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Short reads for choosing gearboxes with fewer forum tabs open and slightly less regret.
The numbers are for guidance. Not gospel.
BoxSelector crunches gear ratios, tyre sizes, and RPM figures to give you useful speed and cruise data for your build. The results are theoretical, based on stock ratio specs and your inputs, and assume perfect mechanical conditions, zero drivetrain losses, and a world without speed bumps, traction limits, or your right foot getting sweaty. Real-world results will vary. Probably a lot for automatics.
Use it to narrow your options, start a conversation with your builder, and stop arguing on forums... then go talk to someone who can actually look at your car.
If you fit a TH400 and wonder why there's no 5th gear, that's on you. BoxSelector and its author accept no responsibility for mechanical failures, unsuitable gearbox choices, blown differentials, bent input shafts, voided warranties, shed fires, or any other consequences arising from using this tool. You're a grown adult building a car. Act like it.
We've done our best to source accurate ratio data from factory specs and reputable technical references. Some ratios (particularly on production variants with running changes) may differ from the figures listed here. Always verify critical specs against your actual unit or a current manufacturer source before committing to anything expensive.
The "Transmission Adapter Search" and "Transfer Case Adapter Search" features open pre-formatted Google searches in a new tab. We're not affiliated with any of the suppliers, parts retailers, or manufacturers that show up in those results. We get nothing if you click them. We're just saving you the effort of typing.
The "buy me a drink" button is a voluntary tip, but nothing is paywalled, and nothing changes if you don't use it. If you do, thank you. My 10mm socket fund appreciates it.
Where BoxSelector links to external sites (manufacturers, reference sources, or otherwise) we're not responsible for their content, accuracy, or whether they've gone offline since we linked them.
Questions? Spotted a ratio that's wrong? Fine, you can let me know here.
Use the form below if something needs fixing, improving, or politely mocking.