Measure your rear axle ratio with your phone and your tach — no gears to count, no diff to open.
GearRatio works out your vehicle’s rear axle (final drive) ratio from GPS speed, a tachometer reading, and your tire size. Hold a steady speed with your tach on a round number, tap Capture, and the app does the math — take a few readings and it averages them, and even shows you the closest common production ratio as a sanity check.
It also has a DynoJet mode: it computes the exact gear-ratio value to enter in DynoJet’s Power Core software for a 224xLC chassis dyno, referenced to the dyno’s fixed 24-inch drum — so it needs no tire size or transmission ratio, just the gear you’ll run on the dyno.
Get the app
Features
- Rear axle (final drive) ratio from GPS speed + tach RPM + tire size — no OBD-II adapter, no hardware
- DynoJet 224xLC gear-ratio setting for Power Core, referenced to the dyno’s 24-inch drum
- Enter your tire by metric size, overall diameter, or a measured roll-out
- Test in your direct 1:1 gear, or enter any gear’s transmission ratio
- Average several readings and see the closest common axle ratio
- MPH or KM/H, your choice
- Works fully offline — no accounts, no ads, no tracking, no network access at all
Do the axle-ratio test in your direct 1:1 gear for the truest reading. The DynoJet value is not the same as your axle ratio, and supports the 224xLC (24-inch drum).