on the larger wheel sets such as 33 12.50 u have to put a diffrent ratio speado into the dash. my rover came with this high ratio fitted and when came to fitting standard road wheels it over read. easy enough to find a low ratio speedo and even simpler to fit.

Just saw your post. I had the same problem a few months back with my 2.5 na. I spent weeks going through all the other possablities such as the gasket gone again but only around pot 1, the valve settings, the pistons and even the head again but it turned out just to be as simple as the washeres...