Synchronizers are used to reduce shift time and increase comfort for manual transmission gearboxes. The increased torque in modern trucks as well as overall vehicle refinements places higher demands on synchronizers. The short duration of the synchronization process, the tribological contact with many parameters including oil additives and surface properties as well as the requirement of many cycles before failure makes experimental testing hard and expensive to perform. To reduce the amount of testing needed, simulations can be performed. Few commercial programs exists for calculating the synchronization process and Scania used a self-made Excel spreadsheet. The objective of this master thesis was to develop a new MATLAB program for calculating synchronization of gearboxes, with focus on versatility, accuracy and user-friendliness, and verify it by physical testing.The result is a program with support for more gearboxes than the old spreadsheets. Several new parameters were included to improve accuracy. The tribological contact is represented by using experimentally determined friction properties and material limits, and compare calculated values with the material limits. User-friendliness have been in focus during the entire development, and the program features an extensive manual. Physical testing shows that the program shows good correlation when the synchronizer is not worn. The accuracy decreases as the wear increases, due to a sudden increase of the coefficient of friction at the end of the synchronization.