Gear Ratio Calculator

Calculate gear ratio, vehicle speed, engine RPM, and see how tire size or sprocket changes affect your setup – for cars, bikes, and machinery.

Drivetrain & Tire Setup

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Speed from RPM

How this gear ratio calculator works

This tool combines transmission ratio, final drive (axle) ratio, and tire size to compute overall gear ratio, vehicle speed, and engine RPM. It also supports simple gear trains (teeth counts) and side‑by‑side comparisons of two setups.

Core formulas

1. Overall gear ratio

If you know the transmission gear ratio and final drive (axle) ratio:

\( \text{Overall Ratio} = R_\text{trans} \times R_\text{final} \)

2. Gear ratio from teeth

For a simple gear pair or chain drive:

\( \text{Gear Ratio} = \dfrac{T_\text{driven}}{T_\text{driver}} \)

3. Wheel circumference

\( C = \pi \times D \)  where \(D\) is tire diameter.

4. Speed from RPM

First compute wheel RPM: \( \text{Wheel RPM} = \dfrac{\text{Engine RPM}}{\text{Overall Ratio}} \)

Then vehicle speed:

\( v_\text{mph} = \dfrac{\text{Wheel RPM} \times C_\text{in}}{1056} \)

(1056 = 12 in/ft × 5280 ft/mile ÷ 60 min/hr)

5. RPM from speed

Rearranging:

\( \text{Engine RPM} = \dfrac{v_\text{mph} \times 1056 \times \text{Overall Ratio}}{C_\text{in}} \)

Typical use cases

  • Street / highway cars: Find cruising RPM at 65–80 mph for different axle ratios and tire sizes.
  • Off‑road & towing: See how lower gears (higher numerical ratios) improve low‑speed torque but raise highway RPM.
  • Motorcycles & bicycles: Use the teeth‑based mode to compare sprocket combinations and cadence.
  • Machinery: Design gear trains to reach a target output speed from a known motor RPM.

Worked examples

Example 1 – Highway RPM for a daily driver

Suppose you have:

  • Transmission 6th gear: 0.75
  • Final drive: 3.73
  • Tire diameter: 28 in
  • Speed: 70 mph

Overall ratio: \( 0.75 \times 3.73 \approx 2.80 \).

Tire circumference: \( C = \pi \times 28 \approx 87.96 \,\text{in} \).

Engine RPM: \( \text{RPM} = \dfrac{70 \times 1056 \times 2.80}{87.96} \approx 2{,}360 \,\text{RPM} \).

Example 2 – Gear ratio from sprocket teeth

A motorcycle has a 16‑tooth front sprocket and a 48‑tooth rear sprocket.

\( \text{Ratio} = \dfrac{48}{16} = 3.00:1 \).

Swapping to a 45‑tooth rear gives \( 45/16 \approx 2.81:1 \), which lowers RPM at a given speed.

Tips for choosing gear ratios

  • Highway comfort: Aim for your engine’s efficient RPM band at your usual cruising speed.
  • Performance: Shorter gearing (higher numerical ratio) improves acceleration but increases RPM and fuel use.
  • Tire changes: Larger tires effectively “tall” the gearing; you may need a numerically higher axle ratio to restore performance.

FAQ

What is a “good” cruising RPM?

For many gasoline engines, 1,800–2,500 RPM at highway speeds is a comfortable, efficient range. Diesels often cruise lower (1,500–2,000 RPM). Always check your specific engine’s torque curve.

Can I use this for electric vehicles?

Yes. EVs often use a single reduction gear instead of multi‑speed transmissions, but the same math applies: use motor RPM, reduction ratio, and tire size to compute speed.