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The Honda Grom is a tiny motorcycle with a big personality: short wheelbase, punchy little engine, and an attitude that turns ordinary streets into places worth exploring. In stock form it’s not built to chase sport bikes — it’s built to be fun, easy to ride, and cheap to run. 

Still, riders ask the same question again and again: “How fast will it go?” The honest answer depends on a few facts: the exact model year, rider weight, gearing, road slope, and how you measure speed. 

In this article I’ll explain what the factory says about the Grom’s power and gearing, what real riders report on level ground, how modifications change the result, and the trade-offs you should consider (safety, legality, reliability). 

I’ll include real, verifiable facts from manufacturer and industry sources and practical steps you can take if you want slightly more top end — or simply a more usable bike for daily riding.

Short note on sources

This guide draws on manufacturer specifications and reputable industry testing plus real-world owner data:

• Honda official specifications and press material for the MSX125/Grom.
• Cycle World’s Grom coverage and road test summaries.
• Independent reviews and road tests (V1Moto, long-term tester reports) for real-world top speed ranges.
• Grom and motorcycle owner forums and builder pages for modification practices, sprocket changes, and big-bore kit experiences (practical, community-tested knowledge). 

A quick reality check: what the Grom is and what it isn’t

The modern Grom is a 124-cc air-cooled single with fuel injection and a compact chassis. It was designed as a minibike that behaves like a full-sized motorcycle in controls and handling, but in a very small package. 

The 2022-onward models use a 5-speed gearbox, and Honda quotes modest output figures that match the bike’s city-friendly role: single-digit horsepower in the mid-to-high range and a few pound-feet of torque. 

Those small numbers explain why the Grom’s stock top speed sits well below typical highway speeds on many roads. The manufacturer’s spec sheets and reputable reviews consistently place the Grom’s usable top speed in the neighborhood of the high 40s to low 60s mph, depending on conditions.

What Honda officially lists (engine and transmission facts)

To understand top speed you first need the basic mechanical data. Honda’s published specifications for the Grom show a roughly 123–124 cc air-cooled SOHC single and, on recent models, an engine producing about 9.6–9.7 horsepower and around 7.7 lb-ft (≈10.4 Nm) of torque. 

From 2022 onward Honda moved the Grom to a 5-speed transmission for smoother cruising and more flexible ratios. Those figures set the performance ceiling: small displacement, modest torque, and a gearbox tuned for city acceleration and usability rather than high-speed runs.

Stock top speed — what riders actually record

If you search owner reports and real-world tests you’ll find a range rather than one fixed number. Most reliable reviews and many owner logs report a practical top speed of roughly 55–60 mph (≈88–97 km/h) on level ground with a light rider and favorable conditions. 

On a flat road with two riders or heavier load, that number drops — sometimes into the high 40s to low 50s mph. On long downhills people have recorded higher GPS speeds, but those are gravity-assisted and not representative of sustained top speed on flat pavement. 

Cycle World and other established motorcycling outlets echo that the Grom’s real-world steady top speed is around the 55–60 mph mark for a stock machine under ideal conditions.

Why the spread? Rider weight, aero (a 6′ rider is a lot less slippery on a tiny bike), wind, temperature, and even tire pressure all change real top speed. The tiny frontal area that makes the Grom cute also means it’s easily slowed by wind.

How top speed is limited by gearing, engine power, and rpm

Top speed is a simple physics and gearing question: the engine produces limited power, and the gearing (primary, gearbox, final sprocket) determines how fast that engine rpm translates to wheel speed. 

If the bike reaches the engine’s peak rpm in top gear before it has converted enough torque into road speed, that rpm limit sets the top speed. In short:

• If gearing is too short (low), the engine hits redline early and you get lots of acceleration but a modest top speed.
• If gearing is too tall, the bike may not have enough torque to climb to peak rpm in top gear, so it can feel sluggish and fail to reach a higher top speed.

Honda set the Grom’s gearing to balance city acceleration, usability, and fuel economy. A small change to sprocket sizes (one tooth on the front or rear) is the most common tweak owners use to shift that balance toward a little more top speed or a little more low-end punch — but you can’t get magic power out of a 125-cc motor.

Typical measured results (summary of tests and owner runs)

Across forums, YouTube runs, and magazine road tests you’ll see a pattern:

• Many stock Groms cruise comfortably at 45–55 mph and will reach 55–60 mph on a flat with a light rider. 
• Very few truly hit 70+ mph on level ground without significant modification or downhill assistance; reports of higher speeds are either downhill runs, lighter-than-average riders, or bikes with gearing and big-bore upgrades. 
• Speedo readings (dash) often overstate actual GPS speed, so GPS-verified runs are the most reliable sources. Several GPS runs posted by owners and testers cluster around the 55–65 mph bracket with stock gearing. 

