GPS Speedometer for laptop?

[Six_Shooter]

You guys are making it way too hard.

Set your cruise to 60.

Watch for the mile marker. Note the second that you pass it. Next mile marker note the second again. Do it for several miles if you really want to be accurate.

If you are hitting the same second every time you pass the next mile marker, you are going 60.

If you are dropping seconds, then you are going faster than 60. One MPH for every second that you are dropping. So if you are at 58 seconds a mile consistently, you were travelling 62.

And if you are adding seconds, again one mile slow for every second you add.

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Take it for 10 miles and see how accurate you are. Of course divide the total number of seconds added or dropped by 10. Your accuracy then is about 1/10 of a MPH, which my guess is more accurate than you can visually read most speedos, and also more accurate than you can set your cruise.

That's all fine and dandy if you can actually be on an open stretch of highway, where no one is cutting you off, or traveling random speeds etc.

GPS speedometer is great for verifying other speedometers.