You don't need a special tripmeter in your car when you start rallying. If you're a newcomer to rallying, a major achievement on your first few rallies will be to find your way around the route without missing any marshalled controls or code board route checks. Maintaining the required average speeds so that you arrive at controls precisely on time will not be important. Indeed if the navigator gives up concentrating on the route to check the mileage and time, it's quite likely to result in a missed junction. Better to be on the right road at the wrong time than on the wrong road at the right time!
After doing a few rallies and you decide that you want to be up with the experts on timekeeping, you'll then need to fit your car with a supplementary tripmeter, a device that can be accurately calibrated against the organiser's measured mile or other calibration distance before the start.
A few historic rally organisers will only allow the use of additional tripmeters that are period mechanical instruments, such as the Halda Tripmaster or Twinmaster. Unfortunately, as these have not been manufactured for decades, they are in great demand and command very high prices. Much more reasonably priced are the Brantz or Terratrip electronic devices and the basic versions that just have a dual display of distance only (not any form of speed) are widely accepted now. These are also a lot easier to calibrate than the Halda for which you have to change cog wheels to get an accurate reading. Installing these electronic tripmeters requires fitting a pick-up to detect rotation of the speedo cable or a pick-up on a wheel hub that gets electrical pulses from passing bolt heads on the wheel hub.
Rally tripmeters are now becoming available with a GPS sensor to detect car movement, with the obvious advantage of a much simpler installation. However most historic rally organisers have categorised GPS tripmeters with sat-navs and hence don't allow them to be used. We may have to go along with this for our Hughes Rally, but a GPS tripmeter would be OK for our Sunday morning Regularity Runs.
If you have a smartphone (Apple or Android) and search the app store, you'll find several rally tripmeter apps that use the phone's GPS sensor, but there are very few historic rallies that would allow you to use them. However, if you're a novice on one of our Regularity Runs and don't have a tripmeter fitted in your car, you may be permitted to use your phone with one of these apps, so long as you get the prior approval of the organiser of the event and can show him that the app does not display your average speed.
Here's a tip for setting up your Brantz or Terratrip if you don't have an accurate measured mile available but do have a smartphone with a GPS tripmeter app that displays distance to 1/100 mile precision. Find a main road which goes for several miles without any sharp turns or roundabouts and from a standing start in a layby you drive several miles to another layby where you stop and read off both the mileage travelled (on the smartphone) and the calibration mode reading on the tripmeter. You must zero and take readings while stationary as GPS mileages do significantly lag when moving. Then just divide the number on the tripmeter by the GPS measured mileage and you have an accurate calibration figure to set into the Brantz or Terratrip. In theory you should be able to do this with a satnav instead of a smartphone, but I haven't yet found a satnav that will display mileage travelled to better than 1/10 mile precision.