This tool exists to answer a very important question: will this gearbox setup be brilliant, unbearable, or merely character-building?
Most people pick a gearbox from a forum thread and find out what it drives like after the money has gone. That is the wrong order. Tyre size, diff ratio and the gear ratios decide how the car feels in every gear, and all three can be worked out on paper before anything is bought. BoxSelector does the paper part.
What the calculator does
Enter your tyre size and RPM targets, pick up to four gearboxes from the list or type in your own ratios, set the diff, and you get the mechanical top speed in every gear, the cruising RPM at road speed, and a gear table you can hand to your builder. Transfer cases and low range are in there for the 4WD crowd. There is a PDF export for people who prefer to argue with evidence.
What it is not
It is not a substitute for someone who has seen your car. The numbers assume perfect traction, zero drivetrain loss and a right foot with no opinions. Use them to narrow the choice and start a better conversation with your builder, then go and have that conversation. Check ratios against your actual unit before spending real money. Manufacturers make running changes and rarely send a postcard.
Design principles
- Fast enough to use mid-idea, mid-build, or mid-regret.
- Readable enough that the maths stays helpful, not mystical.
- Simple enough that fewer moving parts means fewer moving failures.
Who runs it
BoxSelector is a small independent project from the same garage as StandaloneHQ, the ECU pinout planner for standalone swaps. It is funded by the adverts around the tool and the occasional pint. If a ratio is wrong, tell us which one and where the factory figures say otherwise. That is how the list gets better.