Units
Scale presets
1Gear ratio solver
Enter pinion (motor) and spur/crown (axle) teeth. Watch the gears mesh.
3.00 : 1
2Rollout + scale speed lab
This slot car rollout calculator turns tire diameter and gear ratio into rollout (distance per motor revolution), then adds motor RPM for real and scale top speed.
20.94 mm / motor rev (mm mode)
Real 1:1 top speed
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Scale speed 1/32
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Scale speed 1/24
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Axle RPM: —
3Target-ratio & rollout finder
Know the ratio (or rollout) you want? Get every pinion/spur combo that hits it, ranked by accuracy.
Tap any row to load it into the solver.
| # | Pinion | Spur | Ratio | Error | Stock |
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4Interactive gear chart
The classic slot car gear ratio chart — tap any cell to load it into the solver. Red = short gearing, blue = tall.
short (≥3.40)
neutral
tall (≤2.40)
548 vs 64 pitch lab
Pitch = teeth per inch of gear diameter. Finer pitch = smaller gear at the same tooth count.
Pitch diameter
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Module
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48 pitch — the tough one
- Stronger teeth — survives big power and crashes.
- Forgiving mesh — easier to set up, tolerates wear.
- Slightly noisier, a touch more friction.
- Go-to for 1/24 and high-power classes.
64 pitch — the smooth one
- Smoother & quieter — finer teeth, less drag.
- Smaller diameter — more ground clearance, finer ratio steps.
- Teeth are weaker — mesh must be spot-on.
- Favorite for 1/32 club racing.
Mesh compatibility checker
Ideal center distance (shaft to shaft): —
6Track-type gearing advisor
Twisty track or fast track? This pocket gearing guide gives you a gearing direction and concrete pinion/spur combos to try.
Tire size converter
How the math works
- Gear ratio
ratio = spur teeth ÷ pinion teeth— motor revolutions per one axle revolution. 9T/27T → 3.00:1. - Rollout
rollout = π × tire diameter ÷ ratio— distance traveled per motor revolution. 20 mm tire at 3:1 → 20.94 mm. - Real top speed
rollout × RPM × 60 ÷ 1,000,000→ km/h (or ÷ 63,360 → mph for inches). - Scale speed = real speed × scale factor (×32 for 1/32, ×24 for 1/24).
- Pitch diameter
teeth ÷ pitchin inches; module = 25.4 ÷ pitch in mm. Pinion and spur must share pitch to mesh. - Center distance = (pinion pitch dia + spur pitch dia) ÷ 2.
Calculated top speeds are theoretical (no-load RPM, no slip) — real speeds run a little lower.
Slot car gearing FAQ
How do you calculate slot car gear ratio?
Divide the spur (or crown) gear teeth by the pinion teeth: gear ratio = spur teeth ÷ pinion teeth. A 9-tooth pinion with a 27-tooth spur gives 27 ÷ 9 = 3.00:1, meaning the motor turns 3 times for every 1 turn of the rear axle. A bigger number means shorter gearing (more punch, less top speed).
What is rollout on a slot car and how is it calculated?
Rollout is the distance your slot car travels per motor revolution. Rollout = π × tire diameter ÷ gear ratio. Example: a 20 mm tire with a 3:1 ratio gives π × 20 ÷ 3 ≈ 20.94 mm per motor rev. Racers use rollout to compare gearing between cars with different tire sizes — the same ratio with bigger tires is effectively taller gearing.
What is the difference between 48 pitch and 64 pitch slot car gears?
Pitch is teeth per inch of gear diameter. A 64-pitch gear has finer, smaller teeth than 48-pitch at the same tooth count (a 27T spur is 14.29 mm across in 48p but only 10.71 mm in 64p). 64p runs smoother and quieter with slightly less friction — popular in 1/32 racing — while 48p teeth are stronger and more forgiving of imperfect mesh, common in 1/24 and high-power classes.
Can you mix 48 pitch and 64 pitch gears?
No. The pinion and the spur/crown must be the same pitch to mesh — 48-pitch teeth will not engage 64-pitch teeth and will strip or jam. If you switch pitch, replace both gears as a matched pair, and re-check the gear mesh and center distance.
