The basic Continental TSIO-550 engine is rated for 350 hp, whereas the 550K version is derated to only 315 hp. This means that max cruise can be established at 85% without violating the traditional 75% rule. Seventy five percent of 350 hp equals 262 hp, but expressed as a percentage of the 550K’s derated 315 hp, the number works out to about 85%. For pilots more interested in economy cruise to stretch nmpg to its limits, 55% at 25,000 feet reduces fuel burn to 12.7 gph while maintaining 175 knots cruise.
Of course the TSIO-550 is fuel-injected, which probably reduces its susceptibility to carb icing somewhat.