195 degree ..... 18.00 MPG
175 degree ..... 17.91 MPG
155 degree ..... 17.82 MPG
135 degree ..... 17.74 MPG

For whatever it is worth
the above is what Kevin Gertgen's
'Fuel Economy Calculator' program

http://performancetrends.com/fuel_economy_calculator.htm

predicts for a
stock 1995 2wd 5.9 V8 46RH auto Ram pickup with 3.55 diff,
travelling down a level concrete highway at a steady 70 mph on a windless 42 degree F day

The program does not allow thermostat temps above 195,
but one might speculate that each
20 degrees of coolant temperature increase
might be worth
0.1 MPG

I know that if you idle a 1995 Ram for ten or so minutes
with the factory radiator fan removed,
but with Evans NPG coolant to prevent boil over,
the coolant temperature goes up from
193 to 240,
the idle smoothness noticeably improves,
and the fuel consumption drops from
0.66 gallons per hour to
0.60

When at speed on a highway,
if you raise the coolant temperature by blocking airflow through the radiator,
rather than by thermostat action,
you might get an even better MPG gain,
because air flowing through the front grille and then out the bottom of the vehicle is a major source of aerodynamic drag.

NASCAR teams of course routinely block off as much of the grille as cooling will allow,
and the new GM Cruze has shutters on its radiator opening.