Gas mileage help

[brtnstrns]

So I've had my 99 GT for about 3 or so weeks now and am on my second tank of gas. My gas mileage on my first tank of gas was pretty poor but I assumed it was because I was still getting used to the clutch and was burning off a bit more gas. Now I'm on my second tank and I'm at half a tank of gas and have only gone 70 miles. I know that I've gotten used to the clutch now so I don't think thats it. Granted, all of this driving has been local driving and not much highway time at all since I typically drive from my apartment to campus and around the area (~3-6 mile trips) so its pretty much all stop and go driving right now. I know these aren't fuel efficient cars but 70 miles to half a tank of gas seems pretty sketchy to me, but I'm not sure since this is my first GT. If this seems like it could be normal, thats fine (let me know), but I would assume I could at least go ~100 miles on half a tank even if its city driving. Since I've had the car only a few weeks I haven't checked out anything that could be affecting the fuel efficiency so if anyone can clue me in on what to look at that would be awesome. Otherwise I'm either still bad using the clutch or the car just straight up gets bad mileage. Anyway, thanks in advance for any help.

 

[deftsound]

Something is definitely wrong. If your car is stock and your not driving the piss out of it you should be seeing upwards of 250 miles easy. I used to get atleast 275-325 miles from a tank, actually they are pretty fuel efficient little cars considering they are v8's.

First thing I would do is give it a good tune-up. Replace the fuel filter, spark plugs, air filter, run some sea-foam through it...im sure im forgetting a couple things but you get the idea.

But yea, you should be getting much better mileage than that...

 

[ChillPhatCat]

There's no way to determine what is wrong until we actually know there's a problem. You can't just look at the gas guage and say... well it looks like I'm burning a lot of fuel here, especially with a new to you car. Fill the tank up, drive about 100 miles, then fill it up again and divide the number of miles by the number of gallons, then get back to us. I've seen as low as 16 mpg and as high as 27 mpg with my car.

 

[patman0911]

It's possible to achieve some truly atrocious gas mileage if your only driving a couple of miles around campus at low speeds and constant stop and go.

You'll get better mpg data if you run through several complete tanks but if you're only driving a few miles a day, you may not have the patience for that.

My first few tanks were pretty bad too - too hard to resist flooring it at every opportunity

Check the basic tune up stuff - plugs & filters - as suggested above and keep track of your MPG for a while and see where you're at.

 

[stangGT97]

Does your car feel like it has a miss? Smells rich? A good tune up usually solves poor gas mileage; if you use seafoam, just be sure to run it through the system BEFORE you change your spark plugs. I fouled 2 plugs when I used it last time.

 

[Dragstr05]

Honestly, you can change EVERYTHING - plugs, fuel filter, etc. If you see a difference, it wont be much.

You're driving really short trips, and I mean REALLY short. You are practically ONLY driving the car in open loop. It's probably hitting closed loop by the time you get there, but driving three miles and then parking it for a couple hours or so will kill fuel economy if thats the only way you drive it.

Straight city driving, MPG will suck. Driving straight city I'll see 12 - 15mpg, the more highway I do the better it gets. I've gotten well in the 20's on highway duty, but thats highway. Mine has never been great in city driving.

When I used to driving nothing but stop and go, I'd see around 150mi / tank. You arent far off. Throw money at it if you'd like, but you arent going to save millions at the pump doing so.

 

[40oz]

^ what he said.

Highway I get ~26-28 depending on speed and kind of highway (2 lane = lower speed = better mileage).

The kind of city driving you're talking about, I get crap for mileage, exactly how bad depending on how I drive. I don't worry about it because I'd spend the money on beer if I wasn't spending it on gas.

 

[trinity_gt]

Now I'm on my second tank and I'm at half a tank of gas and have only gone 70 miles. I know that I've gotten used to the clutch now so I don't think thats it. Granted, all of this driving has been local driving and not much highway time at all since I typically drive from my apartment to campus and around the area (~3-6 mile trips) so its pretty much all stop and go driving right now. I know these aren't fuel efficient cars but 70 miles to half a tank of gas seems pretty sketchy to me...

I tend to agree with the others that are saying the super-short trips are probably hurting the fuel economy. A cold engine means enrichment plus a good portion of the run time being open-loop. Having said that, your economy does seem a little on the low side though a lot depends on how the car is driven. You may want to check for dragging brakes (especially rears because of the parking brake...) which can really hose gas use.

Why not take the car for a quick hundred mile highway run to get a better idea of what it'll do?

Keep in mind that the Mustang's fuel tank capacity is only 13-gallons. If you averaged 12MPG -- not out of the realm of possibility given the type of driving you do -- you'd only see 156 miles before running the tank dry.

 

[Ateup]

I/m on my fourth tank of gas with only city driving. No mods other than a KN air filter.

1st tank-19.7 mpg
2nd--17.9
3rd--20.2

 

[40th GT]

Keep in mind that the Mustang's fuel tank capacity is only 13-gallons. If you averaged 12MPG -- not out of the realm of possibility given the type of driving you do -- you'd only see 156 miles before running the tank dry.

The owner's manual and edmunds.com indicate the capacity is 15.7 gallons.
I wouldn't try to verify that with my car in reality, just saying.

 

[trinity_gt]

The owner's manual and edmunds.com indicate the capacity is 15.7 gallons.
I wouldn't try to verify that with my car in reality, just saying.

