I bought a 91 firebird with the 3.1 liter figuring it would be decent on gas as a daily driver, but now, I'm getting about 12 mpg. I was getting around 18 mpg in the summer, and it has slipped since. In that time I've replaced plugs and wires, cap & rotor, air filter, PCV valve, cat. converter, and EGR valve. I don't have any trouble codes. It has always run smoothly, I replaced the converter because it wouldn't pass emissions. I know here in WI you always lose a couple MPGs with winter formula gas, but this is rediculous. Just about anything will get me 12 mpg. Any ideas? What do other people get with the 3.1 liter?

 

TWELVE MPG???? yikes, something is seriously wrong. Even 18 is incredibly poor.

My 355 with a holley 750 double pumper and mech secondaries, gets around 11-13 depedning how spirited im feeling. It basically dumps fuel down the engine's throat.

Is it auto or manual?

there must be something your not telling us, like are you hauling a trailer with heavy weight? Or do you drive at WOT all the time? Or did someone swap in a 456 rear gear? or....?

My Lumina Van, which is probably heavier than a camaro or a firebird with a 3.4 and aod, gets about 26-28 MPG with 50/50 highway, and stop and go city driving.

You should be getting close to 30.

Tell us about your transmission, i think it may be the culprit. Also, how did you calculate this 12 MPG?

 

my 305 gets better than that.... with a 3.42 rear....

 

My mostly stock 305 gets anywhere from 20-25 city driving. ANd I beat on it sometimes! I just like hearing it rumble.

 

I got high teens in my 3.1 and 3.4 summer driving(9 months outta the year).

Check your tire pressure.
If you let it idle to warm up, that will kill it.
O2 sensor
watch how you drive it.
If its an automatic, possibly change the fluid n filter.

Otherwise, thats the best your gonna do. They didnt get any better mpg then a v8.

 

High teens again? Yikes, i find that hard to beleive.... That 26-28 MPG i quoted is half half traffic and highway during winter. All highway, my lumina would get a little over 30.

Stock 305s get mid 20s, like said above. I dont think high teens is something to settle with.

Changing the fluid and filter wont accomplish anything if the transmission is slipping, or if the converter is incorrect or failing. Or if you have a burnt clucth in a manual transmission. Tire pressures wont make much of a difference, even on long trips, it has to really add up to make a difference in your wallet.

Were not talking 1 or 2 MPG difference, were talking like 15+....

Ive never owned a 3.1 camaro, so maybe im missing something, but even when gm makes a new engine, they dont get it THAT wrong... I still think theres a serious problem, but we need more info.

 

What about the O2 sensor?? Also inflated tires and clean oil helps but not gonna help THAT much.

 

Trust me, everything but inside the motor(190k) was redone with new or tested parts. It got high teens all in town. Pulled high 20's all interstate. This is in a hard top, v6, with the only options being an auto transmission.

3.4 motor(68k), when it was running right got about the same. But then my foot was in it more as well. I swapped to a 5sp at close to same time. I started doing weight reduction mods tell the motor acted up.

my 350 TBI(vert, full options) got approx high 20's on driving it home(was getting slightly less then my gtp which was 28/29.). In town it was getting about 18. Now that Ive swapped over to TPI and still dont have my programing right, I dont know, but its worse(dont have speedo yet).

I didnt buy these cars for MPG, anyone that does is crazy IMO.

Your Lumina is a complete different motor, in a complete different drivetrain, with a complete different ecm/ignition. The only thing thats the same is the block and crank. Heads, intake, cam, pistons, trans, drivetrain, all different.

 

My 3.1L is getting about 15MPG as well, but I'm suspect of the o2 sensor being DOA.

Of course that was with a fuel leak at the fuel rail as well

 

I had fuel rail leak problems with my oldsmobile, the ****ing thing caught on fire!! Get some new o-rings for that!! And put a light coat of vaseline on em first too.

