2003 GT - 12 mpg?

Aquarius · Jun 25, 2008

I have a 2003 GT, 5 speed with the following mods:
BBK Long Tubes
BBK Cold Air Intake
BBK Throttle Body

I've been noticing the car has been drinking gas lately, going through about a tank of gas a week. I drive 24.3 miles to work (and back) every day and I go through about a 1/4 tank of gas a day. Sticker says the car should be making 16 city and 23 highway? Why is my gas mileage so bad?

SVT32VDOHC · Jun 25, 2008

Is the car tuned with that exhaust? IF not, that might be why. I have a complete 2001 drivetrain in my 91 notch with a CAI, stock manifolds, BBK o/r H-pipe, MAC catback, T-45 trans and 3.73's. I get about 14MPH untuned.

Stark77 · Jun 25, 2008

I would do a full fluid change - trans fluid, oil, oil filter, FUEL FILTER - I know a lot of people here probably don't buy into this last one, but buy some Marvel Mystery Oil and try some out in the oil and gas. Check your injectors or buy a bottle of injector cleaner. Your set up alone shouldn't necessarily require a dyno tune - do you have a handheld tuner? Are you running super super rich? Smoke out of the pipes at startup? Dirty air filter? Lead foot?

SVT32VDOHC · Jun 25, 2008

I agree with the fuel filter, but buy Motorcraft only, don't mess with the FRAM. That thing is more harm than good.

My car doesn't let out any black smoke, but I do have a slight black dust on my rear bumper. Not too bad, plus my car is yellow so it is spotted easier. I don't have my rear 02 sensors plugged in either. No cats, so no use.

twogts4us · Jun 25, 2008

DBMSTNG · Jun 25, 2008

did this just start recently? if so, it's not the tune. the ECU should be able to compensate for the changes you have anyway. how is your driving style? where do you shift at and what rpm and gear do you cruise at? i assume there is no check engine light?

Stark77 · Jun 25, 2008

Would it be worth the $400 for a dyno tune with only that list of mods? What would it gain of a canned tune?

Mustang5L5 · Jun 25, 2008

You should be doing much better than that. I can flog the hell out of my car and still get more.

I average 18MPG city and 25-26 highway.

How is your driving style. I find that holding a gear over 2000RPM causes your mileage to go down a lot.

TrueBlue03 · Jun 25, 2008

i've been getting 12-14 mpg (all city driving) now for ~ 3 years i usually get around 50 or 55 miles for a 1/4 tank.

do you have 18" rims Aquarius? How long ago did you install the long tubes?

What most people overlook is rim and tire size. I noticed a big dip in my gas mileage when i threw on the saleens with 265's and 295's. A 295 must create a lot more friction due the the larger contact area, right?? .... and then also taking into account both the larger rim and tire take more energy to rotate.

Stark77 · Jun 25, 2008

tires + gears will drain your wallet at the gas pump - bigtime

Aquarius · Jun 26, 2008

Sorry forgot there's 373's in there too. I bought the car last summer, had 30K on it and all the mods already done. One I picked it up, I put new plugs in, cleaned and oiled the filter, new oil/filter, and new fuel filter (FRAM because I didn't learn that they were crap until now). I had all kinds of problems just after I bought it with the check engine light. Lean Bank 2 among others - I made the dealer replace all the 02 sensors, two of which were replaced twice because he thought he just put a bad set in. After maybe 5 trips to the race shop where they outsource the labor to, the mechanic said it was the long tubes throwing the 02's off and I had two options: put stock headers on or physically move the 02 sensor bungs. I didn't do either so I've had the CEL ever since (and it pisses the hell out of me). He did tune it though, and put it on the dyno a few times. I can't remember what the last code was, I'll get that checked tonight. The car actually goes in tomorrow to get a new X-pipe installed b/c my cats are shot. I'm going to have him check the fuel trim levels because I know that last year they were out of wack for a while.

