My 88" with a 3.5 V8 in it now cruises at 70mph @ 2400rpm. Before I fitted a Fairey O/D it was 3000rpm @ 70. It gives me an extra 2mpg. Doesn't sound a lot, but with my mileage it'll pay for itself in a year.
Additionally, it does reduce the cab noise a little, not much to be honest & if you're running a standard doosul you'll probably use up the extra revs getting a little more speed. A petroil would probably benefit a shade more I reckon given the price of the stuff.