What gear ratio should I start with on a 1/32 slot car?
A proven starting point for 1/32 is around 3.0:1 — for example a 9T pinion with a 27T spur, or 10T/30T. From there, change one tooth at a time: one tooth smaller on the pinion (or 2–3 teeth bigger on the spur) shortens gearing noticeably, roughly 10–12% per pinion tooth in this range.
Should I gear differently for a technical track vs a fast track?
Yes. On a short, technical track with tight corners, gear shorter (higher ratio, e.g. 3.3:1) for stronger acceleration and braking out of corners. On a long, fast track, gear taller (lower ratio, e.g. 2.7:1) so the motor does not top out halfway down the straight. Use the track advisor above to get a suggested ratio from your current setup.
How does tire diameter affect slot car gearing?
A bigger tire covers more ground per revolution, so it acts like taller gearing even with the same gears. That is exactly what rollout captures: rollout = π × tire diameter ÷ ratio. If you fit larger tires, compensate with a slightly shorter (higher) gear ratio to keep the same rollout — or accept the extra top speed and softer punch.
What motor RPM do I need for the rollout and speed calculator?
Use your motor's rated no-load RPM. Typical values: 18,000 for stock home-set motors, 21,500 for club motors, 25,000 for race motors and 30,000 for high-end race motors. The built-in motor presets above cover these. Real top speed will be a little lower than calculated because of friction, voltage drop and tire slip.
What does "scale speed" mean — is 1,000 km/h real?
Scale speed multiplies the car's real speed by the scale factor, so you can compare it to a full-size car. A 1/32 car doing 31.4 km/h for real is doing about 1,005 km/h at 1/32 scale — the famous "slot cars are the fastest motorsport on earth" fact. It is a fun comparison, not a claim the little car really goes that fast.
What is the difference between a pinion, spur and crown gear?
The pinion is the small gear on the motor shaft. The spur (or crown) is the large gear on the rear axle. In sidewinder and anglewinder chassis the axle gear is usually called a spur; in inline chassis it is often called a crown because of its shape. The ratio math is identical: axle gear teeth ÷ pinion teeth.
How much does one tooth change the gear ratio?
It depends where you start. Dropping one pinion tooth from 9T to 8T with a 27T spur moves the ratio from 3.00 to 3.375 — about +12.5%. Adding one spur tooth from 27T to 28T with a 9T pinion moves it from 3.00 to 3.11 — about +3.7%. Pinion changes are coarser; spur changes are finer. Try the combo finder above to see exact numbers.
Does gear ratio affect braking on a slot car?
Yes. Shorter gearing (higher ratio) gives stronger brakes because the motor's dynamic braking is multiplied through the gears — the same reason engine braking feels stronger in a lower gear on a real car. If your car feels weak on the brakes into tight corners, try one tooth smaller on the pinion.
What is a good gear ratio for 1/24 slot cars?
1/24 cars are bigger and often run more powerful motors, so they typically use taller ratios around 2.5–3.0:1 (for example 10T/25T = 2.50:1) with 48-pitch gears for strength. Start near 2.8:1, then tune with the track advisor: shorter for technical club tracks, taller for big fast tracks.
How do I convert slot car tire size between inches and millimeters?
Multiply inches by 25.4 to get millimeters, or divide millimeters by 25.4 to get inches. A common 0.75 in rear tire is 0.75 × 25.4 = 19.05 mm, and a 20 mm tire is 20 ÷ 25.4 ≈ 0.787 in. The tire converter in the lab above does this instantly, both ways.
Why do racers talk about rollout instead of just gear ratio?
Because ratio alone ignores tire size. A 3:1 ratio with 19 mm tires (rollout ≈ 19.90 mm) behaves differently from 3:1 with 21 mm tires (rollout ≈ 21.99 mm) — over 10% difference in effective gearing. Comparing rollout lets racers copy a winning setup between cars even when tire diameters differ.
Is this slot car gear calculator free and private?
Yes. It is free, works offline, and every calculation happens inside your browser — your gear combos, tire sizes and motor RPM are never sent to any server.