Synthetic diesel fuels from Fischer-Tropsch or hydrotreating processes have high cetane numbers with respect to conventional diesel fuel. This study investigates diesel combustion characteristics with these high cetane fuels. A military jet fuel (JP-5 specification), a Fischer-Tropsch (FT) synthetic diesel, and normal hexadecane (C16), a pure component fuel with defined cetane number of 100, are compared with operation of conventional military diesel fuel (F-76 specification). The fuels are tested in a AM General GEP HMMWV engine, an indirect-injection, largely mechanically-controlled diesel engine. Hundreds of thousands of these are in current use and are projected to be in service for many years to come. Experimental testing showed that satisfactory operation could be achieved across the speed-load operating map even for the highest cetane fuel (normal hexadecane). The JP-5, FT, and C16 fuels all showed later injection timing. Despite having a significantly higher cetane number, the FT fuel showed a longer ignition delay, probably due to the lower density of this synthetic fuel, which leads to slower penetration into the chamber. Thus, ignition delay was not found to correlate directly with cetane number, and fuel density effects were able to counteract the much higher cetane number of the FT fuel. Peak pressure was lower with JP-5, FT, and C16, relative to diesel, due to both differences in ignition delay and combustion duration. BMEP with the JP-5, FT, and C16 changed little relative to diesel, but was reduced with FT operation (10-20%) in the high-speed, low-load region of the operating map. BSFC was improved for JP-5 and C16 (approximately 5%), but slightly worse for FT fuel (approximately 5%) particularly in the high-speed, low-load region.