Since buying the Garmin navigation module for my Kenwood in dash dvd player, i've been watching the difference between GPS trip meter and the odometer. For instance, the last reset was at 859.5 miles on the odometer, on the GPS it said 835.4 miles. Bottom line is almost a 3% difference between my odometer and the more accurate GPS. The results were that according to the odometer I got 17.25 mpg. By the GPS the results were 16.77 mpg.

On my '95 Dakota, '99 Ram and '04 Ram, all had speedos that read a little high, resulting in better, but not accurate mileage.


Is this stuff just me or is anyone else seeing this stuff?

Those with GPS/navigation systems chime in here.
Mike
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Note:stock tires and wheels are on my vehicle

 

Don't have a gps hooked up right now but I can provide some explanation. Your truck measures accurate rolling miles as you travel. Your GPS measures linear mileage traveled as observed from space. Lets say you traverse a steep grade for 10 miles. your truck would register 10 miles but the GPS would not. It would register much less.

On another note which unit do you have and can you post some pictures. I am upgrading both the truck and the jeep shortly with a Kenwood 7019 in dash deck, sirius tuners and the garmin nav in the jeep. Not needed in the truck as my Edge A2 monitor will have TomTom nav.

 

maybe the gps doesnt take rise and fall of elivation into account? just an idea

 

My truck was off 7% when I first got it. My Hypertech programmer allowed me to fix it by changing the tire size diameter.

 

If I correct for tire size on the Outlook monitor it is different by about 2 mph. At 70 on the speedo, the Outlook says says 68. I have2 devices both saying that the speedo is reading a little high!? The Garmin module is plugged into the Kenwood module and is actually in my glove box along with a 3 way input switch to hook up an mp3 player or Playstation or an alternate video source. I'll try to get some pictures of these items posted tomorrow.
Mike
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It is said that most Dodge speedos are calibrated from the factory at tad high for two reasons mostly. The biggest one is so you run out your warranty that much quicker (bastards) and secondly so some jackass cought speeding doesn't blame dodge by saying he was doing the speed limit, the speedo must be wrong....

 

I know that almost everyone hates a dealer, but they can calibrate the speedo by tirerevolutions per mile. Might get you closer. just a thought....

 

wierd how you didn't say I was wrong about my statement though....

 

No,I think that you are probally right, Tommy, that made the most sense.

 

Another reason could be due to the GPS limited acuracy. As you are rolling very slow in traffic alot of times the gps is bouncing your location around in a radius around you. Mine has a setting that shows how acurate it is at any given moment in feet and has been off by 60 feet before and maybe more that I don't know of. All depends on how many satelites you are locked on to and the weather and location.
So if you are rolling at 5 mph or less the gps is reporting your location around a radius of whatever acuracy you are getting at the moment. It may show you 30 behind where you actually are then 30 off to the side then 8 feet in front, ect and does thisa fewtimes a second.
So if you are only 3% off, it's safe to say you probably spend twice that % in traffic and slow rolling conditions. The faster you go the more accurate it is when it comes to milage.