312 2874cc mercedes sprinter 1999?

Hi the van suddenly wont go above 50mph,and the and it dosent want to rev above 3000 rpm. I recently had front offside tyre changed and the problem seemed to start after that. About a week later i noticed a grinding noise from wheel, on inspection saw carbon deposits coming from the wheel nuts,and found them to be loose,i promtly tightened them and the grinding stopped,but the power problem still remains could there be a link?thanks franki.