If you get 250km from 34l then that is 13.6l/100km. At 12l/100km you should have gotten 283km.

Don't forget to reset the fuel economy reading dash display when you fill up. The dash display measures the average fuel consumption since the last time it was reset, or at least mine does (I presume the newer ones are the same). That means if you've never reset it it'll be reading the average fuel consumption since you bought the JK.