Ratio Calc is a quick and easy tool for radio control car racers to use trackside. Knowing your final drive ratio (FDR) is very useful for comparison with other racers to determine if your car is geared correctly. You can also view your favourite information on Apple Watch.

It lets you quickly select your car by filtering on manufacturer and model. Select your pinion and spur size and the FDR is displayed. Tyre diameter can also be entered to calculate your rollout.

Ratio and roll out charts are also available which plot pinion and spur gear combinations to create very handy reference tables.

- Save your favourite cars for quick access

- Save notes and track name against your favourite cars

- Share ratio/rollout charts

- Download new cars from within the app

- Select measurement units (imperial/metric)

- Select your preferred gear pitch, alternative ratios in other pitches will be shown

- Over 1500 cars/variants in the database

What's New

Version 6.5

We've added a tip jar - the app has been kept up to date since 2011 and we've never charged for an update. Any tip will unlock the ability to change the app's colour scheme and app icon - we've even included some retro icons that may be familiar to long time users.

Also in this version:
-Improved watch app reliability
-Transponder input form improvements
-Other boring little fixes and optimisations

Ratings and Reviews



Well done! I love the amount of cars you have in the library. Great tool!



Great place to keep notes and gearing for all your cars. Enough models for every occasion!!! Store multiple cars and compare final drive ratios or roll out for accurate records. Will definitely save money on spent motors and speed controls due to over gearing.



Great app for finding FDR. Has a bug on ipad at moment where I cannot hit "done", it does not appear on screen, after setting pinion and spur size, so cannot get out of this mode. Otherwise it does everything we need for finding gear ratios on our RC cars. Thanks for making this app. :-)


David Mills
8.4 MB
Requires iOS 9.0 or later. Compatible with iPhone, iPad, and iPod touch.
English, German, Italian, Spanish
Age Rating
Rated 4+
© 2011-2018 David Mills
In-App Purchases
  1. Nice Tip $0.99
  2. Mega Tip $4.99
  3. Generous Tip $2.99


  • Family Sharing

    With Family Sharing set up, up to six family members can use this app.

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