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MetroMPG 11-28-2007 06:34 PM

Using Google Earth to measure odometer accuracy

The old-fashioned way to check your odometer is by counting mile posts on the highway and comparing them to the distance the car's odometer reports.

The new-fashioned way is to use a GPS unit.

The Google Earth way lets you pre-measure a very specific route, and then go out and drive it to make the comparison.

In the google earth program, just zoom in on the area you want to measure, go to the "tools" menu, then to "measure", then select whether you want to measure a "line" or "path". ("path" is more useful unless the road you're measuring is perfectly straight!)

Use the tool to measure out your path, hop in the car, and go drive it. of course, the longer the path you measure & drive, the more accurate your odometer calibration will be.

One difficulty arises if you happen to live in an area which is not covered in a high resolution by the google earth images (meaning, you can't zoom in close enough to discern local roads & intersections).

Why bother with odometer verification? Because an inaccurate odometer throws off your mileage calculations - a 2% error on a 50 MPG vehicle = 1 mpg off. maybe you switched to different wheels or tires (or transmissions). or maybe your odometer is out of whack from the factory. Maybe you're really getting worse MPG than you think; or maybe it's better! you'll never know if you don't check it out.

If you use google earth to measure out a longer drive for better accuracy, take intermediate readings (in both G.E. and in the car) at several points along your route. Hold on to this info and you'll have a reference you can use to check changes to the car in the future (e.g. tire size, transmission changes). The shorter segments give you the option of doing quick tests.

al_capwn 03-21-2010 12:47 PM

there is also the option of walking into a police station, ask if/where they have a radar trailer deployed, and carefully accelerate until the radar trailer display ticks up to a nice round number and then check your speedo. 39... 39... 39... 40, check speedo.

imzjustplayin 03-04-2011 03:49 PM

both of those methods listed in this thread are notoriously unreliable. The best way is to just carry with you a GPS unit that says the MPH you are going. Google earth, when calculating distance, does not factor in elevation and its detection of elevation isn't not exactly "accurate". Google earth has no idea if you're climbing a highway onramp, how steep it is or anything of the sort to give you an example. As for the MPH radar detectors, they're not terribly accurate.

I suppose the utmost accurate way to measure mph, distance, etc. is to make your own speedometer, measure out the wheel, tire, gearing, etc. everything to the mm, then find an area that is exactly one mile long, mark it with paint, then when maintaining a given speed, with a stop watch, calculate how long it takes to make it across. Then Adjust speedometer accordingly. There is more to it than I've said but the point is, if utmost accuracy is needed, I think it can be figured out how to accomplish it with the most basic instruments.

MetroMPG 03-04-2011 04:00 PM

Good point about elevation changes not being factored by the Google Earth distance measuring tool.

When I tested Google Earth against a calibrated odometer, it happened to be on flat terrain, so there wasn't any error detected.

But it's worth noting.

Joenavy85 03-04-2011 04:17 PM

also take into account overlapping photos will mess up calculations as well

Arragonis 03-06-2011 05:20 AM

Is there a tool for elevation as well ?

NeilBlanchard 03-06-2011 06:52 AM

Originally, I thought that Google Earth did take elevation into account; but unfortunately, it does not.

In my (non-exhaustive) search, I found results that indicate that GPS units are far from accurate for elevations, and therefore do not measure distances taking elevation into account...

Arragonis 03-06-2011 11:49 AM

I've seen some elevation maps that people have made from one point to the next. I would like to have a look at that to check the differences between home and work and back. Home is near the sea, work is about 10 miles inland - uphill (and 45-50 mpg ave) there, downhill (55-65 mpg ave) back.

MetroMPG 06-28-2011 10:29 AM

Used the Google Earth technique to check the accuracy of the UFO's (2000 Honda Insight) odometer yesterday:

35.9 km indicated
35.35 km plotted on Google Earth

Assuming no error in Google Earth (* there were a few elevation changes on this route that might skew it to measure lower than actual), that means the odo is over-reporting distance traveled by 1.5%

I'll borrow a GPS to compare...

MetroMPG 07-10-2011 10:39 PM

And I did borrow a GPS. It was very close to the car's odo:

30 km indicated
29.85 km GPS

That means the odo is over-reporting distance traveled by 0.5% compared to the GPS.

And this single comparison to the Google Earth Method (TM) shows it's very close (and seems to confirm the measurement will be short if there are lots of -- or big -- elevation changes on the measured route).

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