Passat speedometer

I have recently bought a vw passat mk3 1.9TD, model 1989. The first engine of the car was 1.6TD later according to first owners words it was changed to 1.9l turbodiesel. The car is in a good condition except some little electrical reparable annoying problems. The speedometer shows 15km/h more than actually it is going eg. You are going 100 the speedo shows 115km/h. The tires aren't the original size they are 165/65/14, and I think they shoud be 185/65/14. But according to some tire calculators even if the correct size is fitted it would improve some 5km/h show less so even if I change tires I will get 10km/h more than actual speed.
What should I check and how can I fix this problem, please help! Any help appreciated!
Thanks in advance