gas mileage

I have a 2003 convertible ford mustang, i used to get 320 miles to a tank of gas, now i am lucky if i get 170!!! I had my car tuned up-points, plugs etc..., new tires, new brakes ...and even put a lock gas cap-(thinking that maybe my gas was being stolen!) it didnt work, what else could be wrong? i drive about 10 miles a day, i live only 1.5 form work!!!! I used to drive about 20 miles to work each way, then i moved closer, this seemed to happen right after i moved. Please help!!!!!

Is it the 3.8 or V8? If it's the 4.6L you never got 320mpg in the city actually 170 in stop-n-go traffic is a little low but if you have your a/c or heater on those numbers are prob right. If its the V6 well.......never owned one or worked on one so im no help to you there.

its a 3.8. Before I moved, the drive to work and back was hwy miles, now i do live in the 'city'. could it make that big of a difference?

yeah it makes a pretty big difference........but you could always bring it to a mechanic so they can look over everything just to make sure.

Yea the highway to city driving really effects it. Sometimes a drastic change in mileage. Cause you got to stop so offten sometimes you got to go and get going then stop again 50 yards down the road. That kills the gas mileage.