Odo "Lag" and Speedo accuracy

[TomoHawk]

I think this is a drivetrain-related thing, although it's mostly about the speedo gauge itself.

In Roadrally, you calibrate your odometer when you run the first leg, which is about 10 miles. The Rallymaster gives you the exact mileage and you calculate your "correction factor." But the testing I've done so far indicates that the odometer isn't as great as you might think...

At 30 MPH, the odometer matches the distance you drive very well. At 60 MPH, it's only 90% on; it goes up by 0.9 miles for every 1 mile on the highway (assuming the mile-markers are correctly spaced.) It gets worse as you go faster.

If you time yourself for the miles, you get 2 minutes to a mile @30MPH, it's 80 seconds @45 MPH, and 60 seconds per mile @60 MPH. At 45 MPH I got 79 seconds, and @30MPH, I got 1:59 to a mile, and Fortunately(?) for 60 MPH, I got 57 seconds, which seems consistent with the odometer.

The question is,

Is this appropriate for a 30 year-old car with 120,000 miles, or should it be something else? It's the original Speedometer, but I know some people have been putting in new gauges after a restoration; the new one could be more accurate.

A second question.

When you zero the Odometer, it usually takes a small distance, like 100-200 ft. before the odometer wheel starts turning. It's called odo "lag." and since you reset tthe odometer for each leg (up to 12 times in a rally,) the lag can affect your final score. Has anyone tried it on theirs? I haven't gotten to measure mine on the Z, but it's that way on the DD.

The goal is to see if the Z would be good enough to use on road rally, which relies on a good odometer, or well-documented correction steps.

thxZ