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I've been using a 60C for over a year, and have found that the ETA to a point that I am driving to is reasonably accurate if there is no traffic. From what I understand, the unit uses road speed, not actual speed, to compute your ETA.

Last week I flew to Dallas and San Antonio, and was using my GPSr on Southwest Airline flights. Since we were in heavy clouds, I couldn't pursue my usual pasttime of identifying ground features from the map, and out of boredom, I started watching the Time to Destination reading, comparing to my our actual speed. I took a few readings on the Dallas flight, and then, flying home from San Antonio, I took more readings. The flight is pretty short from the time of the "OK to use PEDs" announcement to the "discontinue and store all PEDs" announcement.

Based upon my distance to Hobby airport and the unit's estimated travel time, I computed the assumed speed and compared it to my actual speedHere are my findings:

=====================================================

Time to Waypoint 25 20 18 15 12 10 9 6 5

Distance to Wpt 164 138 128 118 87.9 63.6 53.4 31.2 27.0

Actual Speed 460 510 523 541 501 436 399 346 340

Assumed Speed 394 414 427 472 440 382 356 312 324

(I know the table is hard to read since the forum strips extra spaces--sorry about that).

From the data, you would think that an average speed is being used. For example, from 18 to 15 minutes out, the actual ground speed went from 523 to 541 (we must have had a tail wind--I rarely see speeds much over 505 or so). The "assumed speed" changed from 427 to 472. When the actual goes to 501 at 12 minutes out, the "assumed speed" reduces to 440. But look at the last two entries. Speed declines from 346 to 340, but the "assumed speed" goes from 312 to 324.

If you are routing off road, it looks at the closing speed to the destination and calculates the time it takes at that speed. There is a running averaging, so it takes time to adjust to different speed.

For on road routing, it looks at what it has in the database for the travelling speeds of the roads and completely ignores how fast you are moving.

Red,

I didn't think about the distinction between ground speed and *closing* speed. I guess if you are traveling at 500 mph in the wrong direction, it will assume that it is going to take a while to get there.

For on road routing, it looks at what it has in the database for the travelling speeds of the roads and completely ignores how fast you are moving.

Tell me if I'm wrong but last summer, coming back from New Orleans, I had my Legend C on and it seemed that the faster I was going, I could see the ETA change on my screen. If it only considers the speed in the database, what happens if you stop moving for an hour, surely the ETA will change!!

For on road routing, it looks at what it has in the database for the travelling speeds of the roads and completely ignores how fast you are moving.

Tell me if I'm wrong but last summer, coming back from New Orleans, I had my Legend C on and it seemed that the faster I was going, I could see the ETA change on my screen. If it only considers the speed in the database, what happens if you stop moving for an hour, surely the ETA will change!!

Keith

Tell me if I'm wrong but last summer, coming back from New Orleans, I had my Legend C on and it seemed that the faster I was going, I could see the ETA change on my screen. If it only considers the speed in the database, what happens if you stop moving for an hour, surely the ETA will change!!

For autorouting, it looks at curretn time plus remianing estimated time to get there based on the speeds it has for the roads.

So if you stop for an hour, the ETA increases by an hour. And yes, it uses different speeds depending on the vehicle choice.

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