Jaguar Clubs of North America

Guide to JCNA Club Rallying & Program Rules
    Chapter 5 - The Odometer and Tires

(TSD) Rally
Instructions and Terms
Route Instructions
Avg. Speed and Calc.
Odometer and Tires
Rally Equipment
Your First Rally
Rules and Techniques
Variations to the Rally
JCNA Rally Rules
Rallymaster Guide
Appendix A
Appendix B

The odometer part of your car's speedometer tells you the miles and tenths of miles you travel. All Jaguars have a separate section on the gauge called the "trip odometer" , which can be returned to zero at any time.

Your humble tenths-reading odometer will tell you more than tenths of a mile! Observe the action of the tenths wheel: it usually doesnt go from one tenth of a mile to the next in one jump, but rather inches its way around in what is usually five steps for each tenth, or two one-hundredths (0.02) of a mile per nudge. A navigator wi th his eyes open can readily interpolate to within about 0.02 with a garden variety odometer. You can increase the accuracy of Smiths odometers and speedometers on XK and E-type Jaguars by having the instrument calibrated by a reputable instrument repair service (such as Nisonger Corp., see bibliography). Tell them you will be using the instrument for rallying, and that you want it calibrated for absolute accuracy at 60 mph.

The above information on instrument calibration does not apply to 1988 and newer XJ6 or VDP with electronicodometers.

Recent Jaguars with factory provided on-board trip computers are generally more accurate and lend themselves to serious rallying. Realize that the information provided by your odometer may be less than totally accurate, and that the experienced navigator must learn how to compensate.

In a rally, you are not so much concerned with the true mileage of 5,280 feet per mile as with the official mileage of the rally. If your mileages match the official mileages, so much the better. It is most unlikely that they will, however, and it is up to you to face the problem and find a way to combat it successfully.

Every good rally includes an Odometer Check (point) of not less than ten miles before settling down to serious rallying. Many rallyists want to be early at the odometer check to give them time to figure out their odometer error. Assuming that the odometer check is at 20.00 miles, and your cars odometer reads differently (you can bet it will), you can work out a factor to apply to ALL mileages and average rates of speed listed in the RIs for the entire rally.

If, for example, your odometer reads 19.40 at the Rallymaster's official 20.00 miles, your odometer reads 0.60mi early (or 3% ahead) and you will have arrived early. If the Rallymaster's official 20 miles comes up 0.60 before your odometer's 20 miles, then your odometer reads late and you will have arrived late. Without compensating for these differences, you would come in early or late at every check-point. Odometer error is cumulative, increasing with dis-tance. Where a turn is indicated by mileage clues only, you may find yourself turning onto the wrong road due to odometer error.

The solution to odometer error lies in a choice of three alternatives. First, you can forget it entirely and simply enjoy the rally route. As long as you don't get lost, you still have a good chance of winning! But if you're serious, you can either change the average speeds in the Rls by the factor of your odometer error, or determine the error between your odometers reading and the official mileage. Although there are more calculations involved, beginners often find it more logical to apply "corrections" to the official mileages wherever they appear in the RIs by writing in what their odometers will read at those given points. Each method has both advantages and disadvantages and each is used by scores of successful rally contenders. Choose your own.

HERE IS THE FORMULA FOR CORRECTING YOUR ODOMETER MILEAGE READINGS SO YOU CAN COMPARE YOUR ODOMETER READINGS WITH THE OFFICIAL MILEAGE AT ANY POINT ON THE RALLY. This formula is for use in relating distance according to your Jaguar odometer to distance as established by the Rallymaster's car that measured the rally route.

At the rally Odometer Check record your mileage. Official Mileage by your odometer reading to find MILEAGE CORRECTION FACTOR:

Speed Correction Factor  = Official Mileage/Your odometer reading

Assume that at the end of 10 official miles (odometer check) your odometer reads 10.25 miles.

Example:  10 mi/10.25 mi = .97561 Mileage Correction Factor

From the formula you know your mileage correction factor is .97561. Any mileage indicated by your odometer multiplied by this factor will give you the equivalent Official Mileage. Suppose a change of average speed (CAS) takes place at a point where your odometer reads 27.65 mi. and you wish to know the official distance to this point. Simply multiply 27.65 by .97561 (actually 27.65 X .976 is close enough) to get 26.876 Official Miles.

At any point in the rally, you can multiply your odometer reading by the Mileage Correction Factor and you will get Official Mileage.


Your odometer reading at the Official Mileage/odometer check = Your Average Speed Correction Factor

Official Mileage
Your Odometer Reads
Correction Factor

If you apply this factor to the average speeds wherever they appear in the RI's, your speed will then be the official average speed corrected for your odometer error. If your Speed Correction Factor is .941, all official average speeds would become adjusted average speeds like this:

Official Speed
Your Corrected Average Speed
30 mph
28.23 mph
34 mph
26.35 mph

Distances registered by your odometer depend, of course, on the gearing (gear ratio) installed in your vehicle. In turn, mileages are dependent on the rolling radius of the wheels, from the center of the axle to the point where the tire is in contact with the road. The variable member of this chain of command is the tire, which must be made with a certain amount of flexibility in order to absorb road shock. Because of this, bias-ply tires that increase their rolling radius on the road, largely through centrifugal force which makes them grow larger at higher speeds, are not preferred for serious rallying. Also, as a tire heats up with use, and the temperature within the tire increases, so will the tire pressure, further increasing the tire radius.

The answer to this has been provided on Jaguars for many years -- Radial Ply Belted Tires. They are virtually non-expansive in addition to offering superior roadholding and handling. Check your tire pressures prior to the rally and remember not to spin your wheels or drift during the event. This really upsets your odometer's accuracy.

Previous: Chapter 4 - Average Speed and Calculations
Next: Chapter 6 - Rally Equipment