A few things to keep in mind that effect level progress.
You level increases faster with speed classes.
You gain a high % level growth if you win a race against players that are higher ranked than you.
Eliminator games seem to give you a higher % increase if you win,possibly because these races seem to take longer.
You lose level % if you quit a game ,It seems around equal to 4-5 races won,not only that but it seems that you level increase seems to stall for a while before increasing again.