Diesel gauge cluster is easily reprogrammable to tick odometer over at less than half speed while keeping speedometer reading accurate.
Not saying to do it as that would be illegal. But it's possible :x
You said mechanic had replaced the water pump? My guess would be the previous water pump sh*t it self, previous owner cooked the f**k out of it, warped the head enough to blow the gasket.
Stick a straight edge across the head and check it.
Nah something like this.
And install inpa.