 

what!!!! i get like 12-15 in my 305 tbi...is yours tpi or something or does mine just run like s#!%. that seems really high to me!!!

 

As I posted above, my 350 tbi got high teens in town. 12-15 is bad unless your highly modded and beat the **** outta it.

 

Agreed. If you've got a 305 and are not averaging above 20 mpg-something's wrong, especially with a manual. I average 22-24 depending on the season (winter warm-ups and hot summer A/C days hurt) in my LG4/auto, a TBI or TPI should do even better.

A 94 camaro with the 3.4 averaged me 26-28 for the two years I drove it, T-5 trans.

Put a few $ into finding and fixing the problem. Imagine how quick it'll pay for itself with near double the mileage....

 

i always got around 18 in normal driving conditions with my 305tbi. Prolly dropped a little since i pulled the 2.73 and put in 3.27's (havent changed speedo gear though so cant be sure). Best i ever pulled was 30, but that was doin 55 on all highway and pissing off semi drivers b/c i drafted so close.

Like someone else said, if you let the car warm up before you drive it that'll kill your milage. Engine running but car not moving = 0 miles per gallon.

Otherwise check for leaks, make sure you're not driving around town in first gear, neighbors siphoning your tank, etc.

 

After your rear end change you can still check your mileage by multiplying your odo reading by a correction factor of 2.73/3.27. Same correction will work for your mph readings. Registering 60 mph, actually going 60 x 2.73/3.27 = 50.9 mph. 'Course that degree of accuracy in the calc probably exceeds the tolerance in the measurement, so say: 50-52 mph.

 

Nope it's just a regular TBI 305. It runs great. I almost feel bad about retiring it soon.

 

It's got an auto transmission, shifts fine, engine is MPFI. Tire pressure is fine, and everything is stock. 115k on the car. No differential gear replacement or anything, just a plain coupe. I don't drive it hard. I've checked the mileage every tank of gas for the past few years no matter what I've driven. I got 23 mpg with a 96 bonneville (3800 series II), I get 16 mpg with a 305 4 barrel in my camaro driving it mostly WOT, which is what I would expect. Maybe I should go back to driving a ford escort wagon that got me 30 mpg, j/k. Maybe I should try an oxygen sensor. If it were a leak on the fuel rail, I think that it would be fairly obvious. When I bought this firebird, I figured I'd get around twice the mileage I'm getting now.

 

I have owned my 92 Camaro for seven years and average 18 - 19 mpg city and 30 -32 hwy. I doubt it was any better when it was new.

 

Yep, city mileage isn't great with these engines. 12mpg is ridiculous though, you should be getting around 20.

 

Yep, something is screwed up. My 2.8 with 194000 got 28mpg -33mpg on the freeway. My current 3.4 with the 2.8 ecm (that probably isn't adjusting enough) gets 25-26 avg.

Alignment?
incorrect temp or stuck t-stat?
vac leak?
fuel leak somewhere that it's harder to smell?
temp sensor (the one that goes to the ECM)

What rpms are you driving at while in overdrive? (TC lockup)
Is it going into closed loop mode?

There are a few other things I can think of, but I think they'd cause rougher running conditions.

 

wow......My 305 has better mpg than that.....maybe you have a clogged air filter....even then it shouldn't drop it THAT much

 

My 2.8 right now is averaging ~18 but its on its way out for a lightly built 3.4. I do 50/50 driving at like 1/2 throttle when accelerating.

 

my 92 firebird 3.1 gets 21-22 city highway driving mix

 

Note to self..... dont buy a v6 auto for fuel mileage .....

Geeze i had no idea. But if it will average basically what my 355 gets, whats the point?

My 94 saturn wagon with a burnt clutch gets 28-33 mpg on the same 50/50 route. Get one of those.

 

Already taken care of

For me, I didn't buy my car for gas mileage. I bought it because it was a good looking car, very clean, 5 speed manual, and an excellent candidate for an engine swap in the future.

