak dyno power, but for real-world conditions—factoring in rider weight, gearing, and aerodynamic load.

If you’re chasing the fastest Grom or Monkey around, don’t overlook the hidden drag. Reach out to CJR and let’s get your setup dialed in to match your build, and riding style.