I put this over in the special interest by mistake so I'll repost over here.
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I'm talking about up in the left hand corner where it tells you how many miles and minutes left in the trip. The time is way off from reality. So my question is how does it calculate that time? And is there a way to change it? Before you reply too fast, I don't need answers on how the math works. I understand that it takes the distance left and divides it by a speed (mph) factor to determind the time left. The questions are what is the speed factor it's using (average speed that you travel, speed limits from the nav system, etc), and can it be changed.

Also, have you guys noticed that the time is way off? On a long trip recently the Nav was telling me it would take 5 hours to get home, but I knew it would only take 4. On short trips (less than 1 hr), the Garmin will be +/- 1-3minutes, where the RL can be off 15 minutes or more.

 

I don't know about anyone else, but mine is pretty acurate.... plus when you drive a RL, you should have the mentality that says "I'll get there when I get there....."

 

I've seen this inaccuracy before both in my RL and my 05 Lexus. It's usually more inaccurate for long drives. Figure its calculating everything in accordance with local speed limits whereas no one really drives exactly at the limit.

 

I believe that it uses a combination of the types of roads you're on along with the spped that you're traveling. The reason why I say that is that I've seen the ETA increase when I slow down because of traffic.

 

There are many variables to consider.

Which route did you select? Easy, Highway, No Highway, Direct, etc.? The initial time estimates are based on these routing filters.

Obviously they are padded for traffic density, number of intersections, etc into the computations.

But it is also interactive with the GPS. If it calculates a 20 minute ETA and you are making good time, it adjusts the ETA while en-route. If your driving at a more consistant pace or more stop and go, it recalculates the ETA to lesser or greater values.

Lets say you engage a route calculated at the start for 20 mins. Then you get stuck waiting for an accident to clear. the NAVI will recalculate and update the ETA to perhaps 30 mins. Then you get back on your way, you get every green light and you are exceeding the average speed for that route. the NAVI again recalculates and now it may say you need less time. When you arrive, it may only have been 17 minutes total.

Point is the ETA is not fixed. It is an estimate with buffering for normal traffic situations. Once en-route it dynamically adjusts to the actual conditions in which you are driving. You will always arrive with 0 minutes to go.

 

Much more eloquently stated than I.

 

As mentioned, there are a number of variables. Interestingly, this past week I made two round trips between the Philadelphia area and Williamsport, PA. 167 miles and 2 hours, 57 minutes according to the NAV. I didn't check the mileage (it was close) but the time was within five minutes each time. I guess sometimes it gets it right. Got close to 30 MPG as well, even going through the Poconos.

LL

 

I believe it also adjusts for Traffic Flow if you have that feature, i.e., if route invovles congested area, it uses flow info instead of speed limit.

 

I thought I noticed that once, but I haven't really experimented with it.

 

I have seen that happen once when travelling on the I-95 and there was a huge accident. The travel time jumped from 2 hours to 4 hours.