Electromagnetic energy harvesters (EMEH) for tire pressure monitoring systems (TPMS) typically suffer from low output voltages. Step-up converters play an important role in the energy harvesting chain, would allow extracting energy efficiently, and would provide output voltages significantly higher than standard techniques provide. This paper presents a simple but high efficient boost converter to circumvent the limited low output voltage. The circuit principle builds on a switch, being connected to the EMEH terminal, and that periodically charges the inherent EMEH inductor. Based on the unidirectionally electromechanical system description, the energy efficiency is derived, whereas the discontinuous current mode (DCM) and the continuous current mode are considered. An expression of the ideal switch timing is found for a maximal energy supply and is valid for any linear electromagnetic energy harvester. The expression solely bases on the system's resistance and inductance. Complementary to traditional optimal load resistance characterization, this work studies the circuit performance to maximally harvest and transfer energy to a storage capacitor without a connected load. The boost interface circuit with optimal parameters is applied to an EMEH developed for TPMS and compared with the performance of other interface circuits. Depending on the specific setup and on the boundary conditions, efficiencies of 80% and 65% can be reached easily without and with AC-DC Schottky diode rectification losses, respectively.