This probably doesn't matter much for you, but it can for me. Mile markers are based upon 2-D distance and can be way off in hilly areas. I used an accurate handheld GPS to calibrate both of my vehicles. I did a very good thorough job on the the Highlander. In the Highlander, the odometer readout has to be increased by 3.42% to be accurate.

To summarize the Highlander, my lifetime fuel economy calculated from the display mpg is 32.8 mpg. If I use the odometer reading (what people refer to as manual calculations) and the number of gallons I add to the vehicle, the lifetime fuel economy is 31.6 mpg. If I use my calibrated odometer numbers and the number of gallons I add to the vehicle, the lifetime fuel economy is 32.7 mpg. Maybe it is just a coincidence, but the display is very accurate--more accurate than "manual calculations".

I have not done as thorough a job in my Civic since I have not taken it on any long trips. However, the odometer appears to read about 1% too high. From the display, the lifetime fuel economy in this vehicle is 56.2 mpg. If I do the "manual calculations", the lifetime fuel economy is 56.9 mpg. This ends up being just over 1% high. If I use the calibrated odometer numbers, the display fuel economy is very accurate.

My main point here is that you cannot say that the display fuel economy is inaccurate just based upon what people here are referring to as "manual calculations". To test the accuracy of your display fuel economy, you have to have an accurate measurement to test it against. You cannot use another number that has not been tested for accuracy.

 

The odometer has to represent the true mileage of a car, by law. There is tolerance for things like treadwear, but as long as that outer circumference of the tire stays about the same you should get a reasonably accurate reading. The amount of error is very small.

 

I do not think that is correct. From this document here, it says:

Hmmm, do you consider 4% to be a very small error?

 

I talked about how I calibrated my odometer here. I started out trying to do it with interstate mile markers, but I ended up using a handheld GPS.

http://www.cleanmpg.com/forums/fuel-...eter-5005.html

 

You have some good points, and I'm beside myself at the lengths you have taken to find out the accuracy for your vehicles.

However, I do say yes, 4% is not much to worry about, because in the end what difference does it make? Tracking mileage is for our own information. It will always be a subjective reading, affected by our particular driving habits and driving conditions, so whenever we compare our numbers it will never be as fair as what a central body with a common test like the EPA finds. What does it mean, exactly, to be showing our numbers to the public? However, tracking mileage can reveal trends, and you can ask youself why it is dropping or why it is going up. The actual numbers don't mean much by themselves.

The reason I favour tracking mileage by the odometer and refill amount alone is that these figures derive from simpler, more standard calculations. And every vehicle and gas pump have them, so anyone can do it. I'm assuming that regardless of odometer or fuel pump inaccuracies, the errors generated by onboard mileage computers should be worse, from one car of the same model to the next, and from one brand to another, due to the complexity and differing techniques involved in generating those figures. For example, one manufacturer may read the volume from the fuel injectors, while another may be taking their readings from the fuel tank level. If the one measuring squirts of fuel has any error to it, then that error compounds; The other, reading tank levels, can be innaccurate but will be relatively self-correcting on subsequent readings. Either way, is there any governance to define standard procedure for this optional feature?

 

Well, I didn't make a special trip to do it, I just recorded the data on the trip. It really wasn't that much work and I'm not suggesting people go out and do it. I was just curious.


I doubt that there are any automobiles that calculate the fuel economy by reading the tank level. I have found my display fuel economies to be very consistent. With proper calibration, they can be very accurate.

I just don't see the point in criticizing the inaccuracies in one technique and replacing it with another of unknown inaccuracies. Also, the first relies on the same distance measurement as the second. In my limited experience, the display fuel economy is very accurate and the "manual calculations" are not. I'm not saying that this is always the case, but you cannot just say that one is better than the other.

Also, I don't think it is that important. I agree that the relative numbers are more important. If you want accuracy, you can do what I did. If you want consistency, the best place to get that will be from the display fuel economy. You can get an accurate calibration of this number by calibrating the odometer.

 

I've run 44 tanks through my '06 prius and what I've noticed is that while it seems somewhat true that the display is greater than the calculated mpg, it is not always so. And the difference seems to be around 1.5 mpg display greater, to as much as 6 mpg diff with the difference in favor of calculated.

Call me old fashioned, but whatever the reason for the difference, I think the calculated method is more accurate for me.

 

I'm enjoying this topic because it is fleshing out the "why's" of what we choose to take our measurements from.

You brought up tracking distance through GPS, and here's what I don't get: GPS resolves to two dimensions -- the coordinates -- on the receiver. On a flat road it will be fine to test your distance, just like it is fine to measure your velocity. But a GPS should not be able to account for elevation changes, any more than mile markers or paper maps. I say should not but I can't recall my portable GPS navigator ever underestimating my speed on inclines. Given the tests that you have performed, including long distance ones with the HiHy, just how confident are you that you have found the correct offset to the odometer?

 

GPS is a three dimensional process and it gives three coordinates--longitude, latitude and elevation. Why do you think it doesn't take altitude into account?

I am very confident I measured the correct calibration factor for my odometer. As I posted in the link I provided, interstate mile markers were very inconsistent. Measuring 20 mile marker segments, the odometer readings ranged from 19.3 (relatively flat segment) to 20.1 miles (VERY hilly and steep area). When using GPS to calibrate my odometer, the GPS distance vs. odometer distance was extremely linear.

 

I'm probably over my head here, but I'm bored so here goes:

Altitude is best measured barometrically.

How important is tire pressure in driving that measured mile? Temperature? Road surface conditions?

How important is it to run the same course several times?

I've noticed that my GPS is almost perfect in its distance read-out to off ramps. What does that tell us?

Of my 44 fill-ups, I've had 3 in which the read-out was less than the calculated mpg.