Miata Mailing List: October 1994, Message #43
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   From: RoyWagner@aol.com
Subject: Re: Inaccurate Mile Markers
   Date: Tue, 4 Oct 1994 08:51:41 -0400

In California, and I just found one in Virginia (on a very obscure road),
they have "measured miles", that can be used to check your odometer.  I wish
they would set up more of these measured miles. You can't trust the highway
markers. They are close but not accurate. 

You can check your speedometer, if you can hold a steady speed in traffic.
This is not an easy task and I speak from many years of TSD rallying
experience. Also checking your speed is easiest if you can maintain  60 MPH.
The checks are usually in areas with speeds of 55 or 65 for which you need to
know the "speed factor" or just go it at 60. At 55 MPH a mile will be covered
in about 65 seconds, at 65 in 55 seconds. Pretty easy to remember.

I haven't accurately checked my Miata using a measured mile, but casual
checks lead me to believe it is significantly off, ie. when my speedometer
shows 65 MPH, I'm going about 60. Our local police (Fairfax) set up speed
checks that do a radar check and post your speed on a large display as you
pass. According to these, my Miata's speedometer is off by at least 10% at 25
MPH. Keeps me within the limits, but sure annoys those behind me with
speedometers measuring off the opposite way.

- Roy -