The 680 shows the vehicle speed. At 60 miles per hour on the camry speedo the garmin shows 57 miles per hour. Witch one is most accurate?

Garmin. Search the forum and you'll even find out why.

The Garmin may be closer - the speedometer is allowed to read high, but not low, per UN regulation:

http://www.unece.org/trans/main/wp29...egs/39rv1e.pdf

The relationship between indicated speed (V1), and actual speed (V2), according to the regulation is:

0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h

So, indicated speed will always be equal or greater than true speed, by no more than 4km/h plus 10% of true speed.

However, none of this guarantees the Garmin is correct, just that there is no specific regulation requiring your GPS receiver to be inaccurate. It too has an inherent margin of error, based on signal reception, and any embedded error should the military authorize it, but many GPS units will display the anticipated error on one of its screens.

Why does the UN get involved in this? Who knows? But there we are.

They are acting as an international standards body. Since cars are sold globally, this provides a world standard. AFAIK the UN cannot enforce any of these standards, but member nations have something to adopt. For instance, even though the standard has been around since 1958, Japan only agreed to it in 1998!

Have TCH with factory NAV. How do I read the ground speed on this GPS NAV system? because I can't find this read-outanywhere.
If this GPS DOESNOT give the speed... it has to been the only one in the world that does not have this feature?

I just did a simple mile post check against the odometer (tripodometer) I could not do a long enough test to really tell much , but the car seemed to register 1 mi about two car lengths before i got to the next mile post. this could have a small affect on my MPG. Has anyone done tests with miles per hour or odometer readings against there nav ?? Next up for me will be a tri-fecta. the odometer VS the nav VS the mile posts, for miles traveled. If this stuff is in older posts maybe someone could refer me to them. JOE