Section 2
OPERATION
TIMEWISE 798A
39
Fine Tuning
the Odometer
Factor
Whenever the rallymaster gives an official mileage at a reference, you can use that
information to confirm the measuring accuracy of the 798A. When doing so, you will often
notice slight deviations from official mileages. Such inconsistencies occur when the
rallymaster’s driving style differs from yours. Also, the course may have been measured
under different road and weather conditions.
Although you can align your odometer at each official mileage, you may decide to re-figure
your odometer factor in the midst of the rally. Changing your factor to adjust for such minor
course variations is called “fine tuning” the factor.
To fine tune your factor in the midst of a rally, use the
New Factor
formula as before. You
will need two official mileage references and your odometer reading at both locations. Then
use the distances between the references (both official and measured) in the formula.
Rather than using the
New Factor
formula to re-calculate the factor, it is sometimes easier
to do a quick fix “in your head”. Let’s look at the relationship between a mileage
measurement error and the correction required in the odometer factor to eliminate the error.
Assume your factor is 065000. Then suppose your odometer has increased 1.00 mile after
traveling 0.99 official course miles. Your measurement of 1.00 was in error by 0.01 mile.
That is, your measurement error is 0.01 mile per 1.00 mile, or 1% (0.01 ÷ 1.00). This 1%
error figure is also the error in the factor.
A decrease of the factor by this same percentage is necessary to correct the factor.
(Remember — decreasing the factor makes the 798A measure fewer miles.) Thus, reduce the
factor 1% of 065000, or 650 counts. The correct factor is 064350. You must also correct the
inactive factors, as well as adjust your odometer and calculated time to match the
rallymaster.
Had you traveled 10.00 miles before accumulating the 0.01 mile error, you would have a
0.1% error (0.01 ÷ 10.00). Your factor would then be reduced 0.1% of 065000, or 65 counts,
to a new value of 064935.
Unfortunately, when an opportunity to fine tune the factor does occur, you will likely not
have such easy to use numbers for the mileage or factor. Still, the principle is the
same…figure the percentage error of the mileage, and correct the factor by that percentage.
Here’s a more realistic example: Assume your factor is 072563 and that you reset the
odometer at the start of the leg. You are currently working on an instruction that reads,
“Turn on Rinkel Road at 4.32 miles”
When you turn on Rinkel Road, you note that your odometer displays 4.30 miles, giving you
an error of 0.02 miles. This is a 0.02 mile error in 4.30 miles, or approximately 0.5%. To
correct the factor, increase it by 0.5%, or approximately 360 counts. Set your new factor to
072920, rounding off to ease future recalculations. Don’t forget to adjust the inactive factors
by the same percentage. Also, remember to correct both the odometer and calculated time!
CAUTION! As all experienced rallyists know, mileage references at anything other than
clearly labeled signs should be suspect as to their accuracy. Do not change your factor every
time a reference mileage is given! Rather, align your odometer, if you must, and watch for a
repeating trend. If your odometer continues to drift, you may then wish to change the factor.
