Thanks for your response plog,
So, I wanted to test my caluclated field Kilometers for 0 first but I get syntax error message.
=IIf([VehicleReading]-[PreviousReading]<>0 and IsNull([Kilometers])= False, [Kilometers], "N/A")
I put [VehicleReading]-[PreviousReading] instead of [Kilometers]...as...