I just got my 05 MCS (oem 17"'s if it matters) and when i have my onboard digital display on the tach set to current speed I noticed that there is a 7-10mph (more) variation between the display and the actual center speedometer? Is anyone else having this problem?

 

7-10? no, but my Scanguage shows 2-4 different than my speedo.

 

Neither one is correct, so add them together, divide by 2, carry the 7 and pitch the whole thing out the window.
GPS will give you an accurate speed. So will the Scanguage II.

 

Many car manufacturers purposely set the speedometer a little high to keep people from speeding. Both my old '02 MCS and current '06 MCS are off by ~7% from actual speed compared to what radar and my GPS show. There is no way to change this (i.e. no one is willing to edit the core computer, as this can be considered tampering and is illegal). Despite this nuisance, it has been verified by someone in my local club the mileage is true.

Best bet is to compare against your GPS and make a mental note of how fast you are actually going vs. the digital or analog displays.