Speedometer problem

[astondg]

I have just been out taking readings from the speedometer and tachometer in my 1985 323i. I also checked the actual speeds for the same rpm in an Excel spreadsheet I downloaded. The results were:

Gear: 4th
RPM | Speedo | Actual
2000 | 60 (37) | 56 (35)
2500 | 70 (43) | 71 (44)
2900 | 80 (50) | 82 (51)
3750 | 100 (62) | 106 (66)

Gear: 5th
3050 | 100 (62) | 106 (66)

I am assuming that the tachometer is correct although it could be out by a little bit too. Also the results are probably not entirely accurate, especially with the mph conversions because I left off the decimal places, but should be within 1 or 2 km/h.

So the results show that it is overreading at 60km/h, fairly accurate at 70km/h, starting to underread by 80km/h, and underreading by 100km/h.

I have 205/50/16 tyres Vs the factory 195/65/14 (although my car came from the dealer when new with 205/55/15) so that would affect it slightly but I thought the speedometers were supposed to overread by a lot? Also why does mine change from overreading to underreading?

I know there have been other threads about the speedo but I can't find any where this problem was mentioned. There was mention of the cables being damaged and causing it to be erratic but mine seems fairly constant, just wrong.

Any ideas?

Aston