How modifications change the picture (and the trade-offs)

If stock top speed isn’t enough, riders commonly choose one or more of these paths — each with predictable effects and costs.

1. Gearing changes (sprocket swaps)

Dropping one tooth on the front sprocket or reducing teeth on the rear changes final drive ratio. A smaller front sprocket (e.g., from 15→14T) raises top speed potential at the cost of a tiny loss of initial acceleration — and one tooth can make 5–10% difference in gearing feel. 

Many owners report the 14/34 or similar combos make 5th gear usable and increase cruising speed without adding engine modifications. The change is cheap and reversible, but it doesn’t increase peak power. 

2. Intake/exhaust/tune (mild performance mods)

Freeing airflow with a high-flow air filter, a freer exhaust, and a matching fuel/ignition tune can add a couple of horsepower and improve midrange. That can translate to slightly higher top speed or a more usable roll on. Gains are modest unless you rework other parts of the motor and tune the ECU properly.

3. Big-bore kits and internal engine work

Companies sell big-bore kits that enlarge displacement (139, 149, 170, 186 cc and beyond). A properly built big-bore with correct fueling and cooling can multiply power — some kits claim 30–70% more horsepower. 

With those gains and a taller final drive, street top speeds of 70–85 mph become plausible, depending on the build and gearing. These builds add cost, complexity, and stress on the stock transmission and brakes; they often require ECU reflashes, upgraded cooling, and stronger clutches. 

A well-known builder lists packages that move a Grom into the 16–18 hp range and reports high-speed runs in the 70s with matching gearing.

4. Weight reduction and aerodynamics

On a tiny bike, small weight savings and a smoother tuck can help. A lighter rider or a more aerodynamic riding position improves top speed more than you might expect because drag and power are tight at the bike’s small power level.

Trade-offs: More speed usually shortens engine life, increases heat, and may compromise legal status and safety. Bigger power requires better brakes, better tires, and often upgraded cooling. Law and insurance implications also apply in some areas.

Practical steps to measure your Grom’s top speed accurately

If you want to test your bike, do it safely and legally:

1. Use a GPS logging device or phone app (not the speedometer) to record true speed. Dash speedometers can be optimistic.
2. Test on a closed road or track, or a long, empty, legal road with minimal traffic. Never attempt top speed on public streets with traffic.
3. Test with a light rider, correct tire pressure, and no luggage to get a repeatable number.
4. Record weather — wind and temperature matter.
5. Repeat runs in both directions and average them if possible; this cancels wind advantage.

Many riders post GPS-verified runs online; these are the most reliable comparisons.

Safety and reliability considerations

If you modify a Grom for more speed, plan for the supporting systems:

• Brakes: stock discs do a good job for stock speeds, but higher top speed and power mean you should consider better pads, braided lines, and perhaps larger rotors (or more aggressive pads).
• Tires: choose tires rated for the higher speeds you plan to run; many small-wheel tires have speed limits that can be exceeded by aggressive modifications.
• Cooling: bigger engines and higher sustained speeds generate heat — added oil coolers or improved ventilation help reliability.
• Legal and warranty: altering the ECU or installing big-bore kits usually voids factory warranty and can affect legal compliance for emissions in some areas.

What a typical buyer should choose

If you want a simple commuter/minibike with the ability to hop onto faster roads occasionally, buy the stock Grom and accept a realistic cruising speed of around 45–55 mph. 

If you want a sportier, faster ride and are willing to invest time and money, a combination of a mild tune, intake/exhaust improvements, and a single-tooth gearing change will give a higher usable highway speed without brutal compromises. 

For radical speed and acceleration, a professional big-bore build with ECU reflashing and upgraded running gear is the way — but it turns the Grom from a simple commuter into a hobby project and must be done by informed builders.

Real facts you can rely on (short list)

• Honda’s modern Grom uses a ≈123.9–124.9 cc air-cooled single and (on recent models) a 5-speed transmission. Reported output is about 9.6–9.7 hp and roughly 7.7 lb-ft of torque.
• Industry reviews and owner reports put a typical stock flat-ground top speed near 55–60 mph for a light rider in ideal conditions; heavier riders and headwinds reduce that number.
• Gearing changes (sprocket swaps) are the most common low-cost way to alter top speed or make the new 5th gear useful; a single tooth change in the front sprocket often makes a noticeable difference.

Final recommendations

• If you want reliable, legal, and low-cost urban mobility: buy the stock Grom, learn its limits, and enjoy its agility. It’s a brilliant small motorcycle for short trips.
• If you want more usable highway performance while staying conservative: change gearing (front sprocket −1 tooth), maintain tires/pressure, and consider a mild intake/exhaust tune. Keep supporting upgrades in mind (brakes, tires).

If you want big power and top speed: plan for a full build, budget for ECU work, cooling, and stronger hardware, and accept warranty, emissions, and safety trade-offs.