For anyone that may be interested.....
Fitted an ali 14.5 x 19 Black Max today.
No speed reduction (40MPH on GPS with no current, flat calm water, 2 people & full fuel tank)
WOT is 5450RPM which seems ideal
No acceleration problems and no 'alarms'
When I say 'alarms', I was told by the other drivers that when they'd floor it to WOT alarms would go off and engine would go into limp mode. I'm guessing that the thing may have been under-pitched and engine was hitting rev limiter, hence possibly going into limp mode.