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318 messages, Last post on Sep 29, 2009 at 8:00 PM
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Replying to: hiwayman (Jan 08, 2008 2:31 pm) Don't make the assumption that the odomenter is correct. Using my GPS, which I believe is very accurate, I've determined that my NAH odometer is off by about 2+% - under reporting. That's about the same difference between manual calculations and the computer. I was driving south on 495 in MA and the GPS said 50 miles until the next turn at my exit. I reset the trip meter and when I arrived at the exit 50 miles later the trip meter was reading about 48.5 miles. |
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Replying to: saabgeorge (Jan 13, 2008 7:02 am) I drove my NAH with GPS directions on a highway. I had a second portable GPS I just bought as a gift for my dead but wanted to test it first; at the same time, 2 miles before an exit, NAH GPS says "exit in 2 miles", the portable GPS says "exit in half a mile". Maybe an extreme example but this discrepancy did happen couple of times. I'm not saying your GPS is not accurate but I don't think it's a valid measuring tool no matter how great it is. 'Drive by wire' GPS, such as in NAH, would be probably better, as if you loose a signal it still shows your correct location. I drive Holland Tunnel from NY to NJ daily and it shows precisely where you are at all time. If you come out of tunnel and get a signal again, the car icon doesn't even have to adjust itself, it points exactly where you are. Pretty cool. But I guess we can leave GPS talk to another forum...
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Replying to: karolpl7 (Jan 14, 2008 6:04 pm) I use a GPS receiver/logger connected to TomTom Navigator 6 on my Treo 755p. The GPS sends lat, long and measured speed to the software. The speed is displayed on the screen. As a longtime GPS user, I know that the speed from the GPS receiver is the most accurate. Having said that, when I used my portable GPS in our NAH, I found that the speed readout on my portable GPS matched the speedometer. Assuming that the odometer and speedometer are tied together, I believe the NAH odometer is accurate. On my RAV4, the speedometer indicates about -1-2 relative to the GPS readout. Regarding the differences in "distance to a turn," that is a software based algorithm that is dependent on the mapping database. The system may use a plus or minus 50 yards to give a warning, depending on the system being used. Accuracy is not as important. |
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Replying to: karolpl7 (Jan 14, 2008 6:04 pm) When using the GPS, as I described to check my odometer, it's the map data that determines the distance between two different points such as my starting point and the end point, which was my exit. The map data, in my opinion, is pretty accurate - similar to the maps you'd find on MapQuest. These are the detailed maps that you have to load - not the base map. The GPS just determines how close you are to the starting point and how close you are to the end point when you get there so, at the worst, it would be no more than 400 ft off. My odometer was like a mile and a half off. |
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Replying to: saabgeorge (Jan 13, 2008 7:02 am) |
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Replying to: saabgeorge (Jan 13, 2008 7:02 am) GPS is, of course, actually a military system. There are two levels of accuracy transmitted by these GPS "constellations" (groups of GPS satellites). The military broadcast is accurate down to about a yard, or even less, depending upon other factors (I'll get to those in a sec). The civilian broadcast is much less accurate, and the military actually imbeds what's called a "dither factor" into that signal, so that the accuracy continually changes from a yard or two, to easily dozens of yards. This is to prevent a foreign nation from using our own technology to guide their weapons. What you will see in civilian GPSs of ANY kind or maker, is a changing accuracy caused by this dither factor. But wait, there's more! GPS accuracy is also depended upon the number of satellites your unit can "see". The minimum number of GPSs your unit must be able to see to get lat/long and atiltude are 4. Cheap GPSs, then, often have just 4 channels. But your GPS can be a lot more accurate, if it can see more satellites. Thus, good GPSs often have 12 channels. Now here's the rub. GPS satellites are not geosyncronous, meaning they do not stay in the same place in the sky at all times, like TV, weather, and communications satellites do. So, at any given time, there will be a number of GPS satellites in the sky, with others popping up over the horizon, and others going out of sight over the horizon. Your GPS unit keeps track of where these satellites are supposed to be with something called ephemeris data (it's a file that uses time of day and lat/long of the unit to generate a sky map for the GPS of where the birds are). Downloading this file from a satellite is why your GPS takes longer the first time you turn it on, or after it's been stored for a while. The accuracy of that ephemeris data and how it is updated is actually closely guarded by GPS manufacturers. It is the keys to the kingdom, so to speak. Becasue of all these factors, your GPS, ANY GPS (without the inertial navigation system that NAHs have for tunnels and deep valleys) will have varying accuracy from day to day, and yes, even from minute to minute. Interesting side note here. In the beginning of the second Gulf War, when we weren't sure when the military was actually going to attack, I had my GPS turned on. It's an older Garmin model which displays the dither factor. Minutes before the first cruise missles slammed into downtown Bagdad, we saw the dither factor go nuts. We knew something was about to happen. OK. So you have two GPS units in your car which are telling you different things. My guess is that there is a part of the car or surrounding environment that is blocking the sky view for one of the units, making it less accurate. If your TomTom is more accurate, that's cool, but I bet if you kept using the GPSs, together, you'd find days where the NAH is more accurate. One other thing you have to consider is that the two GPS systems may actually be affecting each other. GPS is nothing more than fancy radio signals. Units on the ground lock onto radio signals by generating their own radio signal, and then comparing it to the signals in the environment until it finds what it's looking for (It's actually called "superhetrodyne" for you electronics geeks). Since there is a radio signal being broadcast by each GPS unit, the OTHER unit in close proximity to it can actually be picked up, and confuse the unit. It doesn't happen often, but it does happen.... Additionally, the designers of the GPS systems have to make judgement calls as to things like "what is an exit?" Sounds like a simple question. But where do you place the "X" that says "exit" in your onboard maps? At the tip of the lane split? At the first curve? Half-way between one highway and another, along the exit ramp? This can be a very difficult decision, since there are so many different kinds of exits, ramps, merges, etc., etc. Obviously, since this is proprietary software, TomTom and the people that make Nissan's GPS make different decisions.... All that being said, I've been particularly underwhelmed with my Nissan's GPS. It has taken us on some sight-seeing trips that got us to a destination nowhere near where we wanted to go. Odd. Very odd. Now, we basically use the GPS on long trips to let us know how much farther it is, miles/time-wise to our destination (Are we therrrrrre yet?!?!)
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Replying to: hiwayman (Jan 15, 2008 9:46 am) http://www.eagerodometers.com/advise.html
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Replying to: saabgeorge (Jan 15, 2008 12:23 pm)  . I had an interesting situation last summer. I ride motorcycles. A buddy of mine and I both have Garmins on our bikes. His is a Street Pilot (can't remember the exact model) and mine is an older GPS V. We would always show a variance between the two GPS units regarding how far we had to go to the next waypoint. I assume his was more accurate, since it was a newer unit, but over a day's course, we'd often show a difference of a mile or better. For fuel calcs over a long haul, this wouldn't even come up into the first order of magnitude, calculations-wise (wouldn't make any difference), but it's a variance, never the less. |
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I reached 10,000 miles on my 2007 NAH today. Here are some stats. Average MPG was 36MPG. Drove 400 miles on a half a tank 2 times. Best MPG 39 and worst 32. Overall the experience has been great. The only problem was car pulled to the right when I first bought it, but the problem seems to be fixed. Will switch to synthetic motor oil and see if there is any difference in MPG. Very smooth, quiet and handles well.
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Replying to: mazda6dude (Feb 06, 2008 9:58 am)
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