You have to make sure that one of the "trainable" attributes is only at 19 until experience hits 20.
In other words, assuming you leave Mental training until last, keep training until three of the four Mental attributes are at 20 and one remains at 19. Then suspend training and wait until the experience attribute gets to 20. When this happens resume training and when the final Mental attribute clicks to 20 the driver level will also go to 20.
The driver below is level 18, but if I resume training so that knowledge rises from 19 to 20 he will become a L20 driver.
However, it's just a visual issue, a L18 driver that has 20 across the board in all attributes will be just as fast as a L20 driver. The one below I haven't bothered to train to L20 and is a 3 times champion.
