I recalculated the error-difference and I should've said the speedo is 13% off. (I was measuring the wrongway around)
When the speedo reads 60mph, I'm actually doing 54mph on the GPS.
At 70 speedo-miles, it shows 63 on the GPS.

I made an 11 mile drive yesterday (on the speedo), which was 9.7 miles according to the GPS triptracker-app in my phone.
And I just rechecked the same route with Google Maps and come out at 9.57 miles.

Rear tire diameter is a measured (horizontally) 25.8" (235/60/15" BF Goodrich).
(Maybe I should've measured the tire height vertically to take tire compression into account.)