Ok... did it...

Ran 25kms @ 100km/h
Should have taken me 15mins.
Took me 14mins 34 seconds.

Used highway markers, and my trip odometer, which was pretty much bang on.

That indicates your speedo is reading about 3% low (reading 100 actual 103). Unusual (normally factory speedos read high - so common that road tests of the 60'* included an entry for actual vs speedo reading). So when you were reading 120, the actual was 124 and if the other car was reading high, it could easily have shown close to 130.

It is more in perception than reality.