GPS Rally Computer

GPS Rally Computer is a Universal app designed to simplify computations necessary to navigate a traditional time-speed-distance (TSD) automobile rally. The design and implementation is based on specifications outlined by Clarence Westburg and Rich Bireta, commonly know as the Simple Rally Computer or by its acronym SRC. The design mimics in digital form the functionality of the classic Zeron 660 and 880 rally computers.

A TSD rally is comprised of stages set along a route defined by written instructions. The first stage is the Odometer Check stage that allows you to calibrate your car’s odometer with that of the Rally Master’s odometer.

In later stages you will follow a course at a particular average speed that will change from time to time at places that are generally unknown in advance.

In order to drive the course at the prescribed speeds. Odometer measurement is obtained from Apple’s GPS receiver or third-party devices via Bluetooth.

The GPS Rally Computer allows you to do all this and much, much more. For more documentation, download the User's Guide.

Features of the GPS Rally Computer:

  • Clock time in either minutes and seconds to tenths of a second or decimal minutes to thousandths of a minute
  • The clock is a 24-hour clock to simplify computations
  • Distance measurements in either miles or kilometers
  • The car's odometer distance measurement may be the same or different than the official rally odometer distance measurement
  • One button click to reset default conditions for starting a rally
  • The internal computer clock can be adjusted to match the official rally clock
  • A "sprocket" button in the top menu bar allows access to the app settings from within the running application including settings for display of certain items
  • Calibrate the car's odometer to the official rally odometer on the odometer check stage via a popup
  • Change average speed (CAST) on a custom popup keypad
  • Preselect the next CAST to simplify the process of changing speed on each leg
  • As time passes and as the car's odometer is incremented, the actual speed over the course is compared with the expected speed. The difference in seconds or hundredths of a minute is displayed along with an indication of either "Early" or "Late" to stay on pace
  • Time errors may be displayed in either of two modes selectable in the app Settings
  • Pause or Gain time or take a Time Allowance on any stage
  • Handle off-course corrections to time and distance with Reverse button
  • Record "splits" of time and error at any time with a Control/Split button
  • Record "control" and reset time error and distance to zero and increment next starting time 5 minutes later (distance and time are modifiable) with a Control/Split button
  • Log table that records time, speed, distance, and error for various actions such as splits and CAST changes
  • Set next starting time for the Next CAST and have the time tick down on the master clock and autostart CAST, or override with a manual button click
  • Optionally, use a Dual GPS receiver via Bluetooth to supplement or substitute for the Apple internal GPS

Note: Use of the GPS Rally Computer during any particular rally will be subject to the rules and instructions governing the rally and will likely define your category or class among participants. Some rallies may, in fact, prohibit use of this app. Also, some rallies may have classes strictly for those with Simple Rally Computer (SRC) apps.

For better GPS reception and improved accuracy while rallying, third-party wireless GPS receivers are recommended. This app has been tested with Bluetooth connectivity to a Dual XGPS150A Universal GPS Receiver.

Disclaimer: Continued use of GPS running in the background can dramatically decrease battery life.


change in version 4.3
  • Added optional display of time errors with an animated dial graphic with colors to show pace ahead or behind ideal. The dial will display errors in whole numbers when greater than or equal to +/– 1 second and decimal numbers when nearer to zero. The options may be chosen in the app Settings.
change in version 4.2
  • Added support for Rally Computer Remote app that serves as a driver's heads-up display connected via Bluetooth.
changes in version 4.1.2
  • Fixed bug related to setting new odometer distance for Next CAST. If there is a user input new odometer distance, the odometer distance will reset to this distance when the CAST xx.xx button is touched or when the CAST xx.xx is executed when the countdown time reaches zero time. Additionally, any distance difference between the new reset odometer distance and the displayed odometer distance will increment the time error based on the difference at the prior CAST
  • After an odometer check, the starting distance for the first stage will be reset to zero or another distance that the user may chose. If the distance is not equal to zero, then the time error will register error to that distance when the CAST xx.xx is touched
changes in version 4.1
  • Changed minimum required iOS version to 10.3 to resolve some issues and to accommodate new technology
  • Reduced from 10 meters to 1.5 meters (33 feet to 5 feet) the minimum distance computed from a pair of GPS coordinates suitable for incrementing distance. This improves displayed distance measurement to 0.001 mile accuracy, which is especially important at speeds less than 15 mph
  • Various changes to accommodate iOS 11
changes in version 4.0
  • Substantial revision to app functionality, to the processing of various rally actions such as Transit Zone computations, and to the behavior of CAST variables and their display
  • For both current and new users, study and reference to the User's Guide is strongly recommended. A link to the downloadable User's Guide in PDF format can be found at

revised: January 22, 2018