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post #1 of 5 (permalink) Old 02-26-2013, 06:22 PM Thread Starter
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Location: Abilene, TX
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Bad City MPG

Title says it all.

I cant believe how awful the gas mileage is on my new mustang. ALL city, but I honestly do not even drive it hard! I know there are tons of factors, but I have always had a light foot when i want it and no how to get good mpg out of a car, but definitely not this one!

Maybe its an overly rich tune? Is it because the TC doesnt lock until 34 MPH?

12 MPG city average seems a little absurd. I have 4.10s so it doesnt take much throttle to accelerate or maintain speed.

Any ideas? I should clean my MAF for sure, but i cant imagine it would be THAT dirty to drop the mpg that much!

Any help is much appreciated.

2000 Mustang GT-HiTech Stage 2 Cams, 4.10s, 31 spline, 3500 Edge, Lowered, 315 DRs, Custom Tuned, Full Exhaust
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post #2 of 5 (permalink) Old 02-26-2013, 07:38 PM
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If your TC isn't locking until you get into OD, that would certainly account for a good portion of your loss.

In my car which weighs a good 400lbs more than yours, with 3.73s and 3500 stall, I still averaged 20-21 MPG city. I lock the converter a few MPH after the 1-2 shift.

Also keep in mind with 4.10s your cruising RPM will be higher and because of that you have higher cylinder vacuum generating more pumping losses. Still running EGR? You can crank the EGR in the tune to max at cruising load conditions to help economy a bit, and you can also shift the O2 bias 3% lean at cruising load/rpm as well for an extra little boost.

If the front O2s are old and have lots of miles, there's a chance they're not switching at true stoich which would throw off your A/F while in closed loop. Also make sure you don't have vacuum leaks. Unmetered air entering the engine requires extra fuel to be added to compensate and will lower MPG.

-Brandon
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Last edited by theterminator93; 02-26-2013 at 07:44 PM.
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post #3 of 5 (permalink) Old 02-26-2013, 09:44 PM Thread Starter
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Those are great mpg numbers! I am still working on getting tuned, but once i do i would love to have lockup come that early. Does lockup that early affect any driveability or performance?

The max MPH i drive in my commute around town is ~50 MPH so i wouldnt think the 4.10s would hurt it at that speed :/

Unfortunately I have long tubes and no egr, so i cant do that! And once again, when i get tuned I will request the o2s be leaned a little as well. The car only has 80,000 miles on it, BUT it is about 13 years old. Thinks its time for new o2s?

2000 Mustang GT-HiTech Stage 2 Cams, 4.10s, 31 spline, 3500 Edge, Lowered, 315 DRs, Custom Tuned, Full Exhaust
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post #4 of 5 (permalink) Old 02-27-2013, 05:36 AM
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It wouldn't hurt any, but the only way to know for sure if they're acting up would be to check their voltages and compare their switching point against a wideband.

Locking the converter after the 1-2 was a bit harsh on the drivetrain at first, but I've sorted it out (mostly). Also locking the converter has a positive impact on performance.

Also, in my car with 3.73s my peak MPG cruising speed is 40 MPH - once I start moving faster than that I start losing MPG. IIRC my "average" instant MPG for 40 MPH was about 28 MPG. If I'm rolling at 50 that figure drops to ~26 MPG. At 60, ~24-25. And at 70, ~23-24. I would think with 4.10s those figures would be more pronounced and shifted downward a few MPH.

-Brandon
97 Laser Red Thunderbird LX 162k, Stage 2 4.6L 2v N/A | 300 BHP (255 RWHP, 290 RWTQ) | 13.95 @ 97.58 | Build details | Pics at the Lorain Assembly plant
98 Black Mark VIII 160k, stock daily driver
07 Redfire Fusion V6 SEL 178k, for the wife
Gone but not forgotten: 96 Mark VIII, 94 Cougar XR7, 93 Mark VIII

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Last edited by theterminator93; 02-27-2013 at 05:51 AM.
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post #5 of 5 (permalink) Old 02-27-2013, 09:45 AM
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Below are the things that are killing the MPG, with the cams (lower vacuum) and tune possibly being rich as the top two. EGR missing is a big one too.

HiTech Stage 2 Cams, 4.10s, 3500 Edge, 315 DRs, Custom Tuned

3500 stall doesnt help, nor does the increased rolling resistance of wider tires. Also, if the car sits outside, that will kill 2 MPG in the winter when all the fuids are thick as molasses starting out.

If it makes you feel better, my inline 6 only gets 12.8, though a new o2 sensor bumped it up closer to 14.

Al

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