Advanced premixed compression ignition (CI) combustion using fumigation has been shown to yield significant improvements in indicated efficiency over traditional diesel combustion strategies while simultaneously reducing engine-out soot and NOX emissions. To better interpret these findings, a breakdown of the ways in which actual performance deviates from ideal engine cycles is helpful. Non-ideal combustion phasing is one cause of such deviations. In this paper, the centroid of the calculated apparent heat release rate is used to estimate an adjusted maximum possible thermal efficiency based on constant volume combustion using an effective compression ratio concept. Using these metrics, experimental engine data are evaluated from a single cylinder direct-injection diesel engine operating in premixed CI mode enabled by gasoline fumigation and a diesel pilot injection. Indicated gross cycle efficiency was found to be higher for premixed fumigation compared with a conventional diesel condition at the same load. A key finding of the work is that the peak indicated cycle efficiency for fumigated premixed CI combustion occurs with combustion phased very close to TDC. Shorter heat release duration and lower heat losses from the cylinder are thought to be the cause of differences in cycle efficiency between conventional combustion and premixed CI fumigation modes.