Just noticed something weird. Had my 2010 Prius over 9 months and have filled the tank 27 times. Each time I note the gallons purchased and miles covered on the Trip A odometer, then reset the Trip A odometer after filling up the tank. I've been keeping the numbers in an Excel spreadsheet. Here's the funny thing: if I sum the mileage from each tank it equals 13094. But I checked the total mileage on the odometer at the last tank and it was 13108. 14 miles is a big difference. I could perhaps understand if each Trip A odometer reading was off by 0.1 miles, and the total mileage was 2 or 3 miles off. But 14 miles surprised me. The odometer had only 1 mile on it when I bought the car. The dealership had the car twice, once to update the brake software and another time to try to silence the numerous squeaks and rattles in the dashboard (unsuccessfully I might add). The good news is that my total mileage is actually a tiny little bit better than I thought since I travelled an extra 14 miles for the gallons purchased. But any ideas why the discrepancy?
Reread your post as car was purchased with 1 mile on the odom. Thought you might have purchased it with 14 miles on the odom. That being the case...in general...the odom is not going to be "dead on" in terms of distance. Hence the reason why you see deviations between calc'd and displayed mpg in the Prius...and any car for that matter. I'm surprised it's off by 14 miles but in math speak...it's only .01068 percent of your total distance traveled...which is just a little over 1% of 1%
Also, think about this... The tires on your car are not made specifically for a Toyota Prius...meaning...the tires weren't built to the specs of the car so the odometer would be 100% correct. They were built to fit a car of it's size, weight, load capacity, etc.
Maybe when you took the car in for service, they did a test loop and reset your trip so now you're off by 14 miles. If not then I'm not sure how the ODO and Trips could be different since they must use the same algorithm to calculate your distance traveled.
27 refills limits the Trip meter truncation error to 2.7 miles. Add another 1 mile for main odo truncation error -- for not knowing tenths & hundredths on the main odo when the car was delivered -- and the accumulated error should not be more than 3.7 miles, with a typical error of half that. Does your fuel log have both TripA and Total Odo together for each fill? If so, go back and crosscheck for any data errors. If not, you are missing a valuable check on data integrity, which has flagged several errors in the fuel logs of my past cars. On my Prius, with 14295 miles and 38 refills, the accumulated error is just 2.2 miles, compared to a theoretical max of (0.1 * 38) + 2 = 5.8 miles. (The second whole mile represents data lost by a grease monkey who couldn't reset the maintenance reminder without resetting my trips meters . That entry is filled with an integer-mile estimate).
That contributes only to the total odometer not matching the true distance on the ground. It is wholly unrelated to two internal meters not matching each other.
Unfortunately no. I must have made a mistake somewhere. But I'll keep the total Odo readings from now on just to double check.
Just a couple of thoughts: A ) trunchation - resetting a trip meter will lose a fraction that ranges from 0.009...1 to 0.099...9. The worst case would be 0.0999...9 * 27 resets < 2.7 miles. However the expecte error would be closer to 1.3 miles. B ) reset delay - resetting a trip meter leads to a short interval where "--" is displayed. I have not made a study of how long in either time or distance this reset interval takes. I'm using ODO to backup for 'fumble fingers' resetting of a trip meter. So I've not tried to compare relative accuracy. However, with the original tires, I consistently saw the trip meter average speed being 'ground truth' accurate compared the indicated speed. I'm running Garmin nuvis in parallel with my tanks but due to the way the Garmin appears to calculate distance in curves (a series of segments,) it has been consistently low. Figuring out the error is going to be a challenge: 14/13094 = .001069 14/13108 = .001068 To make a ground truth measurement, would take a very long drive on a ground truth route. It may make more sense to add a 5th wheel to accurately measure ground truth distance. But then we have the coarse ODO units, miles. I don't have a better answer but just offering some random thoughts. GOOD LUCK! Bob Wilson
