I have a little Garmin Etrex Legend that does a good job of giving me the speed. Its almost dead on from the tests I have given it. Not to mention its waterproof, and about 100 bucks!

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-Justin

91 560 SEC AMG - other dogs dd
01 Honda S2000 - dogs dd
07 MB ML320 CDI - dd
16 Lexus IS250 - wifes dd

it's automatic.