A larger than stock tire up front will read slower than the actual speed, a smaller will read faster than the actual speed.

If the cable is bouncing around in the casing it will make the needle jump around.

I'd rather give someone a tool than a solution. Give them an answer and they'll always expect it. Teach them how to find the answers for themselves and they will spend a lifetime learning.
As for my mental state?
"Not User Friendly"