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.

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
  • Pause or Gain time or take a Time Allowance on any leg
  • 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.


changes in version 2.2
  • More GUI enhancements especially to the size and display of popup keypad functions. Greater size allows for easier inputs. Better displays across all iPad and iPhone models
  • Fixed bug that sometimes caused odometer factors not to be saved
  • Corrected time error while in "Park" mode and removed estimated distance in Park and Reverse because it caused momentary erratic distance computations and time errors
changes in version 2.1
  • Two bug fixes: one erroneously displayed Computed Time when the time display was set to two digits, and the other failed to adjust time error and computed time for Pause, Gain, or Time Allowance
change in version 2.0
  • Added estimate of distance travelled between GPS distance signals. This improves the accuracy of the distance measurement and importantly reduces the rapid changes in time error computations and display to make it easier to maintain a desired pace

revised: April 22, 2017