Posted Wednesday, April 5/06 in General

The further you sink into MPG nerdity, the sooner it will become clear that you need to know how accurate your odometer is. Why is it important? I'm glad you asked!

It's important simply because an inaccurate odometer will throw off your mileage calculations. Maybe your fuel economy is actually worse than your odometer has led you to believe... or maybe it's better! Maybe you switched to different wheels or tires (with a slightly different circumference). Or maybe all this time your odometer has been out of whack straight from the factory, and you just didn't know it.

A 2% error on a 50 MPG vehicle = 1 MPG off. And a 2% error (or worse) isn't uncommon. The fact is, you'll never know if you don't check it out.

Of course, there are already several established ways of verifying an odometer. Unfortunately, none were available to me. So I had to come up with something different.

Method 1:

The old-fashioned way is by counting mile posts on the highway and comparing them against the distance reported by the car. I recall seeing these markers on the U.S. Interstate on a trip to North Carolina a few years ago. Unfortunately, in Ontario I couldn't find any reference to whether our highway markers are standardized, and an e-mailed inquiry to the friendly Ministry of Transportation on the subject went unanswered.

Method 2:

A more modern approach is to compare your odometer against a GPS unit. These are fun toys, if you happen to have one. Alas, I do not. So, foiled I was. Briefly...

Method 3:

Necessity being the mother of invention, I put two and two together and gave birth to one interesting idea: Google Earth. I had been playing with this fun & educational program around the same time I felt the compulsion to verify my odometer, and it just so happens that Google Earth includes several tools for measuring distances very accurately - right down to the millimeter level.

With the Google Earth method, you measure a very specific route on the computer, and then go out and drive it to make the comparison.

Details:

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". (The "path" tool is more useful - it calculates the sum distance between multiple mouse-clicks as you define a meandering route. Unless the route you want to measure is perfectly straight, you'll use the "path" tool.)

Use the tool to measure out your route, then hop in the car, and go drive it. Of course, the longer the path you measure & drive, the more accurate your comparison will be.

One difficulty arises if you happen to live in an area which is not covered in a high enough resolution by the Google Earth satellite images (meaning, you can't zoom in close enough to discern local roads & intersections). Most U.S. cities and some international cities have high resolution - but not the area I live in.

Foiled again, was I?

Nope. Turn on the "Roads" overlay and you lay out a path over a map. You may not be able to measure the exact distance from your driveway to your parking spot at work - you'll have to settle for intersection to intersection. That's good enough for my purposes.

Results:

On a recent Sunday afternoon, I took a leisurely drive which I'd planned out in advance - mostly highway driving, with half a dozen stops & turns, which I purposely kept simple to minimize the chance for error in plotting the path in Google Earth. The results:

72.7 km / 45.17 mi -- odometer reading

72.10 km / 44.8 mi. -- ScanGauge reading

72.55 km / 45.08 mi -- Google Earth path measurement

Not bad. The ScanGauge is under-reporting Google Earth by 0.6%; the odometer is over-reporting Google Earth by 0.2%. I guess I don't have to worry too much about my odometer. And I'll have to check the manual, but I believe the ScanGauge's internal odometer is adjustable.

Corroboration:

A couple of Ecomodder.com members tried the Google Earth method with good results. One member, Dan, measured out a known distance and it was remarkably close:

"OK, I was a little skeptical that this would work. But I tried it anyway. The SuperMID on the Prius said 41.04 km to work ... Google earth said 41.04169 km. You don't get any more calibrated than that! Dead on!"

A second member discovered his odometer was under-reporting by 2.75%. Meaning, he was actually getting better fuel economy than he thought. Sort of like finding loose change under the couch cushions!

Save it for later...

If you use Google Earth to measure out a route, make notes of intermediate readings (in both G.E. and in the car) at several points along the way. Hold on to this info and you'll have a reference you can use to check against possible changes to the car in the future (e.g. different car, tire size, transmissions, etc.). The shorter segments give you the option of doing (potentially less accurate) quick tests.

Resources:

- Google Earth web site