Apologies, just re-checked my sums and I got my 1.75 and 1.62's back to front. DOH!
Assuming 10% slip throughout - in reality could be a bit either way - I would suspect a bit higher with the antifouling on.
With 1.62 gears, 28" props and 5100 rpm you'd see mid 70's.
With 1.75 gears, 28" props and 5100 rpm you'd see just under 70.
As you are seeing 82:
With 1.62 gears it suggests about 5560rpm
With 1.75 gears it suggests about 6000rpm
One way or another, it all points towards the rev counters telling porkies - would definately check them next before considering any drastic measures.