After each battle, you will accrue vehicle specific experience. If you have an 'elite' armored vehicle, you will have the option to convert the vehicle specific experience to free experience (experience that can be used by any vehicle in your garage) by using gold, at a rate of 1 gold per 25 XP converted.
Please note that elite vehicles have a golden laurel wreath around their vehicle type in the upper left area of their vehicle icon.
Log onto the game client:
Click on the "Convert Experience" button by the top right hand corner of the client:
Enter the amount of tank specific experience you would like to convert to free experience. You can unselect any tanks whose experience you do not wish to convert. You can also select or unselect the "All vehicles" option. At the bottom of the "Convert to Free Experience" window, it will state how much gold it will cost you to convert the selected experience. Click on the "Convert" button to proceed:
Please note that it costs 1 gold to convert 25 tank specific experience to free experience.