As digital calculators and lap-top computers are usually banned on historic rallies, navigators need a set of average speed tables to do their time-speed-distance calculations. The best sets have tables for speeds between 15 and 30 mph at 0.1 mile and 0.1 mph increments. Loughborough Car Club provide basic tables to each competitor for their Easylarity rally, which are sufficient for that event.
However, you may want more detailed sets. These are available from a number of sources including Don Barrow (see his website) or you can generate your own if you can use a spreadsheet such as Excel. There is a connection on the links page to print and create your own. When using the tables, some competitors prefer analogue clocks for timing, although the use of modern hand-held digital stopwatches is universally accepted.
CORRECTING FOR YOUR TRIPMETER’S ERROR
It is important that your tripmeter or odometer is set against the official distance. Quite often your readings will differ from the organiser’s mileages, but the organisers will always be deemed to be correct. So, if you have one of the more modern type of tripmeter, you can adjust the readings to be exactly the same as the organisers.
Electronic display units:
For machines such as Brantz, Terratrip and Monit units, these may need to be aligned to the measurements used by organisers which can be slightly different to the ones that you already have in your machines. Events will provide a calibration route. Drive the route, noting your display readings at various points and especially at the finish point. Then, using a calculator take the distance displayed on your machine at a set distance, and divide it by the distance indicated by the organisers at the same point. Then multiply the result by the coefficient shown on you machine and this will give the new coefficient to the inputted into your machine.
Analogue displays and odometers:
If you are using the odometer (which is not adjustable) or one of the older types of trip, such as a Halda which uses cogs and are difficult to change, you may need to calculate your Corrrection Factor. If there is a discrepancy between your reading and the official one which you cannot get rid of by adjusting your trip, you should calculate the Correction Factor by dividing your trip reading by the official distance. Thus, if your trip reads 10.2 and the official distance is 10.00, your Correction Factor is 1.02. You can manage this difference by using a different speed table for the set average speed. As your tripmeter reads long, it will make you go shorter than the set distance in the required time, which is too slow, so a faster speed table should be selected. All you do is multiply the required speed by your Correction Factor, i.e. 30 mph x 1.02 = 30.6 mph, so use that table instead. This is when a set of tables or your own comprehensive print-out of tables will be a must. Otherwise, if your confident that your trip can match the organiser’s readings, you can use a basic table, such as the Loughborough one.
If you’re thinking of buying one of those pocket electronic devices that have been advertised, which they say can be set to display the mileage you should have travelled and beep every 1/10 of a mile for a preset average speed, FORGET IT. The use of these devices has been outlawed by all the historic rally organising clubs, who consider it as blatant cheating, with dire threats of ex-communication to anyone found in possession of one on an event. It has been known for crews to try to gain an unfair advantage with some demon hidden electronic wizardry, but certainly not amoung the top crews. Believe it or not, but it really is possible for a navigator to accurately monitor the average speed with just a Halda, stopwatch and speed tables, whilst navigating the car through a maze of junctions and plotting the route a couple of miles ahead!
I acknowledge the valuable advice from both John Brown and Andy Gibson which has been used to compile this section