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Ronzi · 07-08-2004, 01:14 PM

The hard-won knowledge noted below was gained mostly in the days of Curta calculators and Halda Speed Pilots (I can hear the "wha...???" already), so how it relates to on-board GPS computers, I haven't a clue.
When a rally is laid out, the distance from one instruction to the next is measured to the closest 100th of a mile. The rally master establishes (based on posted speed limit, generally) a specified speed for that little section. The "perfect time" is a calculation based on the specified speed and the measured distance for that section. All of the section times are then added together to reach the total perfect time for a check point.
The theory of the odo check is to compare your car's odometer to the odo of the car that was used to measure the rally. If the odo check is specified to be say 15.7 miles long, and your odo reads 15.2 miles, you have a difference of about 3 or 4%.
On the actual rally sections, you would theoretically have to drive about 3 or 4% faster than the specified speed to reach the end of a given section at the time calculated to be "perfect" based upon the odo of the measuring car.
If your car said the odo check was 16.4 miles instead of 15.7, you would have to drive maybe 4% slower than the specified speed to reach the end of the section at the specified time.
Whether or not your actual speedo is "off" isn't really an issue, since what you are trying to do is match the distance travelled by your car in the same time as the calculated ideal. That calculated ideal is based upon the odo of the measuring car, so the purpose of the odo check is to allow you to compare your odo to the measuring car's odo.
I hope this explanation is a.) correct, and b.) helpful.
Sometimes us old guys don't remember stuff so good.
Regards, and happy rallying.
Ronzi

PS - If you are wondering how the rally master can measure distance to a 100th of a mile, they have to have a special 100th-reading odometer lashup. Maybe they do use GPS for that stuff nowadays.

07-08-2004, 01:14 PM	#5
Ronzi Registered User Join Date: May 2004 Location: Denver CO Posts: 748	The hard-won knowledge noted below was gained mostly in the days of Curta calculators and Halda Speed Pilots (I can hear the "wha...???" already), so how it relates to on-board GPS computers, I haven't a clue. When a rally is laid out, the distance from one instruction to the next is measured to the closest 100th of a mile. The rally master establishes (based on posted speed limit, generally) a specified speed for that little section. The "perfect time" is a calculation based on the specified speed and the measured distance for that section. All of the section times are then added together to reach the total perfect time for a check point. The theory of the odo check is to compare your car's odometer to the odo of the car that was used to measure the rally. If the odo check is specified to be say 15.7 miles long, and your odo reads 15.2 miles, you have a difference of about 3 or 4%. On the actual rally sections, you would theoretically have to drive about 3 or 4% faster than the specified speed to reach the end of a given section at the time calculated to be "perfect" based upon the odo of the measuring car. If your car said the odo check was 16.4 miles instead of 15.7, you would have to drive maybe 4% slower than the specified speed to reach the end of the section at the specified time. Whether or not your actual speedo is "off" isn't really an issue, since what you are trying to do is match the distance travelled by your car in the same time as the calculated ideal. That calculated ideal is based upon the odo of the measuring car, so the purpose of the odo check is to allow you to compare your odo to the measuring car's odo. I hope this explanation is a.) correct, and b.) helpful. Sometimes us old guys don't remember stuff so good. Regards, and happy rallying. Ronzi PS - If you are wondering how the rally master can measure distance to a 100th of a mile, they have to have a special 100th-reading odometer lashup. Maybe they do use GPS for that stuff nowadays.