Well that is alot of copy and pasting there my friend...

To access this... hold the milage reset button in and then start the ignition and engine (located in the middle of the speedo and rev counter).

This will then take you into the ecu and you keep clicking it until you get to the digital speedo...

yes you are correct it is 10% out... i.e at 30mpg... it will read 27mph.

I hope this answers your question.