Estimated Time of Arrival Calculation

What it does, I think, calculate the time based on speed limits for each particular road you're on and an average speed for the whole route - and if you happen to go a little faster than what it's expecting, you shave a little off the ETA, and if you go slower...you add a little to the ETA...I've noticed the changes over the course of a trip...gaining a little here, losing a little there - but in the end, is on the money.

I have found with my C330 that the ETA has been amazingly accurate from the initial estimate -
especially on trips with no pit stops and minimum traffic lights. On couple hour trips I have found the initial estimate to be between 1-2 minutes from final time ! Very handy when meeting people or calling them with estimated arrival time !
My 2 cents worth !
I echo Keiths comments too !

My girlfriend literally asked me that on the weekend when we arrived two minutes behind what the NUVI 360 posted.

I explained to her that the GPS systems are getting their time from SATELLITE, distinct from your vehicle clock or wrist watch which is governed by a battery.

The time is not exactly over or under. Remember, you are literally being tracked by a satellite.

Well, not exactly "tracked" because your GPSr doesn't transmit its position to the satellite.

I've found it's fairly accurate on highways and roads with a minimum number of traffic lights. To increase accuracy on roads with many lights/stop signs, they should just shave a certain percentage off the posted speed limit to compensate for all the stop and go.

Seems like basing their ETA on these roads at say, 30 mph instead of the posted 35 mph would increase accuracy.

My experience is that if my trip is on local roads with lights, my actual ETA will be about 10 minutes later than the original estimate. And I'm not even talking about heavy traffic situations.

-Tom

I noticed that Nuvi calculate the ETA based on a speed that is an average and not the speed limit.
(ie.: 68 when the limit is 75).

But looks like he calcualte this averged also based on how fast you drove on the same road during previous days.

I'm saying this because I noticed that the route my nuvi is suggesting now is different to the one it was suggesting when I bought it.

In fact I can choose betwen two road to go downtown from my house.
1) Highway = Speed Limit 65 - With Traffic the real average is 55mph - but is the longer path to go downtown
2) Internal Road = speed limit 45 - No traffic, so the real average is 45mph - this is the shorter path to go downtown

So 1 was the suggestion the first days, now Nuvi is suggesitng 2, I guess based on the real average.

I don't know if this is the real calculation nuvi is doing, but this is my impression.

Bye

I've noticed that my nuvi 660 seems to always be within 2 to 3 minutes of it's eta. I can certainly live with that.

Greg

the ETA is best case senario. The NUVI assumes you will make all the green lights with no waiting in traffic. Jake

When I have the traffic received hooked up and a delay pops up, that delay automatically adjusts for a later ETA-->so even in traffic, you can have an idea of when you'll arrive and that is really nice!

Even if I know where I'm going, I like to point the GPS to it so I can get the ETA. On long trips or heavy traffic during the week it gives me some sort of reference point.

On freeway trips, the ETA is pretty close plus or minus 3 minutes. If travelling on streets, it's a little less accurate more like plus or minus 10 minutes especially dealing with LA streets.

It's an approximation and a heck of lot more accurate than me guessing

My Nuvi is always 100% on the money because the clock in my truck is set the same as the Nuvi clock.

If I drive faster or hit lots of traffic I can shave off or add time to the original ETA...

Obviously the closer you get to your destination the more accurate the constantly updating arrival time will be. I was comparing the actual arrival time to the time it predicts when you start your journey.

-Tom

There have been several previous posts on this subject. I believe the conclusion was that Garmin is using set speeds for different types of roads. They do not use the actual speed limit, but on major highways they use a slightly higher speed to represent what traffic is going on. Regarding stoplights and stop signs, they don't know about the presence, only an average speed for the type of road, so if there are a lot of stoplights (and you get caught in them), you will fall behind.
I just finished a 200 mile trip on an interstate at night and was traveling about 8 miles over the speedlimit. Arrived 10 minutes ahead of original schedule. Also, these roads have different speed limits for day and night and the GPS is not aware of that. It is an approximation, but overall it works fairly well.

but I have not figured how to get it off the screen and replae it with speed.. is there a way?

The ETA from my 2720 is amazingly accurate. It is much better than my guesses and memory about distances and trips I've made

Since it knows the exact distances to travel on each type of road, that is a very large part of predicting the ETA. Next, having an accurate "average" true speed on can expect on each type of road is the second component.

With these two pieces of information you can get super accurate estimates of ETA.

If you have very light traffic, travel above the speed limits, make all the lights on green - you will arrive earlier than it's first prediction. And, the oppose is true - lots of traffic, hit the red lights, I'll be later than the estimates.

I really use these ETAs to help me arrive places on time instead of way early or late.

aVOL2 -I don't know about your unit but all I have to do on my C330 is touch the lower left corner which has the ETA and it takes me to screen which has my actual speed, avg. speed, max speed etc !

As was noted above, hit that ETA and it will take you to the trip info screen, where it has speed, compas direction, various times & speed info it has recorded.

But...to replace the ETA on the map screen - no, you can't do that. Personally, I've confirmed that my truck's speedometer is accurate using the GPS...so I rely on that to know how fast I'm going when I'm not using cruise control

I've noticed that my 330 always seems to estimate my ETA based on an interstate speed of 65 when I take the interstate. But the interstate here in Texas has a speed limit of 70 and I tend to drive closer to 80, so I always shave time off the ETA, but it never adjusts how it calculates it.

Where did you find eta on the c330? Found it.

I realize that this is now a very old discussion, but I guess not much changed in this area; and I have found my Nuvi 765t amazingly inaccurate in calculating the time. Before Nuvi I had Tomtom, whose accuracy of predicting the time bordered on surreal - it was practically always spot on, even when the trip had some unpredictable delays (for a human being at least). Now with Nuvi, I see that I'm always behind of what it has calculated, and not by 1-2 minutes, but 25-35% of the trip time, especially in the city. It looks like Nuvi completely disregards the presence of traffic lights, or maybe assumes that they are always green, when in fact in 80% it's the opposite. This is disappointing. I was wondering, if I can somehow "tune" that calculation by editing some file on the device, where it supposedly keeps the expected speeds on different roads.

In the city our Nuvi 1350 is about 2 minutes off (slow) for 20-30 minute trip - easily explained by traffic conditions. My new Oregon 550 is the opposite, 2 minutes early for the same trip. It would appear that the Nuvi "is aware" of being in an area of changing speed limits while the Oregon simply does a single speed urban or rural calculation.

Interestingly they both navigate the same route (as in predict turns) even though they have different map sets.

I find the ETA very accurate. I know it updates as you get closer to your destination. I particularly like the fact it shows the ETA in local time at destination if you're crossing into a diofferent time zone. It's a very helpful and reliable tool.