 

I have a 2.8 and I am getting with a ruogh figure around 18mpg. I filled it up and got about 220 miles to a tank. I have an exhaust leak and and an ses light that says the EGR is bad, I figure that is some of the cause.

 

Lets not forget that some of us have nearly 20 y/o motors that may not have been very well taken care of in the past like I suspect mine wasn't. New motor is in the works now and I hope to get better mileage.

 

im just wondering y but y would letting your car idle warm up kill your milag. letting your car idle while warming up dosnt mean nething when your driving it. maybe your injectors are bad. maybe MAP sensor is bad. Bad air filter.

 

^uhhh...Gas mileage is how many miles you go on a tank. Not how much gas you use your drive. Your using up gas no matter how you look at it.

 

i dont think so...

By that logic if you upgraded your 40 L tank to an 80L tank, you have now doubled your gas mileage??? But how come your wallet still hurts?

EDIT: I hope i understood this correctly, but mileage has nothing to do with tank size. its miles PER galon, or km PER liter. (my apologies if i misunderstood you)

Also... warming up a car has alot to do with gas mileage. The most accurate way without buying expensive instrumentation is, either note exactly how much you put into the tank (volume of gas), and see how many miles or kms you can get out of it. However this isnt accurate because you dont usually start off with a completely empty tank.

So a better way is to fill up the tank, figure out how much your tank holds. So if you filled up 69L, you prolly have a 70L tank... etc.. AND divide how many miles/km you get by the number of galons/liters you filled for a ratio.

That ratio is the mileage.

Now back to warming up, if you use 8% of this gas to warm up your car by idling, per tank, you now have 8% (roughly) error in your ratio. (theres a complicated equation to figure out the exact error, but its not relevant).

Another thing is, in the winter, you may be under the impression that your fuel mileage is roughly the same, regardless of idling, but we have to take another thing into account.

Next time you fill up, note the "correction" value at the pump. In canada, the volume of fuel being pumped into your tank is corrected to 15*celcius.

Fluid expands when hot, and compresses when cold. In the winter, the fuel is colder, so its compressed, more dense, and the same "moles" of fuel on a hotter day, would now occupy a smaller space.

In short, you tank will hold more gas in the winter than in the summer. By the time it gets to the engine, its warmed up, and corrected by the computer to use roughly the same amount on fuel injected vehicles.

So you may be using more gas by idling, but you are also pumping more, giving you the illusion your mileage is roughly the same.

SO warming up has alot to do with it.

Ofcourse, even if you dont idle your car, MPG or mileage is a dynamic calculation, because our driving habits are dynamic. You cant just pick ONE number, and say thats your MPG. Its usually an average of what your getting. Thats why the newer cars will tell you an MPG reading, like the 07 mustang, and it changes depending on how your driving, or road conditions. So there is an error involved with any MPG figure.

 

^What I meant was that letting your car idle hurts your avg. miles you go on a tank. It doesn't affect your gas mileag but how many miles you can get out of a tank since you are not using it to travel. With how the average joe calculates his gas mileage it lowers it (avg gas used for miles traveled). I usual calculate mine this way every once in a while. It was a little vague of what I meant but I see what you are saying.

 

Because your burning gas, even at an idle. Every little bit of gas you use idling is that much less you have to get your car moving.

 

I just top the tank off, reset the odometer, and drive like usual until I'm down to a half or a quarter or whatever.

Pull up to the pump, note how many miles I drove, top tank off again, note how many gallons I put in, and divide the miles by the gallons.

350miles/13gallons= about 27mpg

I think sometimes even the brand of fuel you use will affect your mileage. It does for me anyway.

I suppose if you really get into it, you can say you get better mileage driving south, since the centrifigual force of the of Earth rotating makes things lighter at the equator, and you're being pulled south ever so slightly. I guess.

 

Just ask californians, fuel quality has a huge effect on mileage. I've heard the crappy oxygenated fuel they get cuts their mileage ~20%.

 

^I've heard that too. Alot of things affect gas mileage.