So far as I know, Cessna don't publish a figure (most aircraft in this class don't), but if you've got the Cessna POH to hand, it should be reasonably easy to work it out.
From memory (I've not flown any Cessna aircraft for a couple of years I'm afraid) the POH includes tables of airspeed against fuel consumption. Divide knots (nautical miles per hour) by fuel consumption (gallons per hour) and you'll get values for nautical miles per gallon at each airspeed.
Plot a graph of that (nm/gal .v. IAS) and there should be a visible maximum - that's your best range speed in still air.
To get the value for different head/tailwind components what you need to do is recalculate by calculating (knots TAS ± tailwind component) / (gallons/hr) then plot this against knots IAS and this will give you a different graph, but the maximum will now be at the IAS value for best range for that head/tailwind component. If you got it right, the best range speed should be slightly higher against a headwind component, and slightly lower with a tailwind.
Incidentally, I'd say from slightly distant memory that 75kn is about right and would be slightly nose-high.
G