Odometer vs GPS question

[Al in Arkansas]

Due to my choice of front tire and the issues my I/S speedometer came with, my speedometer registers 5 mph faster than my Garmin @ 70.  (speedo = 70, GPS = 65).  Now I discovered Monday that my odometer only registers 1 mph more after 105 miles.  This is comparing it to the trip odometer on my GPS.  Unless I'm completely confused, there should be a 5 mile difference after only 70 miles.  
I know it's not a big deal, but anyone have a suggestion what's up with this apparent discrepancy between the speedometer and the odometer?
Al in Arkansas

[BonS]

The odometer portion of the speedometer gauge is driven by gears. The speedometer needle is magnetically coupled to this mechanism rather than geared. Therefore the two mechanisms aren't really in lock-step with one another at all. The overall accuracy of the speedometer as a whole is limited by the error in the actual diameter of the tire and this affects both the odometer and speedometer. The gearing in the odometer doesn't introduce any inaccuracy at all and is as accurate as the front tires actual diameter versus the "ideal" tire diameter. This, of course, changes with tire wear and inflation. The speedometer however is just an approximation of your true speed. Its accuracy is affected by the front tire diameter, magnet strength, friction and air gap between the metal needle and magnet. That's a lot of room for error. Even the magnets strength changes over time. By the way, my speedo is optimistic by about 4 mph at 70 mph. I trust my GPS which has no moving parts and is considerably more accurate.