RC top speed calculator
Top speed estimate = motor KV x battery voltage x rollout. KV times volts gives motor rpm with no load. Multiply rpm by rollout, the distance the car moves per motor turn, to get speed. Real speed on the track is lower, because load, drag and voltage drop cut motor rpm.
How to use it
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Why the real number is lower
KV times volts is the motor's speed with nothing attached. On the track the motor pushes the car through drag, tire scrub and the drivetrain, and pack voltage drops under load. All of that cuts rpm, so treat this number as a ceiling.
It works best for comparing two gearing choices on the same car, motor and battery.
Motors rated in turns
Many racing motors are sold by turns, like 17.5T, instead of KV. If your motor maker does not list a KV, this estimate will not be reliable. Compare rollout with the gear ratio calculator instead.
Common questions
Is this my real top speed?
No. It is a no-load estimate. Drag, load and voltage drop make the car slower on the track. Use it to compare setups, not to predict a radar number.
Which voltage should I enter?
Use nominal voltage, 3.7 V per LiPo cell, for a middle estimate. Use full charge, 4.2 V per cell, for the highest number the pack can give.
How do I convert mph and km/h?
1 mph is 1.609344 km/h. The calculator shows both.
What if my motor is rated in turns, not KV?
The estimate needs a KV number. If the maker does not list one, compare rollout with the gear ratio calculator instead.
Does a bigger tire make the car faster?
A bigger tire raises rollout, so the estimate goes up. It also loads the motor more, so real speed gains are smaller and the motor runs hotter.
Keep your gearing with the setup
RacerRX does the gear ratio, FDR and rollout math on every setup sheet. 14-day free trial.