Horrible fuel economy... Put back stock heads?

i have a similar set up as you.....id say check a few things....check your 0-2 sensors, check your fuel filter, check your timing,check your whole ignition system.......i get about 20-22mpg on the highway and 17-18 city...

i really dont recomend going back to stock. it will be a waste of time. once you figure out your problem wich i think it is one of the things i listsed, your car will actually run more eficient thna you think...i do suggest having a good stang mechanic tune your car

Ill trade ya my stock heads for those tw's

What kind of Fuel pressure are you running?

Have you cleaned your MAF sensor out? A faulty MAF will suck up the gas!

With 80k miles on stock 02's I'd definatley say change them. I went until 100K when I did my ehxaust, but I have always got good mileage.

Dump the codes and see what the computer says is wrong…

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

Dumping the codes will show codes 41 & 91 for bad O2 sensors

I gained almost 4 mpg after changing my O2's.

Just a wierd question but why worry about fuel econmoy anyway on a mustang. The more you upgrade the more you lose. I only get about 9-12 in the city and maybe 18 on the highway if im lucky. Its the price you pay to go fast

Something isn't right. One thing nice about EFI is that you can have a pretty agressive motor while still getting decent gas milege. I would definatly change the o2's and scan for codes.

As others have said - pull the codes and see what the ecu tells you - it's set up to help you diagnose. If the O2's are malfunctioning, it should tell you. The biggest single factor in fuel mileage is how the car is driven and the conditions under which it is driven. A perfectly stock 5.0L can be driven in a way and in traffic conditions that will produce the kind of mileage you're getting. If you're really taking it easy, and not in continuous stop and go traffic, there's no reason your car shouldn't be able to get in the teens around town, and in the 20's (probably low) on the road even with your mods. See what the computer says.

GATOR said - "The more you upgrade the more you lose." Seems like it might be true - but it's simply not true all the time. If you're careful about how you modify, you can see big performance increases and equal or BETTER fuel economy under similar driving circumstances. Mine has custom cam, 1.7's, alum heads, Explorer intake, FRPP headers, maf, t/b - the usual HCI mods along with 3.73 gears (shorter tires than a STang which make it like a Stang with 3.90 gears) and aerodynamics that are worse than the Mustang's. I routinely get 17-19 mpg around town, and 24-26 mpg on the road. Better fuel mileage than the turbo 4 in my daughter's Volvo wagon.

my buddy put some TFS heads on his car along with a cobra intake and custom cam and he gets horrible mileage like that also. he should be getting better mileage than he is, but not great mileage