Odometer Accuracy for tracking mileage

[COcyclist]

I just filled up last night and I have kept written records of miles and fuel used since I got the car. I have a ScanGuage II and it showed a 854 mile tank. The VW odometer showed only 836 miles since the last fill-up. I calibrated the SGII to the highway mile markers to get an accurate mph reading. Most VW owners report the speedometer is about 3 mph higher than actual speed. I have since gotten a GPS and just wondered what would be MOST accurate: odometer, SGII or GPS? I could leave the GPS in the car and only reset the trip feature at fill-up to get miles per tank. Would this give me more accuracy than either of the other two?

[Clev]

Originally Posted by COcyclist

I just filled up last night and I have kept written records of miles and fuel used since I got the car. I have a ScanGuage II and it showed a 854 mile tank. The VW odometer showed only 836 miles since the last fill-up. I calibrated the SGII to the highway mile markers to get an accurate mph reading. Most VW owners report the speedometer is about 3 mph higher than actual speed. I have since gotten a GPS and just wondered what would be MOST accurate: odometer, SGII or GPS? I could leave the GPS in the car and only reset the trip feature at fill-up to get miles per tank. Would this give me more accuracy than either of the other two?

I calibrated my odometers using my GPS. Cars were about as dead on as you could get; truck was +4.5% due to the bigger tires. I add 4.5% to my "miles travelled" when putting tanks into the garage.

[MetroMPG]

I'd go with GPS for best accuracy too.

I don't have one, and have measured my odometers very closely by using Google Earth!

Using Google Earth to verify your odometer accuracy - MetroMPG.com

[COcyclist]

So GPS has the best accuracy. I use mine in several different cars though. If I calibrate the ScanGuage II using the GPS, is the SGII accurate enough that I should use that reading instead the odometer when logging fuel economy in my garage?

[MetroMPG]

My car's odometer is off since swapping for the taller final drive transmission from a model with different size tires.

After figuring out the difference against Google Earth, I now do what Clev does: I add something like 6.5% to the distance traveled shown on my odometer for doing fuel consumption calcs in the garage.

But I'm just being a geek. I have the ScanGauge speed/distance offset set at +6% - it doesn't do decimal places, which is why I do the math myself to get that extra 0.5% accuracy.

You'd be fine going with the calibrated SG distance.

[discovery]

I think the best is the GPS, go for it.