A couple of weeks ago I got my oil changed, and before the oil change I was getting rougly 20mpg. After the oil change, I've been getting around 14mpg. Now, I do take into consideration that I'm doing mostly city driving, but I was living in the city for a month before the oil change, and I still got around 20. Also, while accelerating, the needle drops considerably, I've never noticed it before until now. I usually dont get down to the 3/4 mark until around 100 miles, but now I'm already past the 3/4 mark and i've only driven 65 miles. What are some potential reasons for this?