Quote:
Originally Posted by California98Civic
The revs/mile of tirerack are just corporate reporting. Worth testing for yourself in the real world conditions of your car and specific set up. The GPS I'm playing with has confirmed, almost, what ridewithgps.com showed me... so if you don't have a GPS, plot out your daily commute carefully on ridewithgps.com and compare what your OEM odo reports. I'd like to know, because I still find my calibration correction astonishing and hard to believe. I'm waiting for an error to appear.
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I used all of the instrumentation I could to estimate my odometer accuracy and try and get good data. Some interesting things occurred. None of the readings agreed. I used two different GPS sources, the scangauge and the stock trip odometer.
I am running 175-65 R 14s which are only slightly larger than stock.
ODO 64.0 miles
Scangauge 63.7 miles
iPhone 4S with GPS app 63.5
Garmin GPS watch 63.31
The watch and iPhone were both started while the car was parked and before it was running. I waited until I was sure that both had a good fix before they were started. I reset the trip meter twice to make sure it was fully zeroed and the scangauge had been off a sufficient amount of time to start this as a new trip. I didn't do any engine off coast so I wouldn't loose any time on the odometer with the car off but moving.
If we average the GPS readings to be 63.4 miles, that means that my Odometer is reading about 0.946% high and my reported fuel economy would be inflated as much. However, the engineer that I am can't help but question the GPS results. I don't think that the sampling rate is very high for the watch or phone. I suspect that the watch samples at about 1 Hz as that's how often it updates the screen and moving at highway speeds the distance increments every time. Since its battery powered and very small it would be wasteful to sample more often from a power perspective. It could sample even less frequently and estimate the distance between samples for the screen updares which would be a valid approach in the intended low speed (running) application. However in the car this makes problems as you start to get jaggies as you go around turns. The distance is a direct point to point calculation and every time it cuts the corner you loose some. Long story short, if I was sampling at 10Hz I would be confident but as it stands now I'm not so sure. Plus the amount of difference between the otherwise identical GPS recordings makes me believe that the results are inconclusive but perhaps my odometer is still skewed towards the high side. What is interesting is that the scangauge and odometer don't agree. They are seeing the same data in theory so they should be very close. I don't know if that could be a floating point rounding error in the scangauge or mechanical variability in the odometer.
Honda odometers could have a variability of +\- 2% which would be realistic. Not good if you are on the high side but that's just how it is. This would also be indicative of running tires smaller than stock, which I am not. They are slightly larger than stock in fact. I would love to see more civic drivers get some data on this with your tire size.