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[porthole]

According to my GPS, my SEEG is about 4-6 mph off at 75.

I will take my GPS' accuracy any day over a vehicle speedo.

All modern, quality GPS units, and by this I mean the major names, typically update the unit's speed and position about 60 times a second and then send the "average" to the display, with most units updating the display once per second or to a user variable interval. On my portable Garmin (276C) the display updates 1 per second, on the Northstar's I use on boats I have the speed update set for 4 seconds, because of the slower vessel average speed.

Interesting note here, Last year Power & Motoryacht did a "speed accuracy" test. They use GPS and radar to evaluate vessel speed at a given cruise RPM.

P&M compared the Garmin 276C, a police radar, $20,000+ surveying GPS along with 1 or 2 other units.
Below 30 mph the Garmin was real close to the survey equipment, above 30 it was dead on.

[spydglide]

According to my GPS, my SEEG is about 4-6 mph off at 75.

I will take my GPS' accuracy any day over a vehicle speedo.

All modern, quality GPS units, and by this I mean the major names, typically update the unit's speed and position about 60 times a second and then send the "average" to the display, with most units updating the display once per second or to a user variable interval. On my portable Garmin (276C) the display updates 1 per second, on the Northstar's I use on boats I have the speed update set for 4 seconds, because of the slower vessel average speed.

Interesting note here, Last year Power & Motoryacht did a "speed accuracy" test. They use GPS and radar to evaluate vessel speed at a given cruise RPM.

P&M compared the Garmin 276C, a police radar, $20,000+ surveying GPS along with 1 or 2 other units.
Below 30 mph the Garmin was real close to the survey equipment, above 30 it was dead on.

That's good stuff, Porthole.  Thanks!  [smiley=xyxthumbs.gif] spyder