GearRatio

Skunk River Restorations, LLC

About this app

Find your car's rear axle (final drive) ratio with nothing but your phone, your tachometer, and a steady stretch of road. No OBD-II adapter, no jacking up the car, no counting ring-and-pinion teeth.

Enter your tire size, hold a steady speed with your tach on a round number, and tap Capture. The app reads your GPS speed at that instant and pairs it with your RPM and tire size to calculate the ratio. Take a few readings and it averages them — and shows the closest common production ratio as a sanity check.

HOW IT WORKS
- Enter your tire: printed metric size (like 205/55R16), overall diameter, or a measured roll-out for the best accuracy
- Pick a target RPM and hold your tach there in a steady gear
- Tap Capture; the app grabs your GPS speed and computes the ratio
- For the truest axle ratio, test in your direct 1:1 gear — or enter any gear's transmission ratio

DYNOJET MODE
Heading to the chassis dyno? Switch to DynoJet mode and it computes the exact gear ratio value to enter in DynoJet's Power Core software so the dyno can derive engine RPM from road speed. It's referenced to the DynoJet 224xLC's fixed 24-inch drum, so it needs no tire size or transmission ratio — just run the test in the gear you'll use for the pull.

FEATURES
- Rear axle ratio from GPS speed + tach RPM + tire size — no hardware
- DynoJet 224xLC gear-ratio setting for Power Core
- Tire input by metric size, diameter, or measured roll-out
- Average multiple readings; closest common ratio shown
- MPH or KM/H
- Works fully offline — no accounts, no ads, no tracking, no network access

Built for classic-car owners, restorers, and hot-rodders who need to know their gearing without pulling the diff cover.

Please don't operate the app while driving — have a passenger tap Capture, or use cruise control to hold a steady speed.

What’s new

First release. Measure your car's rear axle (final drive) ratio and the DynoJet 224xLC gear-ratio setting from GPS speed, a tach reading, and your tire size.