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Old 11-27-2010, 01:02 PM   #1
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Miles Driven: Tripometer/Odometer vs. GPS

Has anyone actually tested one against the other? What are your results?

After having the tire sizes adjusted as close as possible, I verified Speedometer vs. GPS and both were spot on. Today, I did 3 separate 1-Mile tests putting Tripometer/Odometer up against GPS. Well, the results were interesting, yet consistent.

The Tripometer/Odometer was consistantly off by approximately 528 feet, or just about 1/10th of a mile. On each test, the GPS reached 1 Mile before the Tripometer/Odometer by approximately 528 feet, or 1/10th of a mile.

Over a 200 mile distance, that would equal to approximately 20 miles short than actual and 1200 miles short over a 1 Year/12,000 Mile test.

Can the distance travelled be completely irrelevant to rate of speed when it comes to verifying wiith GPS?

I am inclined to believe the distance travelled using GPS is the accurate measurement.

Thanks in advance.

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Old 11-27-2010, 03:18 PM   #2
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A couple of troubleshooting methods for ya. First does your gps have the ability to display current speed? Drive around with that displayed and compare to your current speed.

Second drive down the interstate and check it against the mike makers.

Third drive the same route you have been with the gps in a different car, or walk it to see if is the same. Those maps loaded are not 100% accurate nor is the measurements by gps.

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Old 11-27-2010, 04:02 PM   #3
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Smaxberry, yes, I verified GPS against cruise control set for 25, 35, 45, and 60 mph. All speeds were accurate with GPS.

On good days when I get a location of 9 sattelites or greater, the fix is accurate to within 3 feet. Today, I am getting 8/10 sattelites and I am within 10 feet. The way I check is, I went to a park and stood in between two fixed tables that can be seen on all sattelite terrain maps. I checked visibility of sattelites and had 8/10. The GPS position was determining my current position to be on top of the table to my right, or 5 average steps away, or approximately 10 feet.

I think the correct tire sizes must be programmed into the CPU to be accurate with distance. If my whacky calculations are correct, every 200 miles logged by Trip A should actually be 220 +/-. If this is correct, then my average mpg would adjust from a disappointing 12.23 to a more respectable 14.60.

Anyone else have some ideas, suggestions?
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Old 11-27-2010, 05:33 PM   #4
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Weird that the speedo would be correct but trip would be off. Why would Jeep not use one in the calculation for the other?
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Old 11-27-2010, 07:50 PM   #5
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Measure your actual tires. I think the differences are related to the tire size being off, compared to what you set it to. Most 32's don't actually measure 32"....etc. If I understand your ? correctly, I think that would affect your outcome.
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Old 11-27-2010, 08:04 PM   #6
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There will be some difference in mileage due to the way that GPS measures vs. the speedomoter. All GPS use horizontal distance and the car measures actual distance (up and down hills). Unless the road is perfectly flat there will be a small difference, although not anywhere near the error that you are getting.
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Old 11-27-2010, 08:14 PM   #7
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Iíve used my GPSr to check and set my speedo, but havenít used it to check the odometer. I bet like lighthorse said itís elevation changes and/or turns. If you want to check you odometer use the mile markers on the side of the road.
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Old 11-27-2010, 08:34 PM   #8
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Quote:
Originally Posted by JackdUp
Has anyone actually tested one against the other? What are your results?

After having the tire sizes adjusted as close as possible, I verified Speedometer vs. GPS and both were spot on. Today, I did 3 separate 1-Mile tests putting Tripometer/Odometer up against GPS. Well, the results were interesting, yet consistent.

The Tripometer/Odometer was consistantly off by approximately 528 feet, or just about 1/10th of a mile. On each test, the GPS reached 1 Mile before the Tripometer/Odometer by approximately 528 feet, or 1/10th of a mile.

Over a 200 mile distance, that would equal to approximately 20 miles short than actual and 1200 miles short over a 1 Year/12,000 Mile test.

Can the distance travelled be completely irrelevant to rate of speed when it comes to verifying wiith GPS?

I am inclined to believe the distance travelled using GPS is the accurate measurement.

Thanks in advance.
GPS is actually off.
GPS models are specifically designed to be off by a certain amount as to keep specific cordinants off.
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Old 11-27-2010, 08:51 PM   #9
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Here lies the challenges. The road used was straight and level. If GPS speeds were equal to Speedometer speeds set on cruise control, how would you explain a margin of error for distance? Would that be possible?

It would help if someone else tried this to see their outcome. Verify vehicle speeds match GPS for about 3-5 miles. Then test your Trip A at 0.00 vs GPS. Note the differences, if any, for each mile travelled.

Mine has a difference of approximately 1/10th of a mile.

If anyone tries this, please post your results. Thanks.
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Old 11-28-2010, 12:29 AM   #10
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Elevation changes would be my first guess. Slight GPS errors would be my second. Odometer (stock) would be my last guess though, unless you're losing traction (snow?)

You might want to see if someone else can do the same route in a different vehicle with GPS and see what their results are..

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