Why the difference? - BMW Luxury Touring Community
 
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post #1 of 13 Old Oct 20th, 2008, 12:17 am Thread Starter
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Why the difference?

Over the weekend I took my annual birthday ride and rode some very nice roads in SW New Mexico and Eastern AZ. Before the start I reset my trip milage on my Garmin 2720 to zero. Upon return this after noon there was a difference of 16 miles. The Lt milage (741) was higher than the GPS (725).

Anyone have an explanation? I have done the speedo re-calibration so I do not think that is the issue.

What say the intellectuals?

Rob Asay
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post #2 of 13 Old Oct 20th, 2008, 1:25 am
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Re: Why the difference?

Even if you have done the speedo adjustment, it will still be a marginal error.

The GPS calculates more on where you are, and adds up the mileage driven.

At least thats what i think of it...
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post #3 of 13 Old Oct 20th, 2008, 5:59 am
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Re: Why the difference?

That is an easy one. GPS mileage is very accurate, when basically riding straight line. However, when riding a lot of twisty roads it will be short.

The reason is that GPS calculates speed every fraction of a second so that will be accurate no matter the path, but distance is done on a much longer sample rate. OK for straight line, but on curves the distance between sample points will mean that you have gone around the curve some distance, and the sample points are connected by straight lines. So, the calculated distance is the sum of the straight line segments around the curve, while the actual distance traveled around the curve is longer.

Also contributing is up and down hills. GPS distance will be shorter as the distance calculation in most civilian GPS units does not calculate altitude change well, if at all.

GPS is very accurate for speed, not as good for distance.

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post #4 of 13 Old Oct 20th, 2008, 7:22 am
 
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Question Re: Why the difference?

Quote:
Originally Posted by dshealey
That is an easy one. GPS mileage is very accurate, when basically riding straight line. However, when riding a lot of twisty roads it will be short.

[snip]

GPS is very accurate for speed, not as good for distance.
David, would not the inaccuracy of the LT's odometer be yet another factor? Or is it your understanding that the LT's odometer is fairly spot-on?
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post #5 of 13 Old Oct 20th, 2008, 9:16 am
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Re: Why the difference?

I have don't some looking at this and have found when my buddys and I reset our odometers at the same time that I am always higher then them. I figure I am about 2-3% off. So if we did 100 miles they will show 97. My old bike was even more off.

So when you buy a used LT with 50,000 miles on it you are really getting a bike with about 48000. Look of it as a benefit.

Do a 5 mile test of mile markers to odometer and you will be able to see how much it is off.

Tom Ress
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post #6 of 13 Old Oct 20th, 2008, 11:51 am
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Re: Why the difference?

Quote:
Originally Posted by Tom1200
I figure I am about 2-3% off. So if we did 100 miles they will show 97.
That 2-3% figure is reasonably close for the LT based on all the reports here over the years. The speedo has a higher margin of error, usually 5-8%.

I tested my GT on a 100-mile stretch of straight interstate when I first got it. It came in under 1% error for both the odometer and the gps. Good enough for me.

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post #7 of 13 Old Oct 20th, 2008, 12:29 pm
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Re: Why the difference?

FTR - '05 (and up I presume) LT odometers are pretty damn close to GPS's given mileage.

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post #8 of 13 Old Oct 20th, 2008, 12:52 pm
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Re: Why the difference?

My two GPS's don't take elevation as a measurement. So you can be going up a hill or down a hill at the same rate of speed on your bike but the GPS speed will vary. The odometer on my two bikes seem like they are spot on by mile markers

Neil '00 K1200LT '08 KLR
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post #9 of 13 Old Oct 22nd, 2008, 10:57 pm Thread Starter
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Re: Why the difference?

Well there you have it. It appears that by a majority the consensus is that the LT milage would be the more accurate for my trip. As I did a lot of twisties on this trip. So following Dave's explanation I should go by my odometer. By doing that, I also get a better MPG figure.

Thanks for the viewpoints guys hope we all learned something.

Rob Asay
99 Canyon Red Lt
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1982 Honda "Silver Wing"
1986 Gold Wing Interstate "Blue Babe"
1978 Gold Wing
1976 Kawasaki 900
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post #10 of 13 Old Oct 22nd, 2008, 11:19 pm
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Re: Why the difference?

Lot's of valid reasons posted here...but also consider this. Everytime you lean the bike over, you're running on a smaller diameter tire than when you are riding upright. The speed sensor has no way of discerning the fact that you are heeled over and thus reducing tire diameter. The GPS only "see's" linear travel, the bike see's mechanical travel.

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post #11 of 13 Old Oct 22nd, 2008, 11:52 pm
 
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Re: Why the difference?

My '03 LT always had a 3% high odometer error. As stated above, in mountainous or twisty roads, you aren't going to get an accurate figure from either the bike or GPS.
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post #12 of 13 Old Oct 23rd, 2008, 2:53 am
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Thumbs up Re: Why the difference?

We now have multiple reasons for the LT mileage to be higher than the GPS readings. Funny that all the reasons cause higher mileage to be shown on the ODO.

1- Normally higher reading for the LT ODO
2- GPS lower reading in curves
3- Smaller tire diameter contact in curves causing higher ODO reading.

SO, there is no way to get an accurate reading, but given all the above probably the more accurate of the two is still GPS, even though it will still read a little short when a lot of curves are taken into account. The sharper the turns, the greater the shortening affect. Also the faster the curves are taken, the more the shortening affect as you will have gone further between sample points.

If you want to have accurate GPS mileage on twisty roads, ride very slowly around the turns! On a BMW? Yeah, right. Guess for accurate GPS you have to get a Harley.

I don't want to achieve immortality through my work...I want to achieve it through not dying.

David Shealey
Dandridge, TN
EX: '01 Black LT, BAT BYKE (Totaled at 110,000 miles)
IBA SS, BB, BBG, 10/10ths.
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post #13 of 13 Old Oct 23rd, 2008, 10:03 am
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Re: Why the difference?

Quote:
Originally Posted by dshealey
If you want to have accurate GPS mileage on twisty roads, ride very slowly around the turns! On a BMW? Yeah, right. Guess for accurate GPS you have to get a Harley.
Almost spit my coffee!

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