Speedometer reading incorrect!?!

Hi all,
I have a 06 Almera sx 1.5 and the speedo is reading wrong.
It's about 10% out,
When reading 120 km its actually going just over 100!
Checked it against sat navs, and had a friend follow me to make sure.

Anybody know the cause/fix for this?

Cars standard, nothing messed with.

Tires are 195/55-16's not sure if that standard(just incase wrong tire sizes giving wrong reading?)

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