Turning the Arrma Granite Grom Into a 50 MPH Beast: Here’s How!

The Arrma Granite Grom starts as a plucky 1/18-scale monster truck, but with some serious upgrades—like the Furitek Scorpion brushless motor, Powerhobby PH60 servo, Arrma aluminum shocks, metal diffs, and a steel universal kit—it’s morphed into a tiny titan.

First Impressions
Out of the box, the Grom’s got that Arrma grit: a sturdy composite chassis, chunky dBoots Fortress tires, and a clipless body that’s a cinch to remove. At 13 inches long, it’s backpack-friendly, and the stock 2S LiPo with USB-C charger makes it an easy RTR package. Mine shipped with a higher front end than rear—a “raked” stance that looked cool but threw off the balance. Stock, it’s a solid basher, but with these upgrades, it’s a whole different beast.
Performance: Stock vs. Fully Upgraded
Stock, the MEGA 380 brushed motor and 4WD drivetrain hit 20-25 mph—decent for a beginner rig. The suspension was okay for small jumps, but that front-high stance made it understeer and pitch back in the air. Here’s how the upgrades flipped the script, with speed details on 2S and 3S using the stock 16T pinion:

Furitek Scorpion Brushless Motor (5600kV) : Ditching the brushed motor for this sensored brushless setup (with matching ESC) turned the Grom into a rocket. The 5600kV rating means RPM per volt, and with the stock 16T pinion and 57T spur gear (3.56:1 ratio), speeds scale with battery voltage. Real-world factors—drag, weight (~2.5 lbs), and ~80% efficiency—trim the theoretical max, but it’s still fast. The motor and ESC are fully programmable via a free iPhone or Android app—I dialed the punch setting down to 1 to tame the initial kick, making it smoother off the line, especially on 3S. Calibrating the speed control to the Spektrum iXSR radio was key; I noticed a huge power boost afterward, unlocking the full potential—don’t skip this step! The sensored motor’s smoothness gives awesome air control too—jumps feel precise and predictable, no jerky nonsense.

On 2S (7.4V): Theoretical motor RPM is 41,440 (5600 × 7.4). Divide by the 3.56:1 gear ratio (~11,634 wheel RPM), and with ~7.85” tire circumference, that’s ~17.3 ft/s or ~11,800 ft/min. Converted, theoretical top speed is ~43.3 mph, but losses drop it to 33-38 mph. I’ve hit ~35 mph on pavement—plenty quick for this little rig.

On 3S (11.1V): Theoretical RPM jumps to 62,160 (5600 × 11.1). Same gearing gives ~17,461 wheel RPM, or ~25.9 ft/s (~15,540 ft/min), equating to ~64.9 mph theoretically. Real-world caps it at 48-53 mph due to heat, drag, and drivetrain limits. I’ve clocked ~50 mph—nuts for a 1/18-scale truck, and it’s a handful to control!

Optional Spektrum 3S Battery: For 3S power, I’ve also run the Spektrum 11.1V 2200mAh 3S 30C Smart LiPo Battery with IC3 Connector . It’s a compact, affordable option that fits the Grom’s battery tray and works with the iXSR’s telemetry. Priced at ~$54.99 (as of March 12, 2025, from Horizon Hobby or Amazon), it’s lighter than bigger 3S packs (180g vs. 300g+ for 5000mAh options), keeping the rig nimble. On 3S with this battery, I still hit 48-50 mph, and the Smart tech auto-discharges to storage voltage—handy for maintenance. It’s not the punchiest 3S pack (30C vs. 50C+ alternatives), but it’s reliable and budget-friendly for casual bashing.

Powerhobby PH60 Servo . The stock steering felt mushy under load; the PH60 is a beast—fast, precise, and unfazed by the brushless torque. Corners are tight, and mid-air corrections are spot-on. You’ll need a Hot Racing Micro 25T Servo Saver (part #HRAMG48025) available on Parkflyers to make it work right—for some reason, the Powerhobby-branded version incompatible, Parkflyers link comes with the wrong hardware and won’t fit properly. The Hot Racing one bolts up perfectly and handles the PH60’s power like a champ. Also, the Furitek Carbon Fiber Servo Mount (part #FUR2028) available on Parkflyers is a must to secure it properly—stock mounting won’t cut it with this servo’s grunt.
Arrma Aluminum Shock Set Swapped the stock shocks for Arrma’s aluminum upgrade—sleek, durable, and filled with Traxxas 80wt shock oil (Traxxas 5037, 1000cSt, 60cc silicone) to slow rebound speed. I upped it from the stock ~40wt to 80wt available on Parkflyers, and it pairs perfectly with the spring cut (see below) to keep it level and soak up landings—no more bottoming out under the brushless power. The heavier oil makes it settle smoother after jumps and keeps it planted on rough terrain, a noticeable step up from the stock bounciness.

Metal Diffs (ARA310944 - Metal Differential Gear Set): The stock diffs were a known weak point, especially with brushless power. Upgraded to all-metal differentials, and they’re bulletproof—zero slop, no grinding, just pure power to the wheels. For better launch off the line, swap the rear diff fluid to Kyosho 1000-weight grease—it hooks up harder and gets you moving quick.

Steel Universal Kit (ARA310943 - Steel Driveshaft Set): Ditched the stock driveshafts for a steel universal setup.