I just got my 05 r53 w/ JCW tuning kit recently used from toyota. I have no experience with these cars, I was raised on japanese cars.
All was fine with the car until i noticed the speedometer sat above 0 when i'm at a stand still and when i'm driving, it's 5 to 8 mph higher than it should be. It's nowhere near exact.

Has anyone had that problem? And have any clue how it can be fixed?

I also noticed when I started having a passenger beside me or behind my passenger seat, the airbag light turns on and off randomly. Sometimes the light turns off when i have someone sitting next to me and vice versa. Once, I had the light turn on and the passenger side seat belt light lit up when no one was sitting there. I saw on the car fax that it was brought in 2015 for the recall on the airbag sensors. And read that I could just clear the code with a B200. Any suggestions?