Horrible Gas Mileage!

I had a 90k tune-up done, it basically included a quick inspection of car, new fuel filter, and new spark plugs. Honestly though, save your money and do it yourself. The spark plugs are so easy to change on the mustangs. You may need a new fuel filter.

Is your car auto or stick shift?

Calculate your gas mileage. Fill the tank up completely, drive for about 20 miles, fill your tank up completely again. Divide the exact number of miles you drove by the number of gallons you filled up the second time.

I get about 18 miles per gallon. I could get more if I keep low RPMs.

get a new fuel filter, have you changed your plugs recently? Go down to Auto Zone (i dont know where else this is sold) but the product is Sea Foam. Put one can in your tank and you'll notice a huge difference!

Sea foam works great. Other ones Ive tried are Techron, by Chevron, which is very good also. And one that most dont try is the Lucas Fuel System additive. I like this one the most because it also lubricates the upper cylinder. Ive also used Marvyl Mystery Oil, works good.

It probably doesn't make a HUGE difference, but after you change your fuel filter and plugs out (just find some bosch platinums and a oem filter...keep it cheap) avoid filling up at stations during or soon after those tanker trucks fill up the fuel tanks. Stirrs up the goop in the station and then you're pumping it in your car Hope you fix it! Also take a look at your air filter. If it's in horrible condition like mine was a few months ago, a new one makes a world of difference (Thanks, K&N!).