Welcome to Just Commodores, a site specifically designed for all people who share the same passion as yourself.
I've repeated this in many-a-thread before, but I will say it again, don't rely on the trip computer for your information. Actually calculate it yourself from the L's you put in and the K's you drive.
My trip computer says I use 13.7 on average, but the actual average is 11.7L per 100km. I also use an app called Road Trip to keep an eye on it.
Just about to get off the P plates and suddenly your driving skills have increase so much that you need a lot more power?