There is a pretty big difference between f/2.8 and f/8, especially when you're talking about a prime lens used at close range from the subject, as with your motorcycle gas tank.
One of the most difficult aspects of photography is melding the technical (camera settings/software/etc.) with the creative. You're using both halves of the brain, and that is difficult for anyone to do. Creatively, your mind sees something, and you are trying to replicate that when you press the shutter. I think what you're seeing with that tank/handlebars composition is exaggerated depth and perspective. You are trying to bend the rules of reality, which can be done with photography...that's one of the coolest things about this art form.
Here is an exercise I learned in a photography class that will help you:
If you have a tripod, use it. Attempt your shot again, making sure you have ample lighting and are using AP mode. Start at f/2.8 and press the button. Turn your dial and take another photo, and then another. Go all the way from minimum to maximum aperture, then look at your results. I'm sure one of them will be exactly what you're looking for.