Road computer

Hello friends
My car 2005 gasoline 1.4 206xline
For me, the biggest shortcoming is the computer. I can't follow my fuel consumption while driving.
What is the way I add this feature. We will buy 3 lines of screen, or how much efficiency we get if we buy and connect from externally mounted devices etc. In what ways can I obtain this feature of the minimum price?

If you set the Com2000 key and get a 3-line display, it works, there are friends selling the screen on the site.
In order to activate the key, you can find the topics that are opened in the forum, if you search for 3 lines of computers and com 2000.