The 2003 Ford brochure I have in front of me shows the capacity as "Fuel tank litres (Imp. gal.) 59 (13.0)". Note that this in Canada and that they're using Imperial gallons. 13 Imp gallons is ~15.6 US gallons.

 

[wmburns]

It would help if the mileage is reported in Miles per gallon instead of miles per tank.

+1 on the size of the Mustang fuel tank. Just over 15 US gallons.

I have two Mustangs (GT auto vert, GT manual coupe). Almost completely stock. I consistantly get 18.5-20 MPG in the city (when I stay out of it). If traffic is exceptionally heavy, the mileage may drop into the 18's to very high 17's.

On long highway trips, my mileage is about 22MPG. The crusing speed doesn't seam to make any real difference (70, 80, or 90).

The manual does get slightly better mileage than the automatic.

If you want to get better mileage, check the air in the tires, keep you car in good repair, watch the RPM's. You would be amazed how much the mileage will improve if the RPM's are kept below 2100. Another trick is to coast up to red lights (don't power till the last and brake hard).

With careful driving, Mustangs can get OK gas mileage. However, it is true what they say. "YMMV".

The other option is to get a beater 4 cylinder. You will get great gas mileage but you may be bored to death

 

[trinity_gt]

With careful driving, Mustangs can get OK gas mileage. However, it is true what they say. "YMMV".

Except ones with superchargers and fat tunes.

I'm usually nearing 90-100 miles by half-tank on mine but also have a hard time keeping my foot out of it. On the highway on cruise control it seems to get about 23MPG.

The other option is to get a beater 4 cylinder. You will get great gas mileage but you may be bored to death

My "other" car is a VW GTI with a 1.8T engine. Although it's no blown MGT, it's surprisingly quick and quite a bit of fun. The car's tiny OE turbo doesn't have highway legs but it spools very quickly around town making it a very satisfying driver, believe it or not.

Not all 4-bangers have to be looked upon as boring slowpokes.

 

[brtnstrns]

Thanks for all the input. I've checked all of these things and I try my best to keep the RPMs below 2000 or so when shifting, unless I need to accelerate quickly for anything. I think my main concern is how long it takes for the engine to rev down. It seems to hang quickly and then VERY slowly rev down, which I haven't noticed on any other car I've driven (I was a valet so I've driven a lot). I mentioned this before on a thread and people said its normal for mustangs but it seems to take much much longer than other cars which in my mind would mean its probably wasting some gas doing that. Anyone have any ideas on what that could be or if it really is just how the mustang engines are built. Thanks

 

[wmburns]

RPM rev down.

The rev down issue is how the car is designed. Every Mustang does it. There is a kit available that changes the orifice size in the IAC. This will reduce the time it takes for the RPM's to return to idle. Start with an ebay search.

"MUSTANG 4.6 GT COBRA MACH1 IAC RESTRICTOR PLATE"

Bottom line, there isn't a problem per se with your car. They all do that from the factory.

 

[Dragstr05]

If you want to get better mileage, check the air in the tires, keep you car in good repair, watch the RPM's. You would be amazed how much the mileage will improve if the RPM's are kept below 2100. Another trick is to coast up to red lights (don't power till the last and brake hard).

More than likely, its not his driving per se thats killing him, its his trips. The OP clearly states on his first post that his daily trips are 3-6mi. If you drive 5mi and shut it off for 4 hours and drive another 5mi, your fuel economy is going to suck. His mileage would be far better if he drove 10mi per trip vs 3-6. The colder the coolant is, the more fuel that is added. Plus, its in open loop. Driving your whole tank in open loop will not be good for fuel economy.

 

[wmburns]

How long does a car run in "open loop" before switching to "closed loop". I am confused as every time I have used my ODB2 monitor, the PCM switches to "closed loop" within a few seconds after start up.

I fully agree that the short trips can be hard on mileage.

I also agree that 4V and boosted applications would be hard pressed to get really great gas mileage.

All of my comments were directed to stock GT configurations.

 

[deftsound]

Thanks for all the input. I've checked all of these things and I try my best to keep the RPMs below 2000 or so when shifting, unless I need to accelerate quickly for anything. I think my main concern is how long it takes for the engine to rev down. It seems to hang quickly and then VERY slowly rev down, which I haven't noticed on any other car I've driven (I was a valet so I've driven a lot). I mentioned this before on a thread and people said its normal for mustangs but it seems to take much much longer than other cars which in my mind would mean its probably wasting some gas doing that. Anyone have any ideas on what that could be or if it really is just how the mustang engines are built. Thanks

That could have to do with a few things. What i would do is clean your Throttle body real well (it could be sticking) and clean your MAF sensor and see if that helps.

 

[brtnstrns]

Well I got my car inspected yesterday and it turns out the gas cap wasn't creating a good seal and wasn't pressurizing. I guy said that gas could have been vaporizing from it so I got a new gas cap. So far it seems like that may have been the reason for the seemingly bad gas mileage. Since I got the new gas cap I'm not going through an 1/8th of a tank of gas every day I drive and I'm driving the same routes I usually do so I think that may have been the problem.

 

[wmburns]

Glad to hear.

The problem I am having with this post is I can't really understand how good/bad your mileage really is/was. 1/8 per commute means nothing to me.

Can you post in miles per gallon? Reset the trip odometer with each fill up (will make figuring miles easier).

Also, can you recap all of the items you checked and what was found?

OBTW; how are you dealing with the snow forcasted in the Texas area for Friday?