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Old 01-31-2019, 10:38 AM   #1
Rivet Master
SilverCabin's Avatar

2014 27' Flying Cloud
Viera , Florida
Join Date: Oct 2006
Posts: 1,258
TV Trip Odometer vs. Garmin GPS

I've been logging my Airstream travels since I started Airstreaming in 2006. I used my Garmin navigator's odometers to record the TV miles as well as the trailer's miles. A year ago, I upgraded my 2006 TV for a 2018 F-150 with all the bells and whistles.

I'm still logging miles for the truck and Airstream, now using both the truck's trip odometers and it's trailer accumulated mileage readout and the Garmin GPS. What I've noticed is that the truck gives me lower readings than the Garmin GPS.

I just completed a 1,100-mile trip, and these are the readings:

Truck: Trip 1,127.8; Trailer Miles 1,078.0
GPS: Trip 1,144.0; Trailer Miles 1,093.4

That's a 1.4% difference between the two. Both the truck's digital speedometer and the GPS read the same speed, and I checked them with an iPhone app—no disagreement. So, at the same speed reading, the truck's odometers are accumulating 1.4% fewer miles than the GPS's odometers.

I've been doing my Airstream maintenance based on the Garmin GPS distance for years and will continue to do that since that is the more conservative approach, but why would there be different readings between two devices?

Randy & Pat Godfrey
Florida Airstream Club (027)
ACI# 1212 - AIR# 17017
2014 27FB Flying Cloud - SilverCabin II
2018 F-150 Platinum - unnamed

The best part of any trip is when you pull away from home.
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Old 01-31-2019, 11:35 AM   #2
Rivet Master

2007 22' International CCD
Corona , California
Join Date: Jul 2013
Posts: 8,886
Historically vehicle speedometers and odometers are all over the map when it comes to accuracy, digital or not.

Our two Prius vehicles read speed as at least 2 MPH faster than GPS, and distance at least as bad. The 2012 Tacoma reads speed about 1.5-2 MPH LOW depending on tire wear. The 2002 Tacoma reads about 2 MPH high.

If I was looking for accuracy, I'm not going to find it on vehicle displays, obviously.

Feel free to skip the rest of the hat on...

The problem is the difference between 'ACCURACY" and 'PRECISION' when you are talking measuring stuff.

Accuracy refers to displaying information that is correct based on some standard. In other words, if you put exactly 12 volts into some multimeter, it will display exactly 12 volts every time if it is properly calibrated.

Precision is just the ability of a display, given the same input to measure, to give the exact same output. It does not speak to accuracy--just repeatability under the same conditions of input and output.

So if you put exactly 12 volts into a precise digital meter, and it displays 12.1 volts every time you apply 12 volts to it, it is precise, but it sure isn't accurate or, for that mater, calibrated--its just precise and repeatable...

Unless you spend a lot of time (and $$) calibrating your measuring instruments, you can only assume they give a reference measurement, not an accurate measurement. For most of us humans, we tend to believe digital readouts, but if you carefully look at the specifications, they will be listed as Accuracy +- 3%, +- one digit. In other words, it is neither accurate or precise.

For casual use troubleshooting Airstream trailer electrical systems, a cheap Harbor Freight meter is 'just fine'. If you are calibrating industrial process equipment, you use a relatively expensive Fluke meter with an unexpired calibration sticker on it from the last time it was sent to the Metrology Lab and checked against the NIST-tracable and documented hugely expensive standards they used to calibrate it against every freaking six months....

So now you know more about this subject than you wanted to, assuming you read this far...bottom line, use the cheap meter. At least your measurements will be consistent until the next time you overload it or drop it off the roof of the trailer...(grin) Then just toss it and buy a new one. They run on smoke, by the way--that's why when the smoke leaks out they all quit working...
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.
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Old 01-31-2019, 11:54 AM   #3
Rivet Master

2014 20' Flying Cloud
Sag Harbor , New York
Join Date: Jun 2015
Posts: 17,551
Originally Posted by SilverCabin View Post
. . .
That's a 1.4% difference between the two.
. . .
For dragging our small homes around the country, this is an acceptable margin of error IMO.

Avoid Analysis Paralysis.
Keep It Simple, Stu.
Look Up Once In a While.


Happy Trails!

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