The Schedule Performance Index (SPI) is a measure of how well a project is meeting its schedule. It is calculated by dividing the earned value (EV) by the planned value (PV). If the SPI is greater than 1, then the project is ahead of schedule. If the SPI is less than 1, then the project is behind schedule.
So, if the SPI for your project is greater than 1, then all other things being equal, you can anticipate that your project will finish ahead of schedule.