Speedometer error?

[Gixxerboy63]

...So it goes like this; my 4.6is is getting its doors blown off at 70 mph and it feels like I am traveling at a slower pace. Last week, I installed "speedview" on my Android phone... really cool stuff. My wife's car, and our van are spot on at 60 mph and agree with the speedview. When the X5 reaches 60 mph, the speedview shows 54 mph. If I am doing my math correctly, that's about a 10% negative error.

The good news is that this may have saved me a speeding ticket or two...

The bad news is this;

My X5 is showing 127,xxx miles on the odometer. Because the speedometer is faster than the truck and is racking up miles even faster, that means that it should have only have 115,xxx miles on it!

12k miles will certainly have an impact on the resale value;

I feel like I have been robbed.