A modern vehicle gearing calculator, designed to be easy for both humans and machines (AI Agents) to use.
flowchart TD
subgraph Human["Human Interface"]
Website["Website (.js)"]
App["iOS app (.swift)"]
end
Website --> Core(("Common<br/>math core<br/>(.py)"))
App --> Core
subgraph Machines["Machine Interface"]
Agent["AI Agents"]
end
Agent --> Core
See ARCHITECTURE.md for more details on the design and how that's wired up, the physics behind the model, and how to run the app locally.
Have a play on gears.kranky.dev (a served copy of this repo, no setup required).
Also available as a native iOS app: ThirdGear on Apple App Store.
No API, no scraping needed: calc.py is a
dependency-free Python module served straight from the site, the same file
the browser fetches and runs. Fetch it, import it, call compute() with a
plain dict, get a plain dict back. See
The math is a public, dependency-free module
in ARCHITECTURE.md for how that works, and a real example of Claude using it.
Relates engine RPM to road speed through the drivetrain — transmission gear ratio × final drive × transfer case, with tire diameter and torque-converter slip — and shows a tachometer, speedometer, a chart of engine RPM against road speed with one line per gear, a top-speed table, and a shift-point table.
Drag the speed slider to walk a whole acceleration run: the gear follows the shift schedule and the tachometer sawtooths as each shift lands.
Other things it does:
- Standing-start time — estimates 0–100 km/h (or 0–60 mph) from vehicle weight and tire grip, accounting for each gear's own rotational inertia.
- Compare two setups side by side — same gauges, chart and tables, one overlay per setup, so a gearbox or final-drive swap is a direct comparison rather than two tabs to flip between.
- Metric or imperial units throughout, including tire size, torque, and weight.
- Tire size → diameter, computed from section width, aspect ratio and wheel diameter, plus a library of tire grip presets.
- Gearbox, engine, vehicle and tire presets for common cars, or type in your own.
- Export/import a setup as a JSON file, to save or share a configuration.
- Lap map — load a RaceChrono Pro CSV export and trace the lap, coloured by which gear each setup would be in at that point. No data leaves the browser.
- Light, dark, or auto theme.
- Works fully offline — the gearing math runs in the browser itself, not on a server, so once the page has loaded once it needs no network.
Calculator functionalities are inspired by blocklayer.com/rpm-gear.