I typically shift between 2200 and 3000. I'll cruise between 1000 and 2000. I have the Bullitt wheels - 17's if I'm not mistaken. No black smoke on start-up.

mogs01gt · Jun 26, 2008

The long tubes will make it run a tad rich but the CIA should lean the engine out. If you are getting the bank 2 is lean, that is your drive side of the engine. I would slap on the stock upper intake tube, remove the positive battery terminal and let it sit for 15 minutes. Slap the battery back together and take it for a spin. BBK's is know for having crap parts, I continually get a check engine light because the passenger side O2 bung is not in the correct position on my X pipe. Does the car still have the stock MAF?

When I drive my car around town, I typically try to shift right at 2k but I never get good fuel mileage in city driving.

Stark77 · Jun 26, 2008

3.73's still shouldn't drain your gas THAT much. Things you can do to improve mileage - if you haven't dont so already - would be steeda underdrive pulleys. Piggyback pulleys add weight and will break over time. Adjust your timing on the positive side - as long as you aren't running juice. Lightening up the drivetrain (aluminum driveshaft, lighter rear wheels, etc) could potentially increase gas mileage if your still conservative on the throttle.

SVT32VDOHC · Jun 26, 2008

I agree about tires too. My notch with the 2001 4.6L motor and trans has 285's on the back of it and I think that kills the mileage. When I had my 00GT it was stock with 245's and the mileage was much better on that thing. Well, plus my car now has 78K and that 00GT I had had 6K. It was like brand new.

HOOCBB · Jun 26, 2008

Aquarius said:
Sorry forgot there's 373's in there too. I bought the car last summer, had 30K on it and all the mods already done. One I picked it up, I put new plugs in, cleaned and oiled the filter, new oil/filter, and new fuel filter (FRAM because I didn't learn that they were crap until now). I had all kinds of problems just after I bought it with the check engine light. Lean Bank 2 among others - I made the dealer replace all the 02 sensors, two of which were replaced twice because he thought he just put a bad set in. After maybe 5 trips to the race shop where they outsource the labor to, the mechanic said it was the long tubes throwing the 02's off and I had two options: put stock headers on or physically move the 02 sensor bungs. I didn't do either so I've had the CEL ever since (and it pisses the hell out of me). He did tune it though, and put it on the dyno a few times. I can't remember what the last code was, I'll get that checked tonight. The car actually goes in tomorrow to get a new X-pipe installed b/c my cats are shot. I'm going to have him check the fuel trim levels because I know that last year they were out of wack for a while.

I typically shift between 2200 and 3000. I'll cruise between 1000 and 2000. I have the Bullitt wheels - 17's if I'm not mistaken. No black smoke on start-up.

If your cats are clogged, you will see an improvement in mileage by replacing them. I can't say that it will be significant.

When I was still driving my 95 Ranger, I was getting 22-23 on the highway. Had to replace the full exhaust system from the headers back and the hwy mileage went up to 25-26.

Steeda U/Ds, fluids, filters, PCV would be your best bet. maybe a Steeda timing adjuster, not 100% sure though. They say advancing the timing just a bit has a positive effect on MPG.

joshheat25 · Jun 26, 2008

It is well worth it to do a tune! and if your driving stop and go you should be seeing 15 mpg

gip99drop · Jun 27, 2008

Stark77 said:
tires + gears will drain your wallet at the gas pump - bigtime

Not so true. Just keep your foot out of it.

98E9GT · Jun 27, 2008

Im getting crappy gas mileage too, Mine is basically stock, Im pulling 16MPG in town, thats with one, WOT to 0-80, other than that, its shifting at 1800RPMS. AC is off also.

bhuff30 · Jun 27, 2008

My 88 2.3 mustang gets about 12mpg. But that's with 3.73s, NO overdrive gear (C4), bigger injectors that aren't tuned or compensated for, and E85 which has less energy per gallon. I also beat on it relentlessly and love embarrasing anyone who 'thinks' they are fast, and don't mind at all to losing to those who are fast.

I've never done worse than 17mpg in the 4.6, and that was with a bad O2 sensor and all in town traffic. I've done as well as 27 on the highway.

2003 GT - 12 mpg?

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