I'd say the main problem of your site is that it looks old. The banner, as mentioned above, is part of that impression.
Other examples:
The gray background feels like I'm using Windows 98, which definitely doesn't feel modern.
The white menu background fades in nicely but is maybe too plain. Adding a discreet pattern to it could be nice.
You could also enhance the subtitles (Products, Information, etc.). Maybe rounding their corners like the rest of the site would do the trick.
Your top horizontal navigation bar (Home, Contact us, etc.) doesn't have a very appealing tone to me. Maybe you could try a more lively blue font.
The bottom bar feels a bit lonely. Maybe you could just remove it and place its content inside the dark grey one.
On this page <http://www.driveflexfuel.com/store/8VAT.php> and probably all similar pages, you have a "Varify promo" button instead of Verify. Such mistakes can help rebuke customers who want to make sure the seller can be trusted before purchasing.
I think that pretty much covers it for me.
And thank you for the environment