Does it seam right to only get 6.2 mpg in a Jeep ?

No, it isn't right. I bought a 2004 Grand Cherokee Special Edition brand new and even though it's old now, I still get 17.8 MPG and it's and 8 cylinder. You should get that checked out.

check the average over a tank of fuel, I get 16 mpg in town and 23 mpg on highway 4.0 straight six.

Short trips around town might give really low fuel economy. Like Ed says, double check the read out.