Odometer vs GPS
 

I recently found that my odometer is reading 2.65 miles low compared to the GPS for a 100 mile trip. Which would you trust to do your mpg averages?

I would trust the GPS. IF you can find the specs on your unit, it's probably accurate to within something like 15 meters total point to point. If you didn't hit any resets or move without the unit on, it should be the best way to measure distance covered. I found my Jetta odometer reads 10% high, which was a little disappointing, but its good to know the truth :)

GPS.
Odometer varies with tyre size, tyre wear.

Well it's been good to know that my average is actually better than calculated by around 1mpg.

not even a question. the gps.

I have a MPGuino that is calibrated to a GPS distance. The diffrince between the GPS/Guino vs the Odometer is pretty consistent so I have a conversion factor for the odometer to be used when the guino's power gets interrupted.

GPS, because every time I turn my engine off with coasting, there is about 1-2 seconds where the ODO does not record any movement. Over a 400 mile tank with heavy P&G I can end up 15 miles off (low) if I trust my ODO instead of GPS.

I have a question about how a GPS calculates miles. Does it use a "true projection" like a map distance does, thus flattening out hills and valleys? Or does it calculate the actual distance traveled through the valleys and over the hills?

Around here the terrain is so flat that it would make little difference, but in northern GA's hills it could add up to a percent or two.

I know that there is 1.5 mile difference between my morning commute and the evening drive. How can that be when I start and end my trips at the same points? The answer is the highway interchanges that I go through. On my way to work I go through the inside curve and on the way home I have to go around the outside of the intersection. You wouldn't think such a little thing would make a difference, but we all know it's the little things that add up.

Just because your GPS and you odometer read different distances doesn't necessarily mean that either are wrong. They may be measuring different things.

Every second or so your GPS calculates position XY and Z. Their is usually some error lat/long and a fair bit more for altitude. If you are stopped your Odometer will note that, but the error in the position calculations will add up. Best I can come up with is check your GPS vs your Odo for a couple different routes, at speed. Then compare both to a measured distance.

...it's a three-dimensional (lat, long and alt) "TIME" sphere, which measures the "TIME" from each satellite to your receiver to determine your location.

...GPS (used to be called NAVSTAR) is a digital, high (very high!) tech, 3-dimensional, version of the old two-dimensional Navy/Maritime LORAN systems.