Why is my fuel economy poor? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 12-09-2009, 04:46 PM   #1
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Default Why is my fuel economy poor?

this has been bothering me for a long time now, but ever since I got this car the fuel economy has left much to be desired. on average i get around 21-23mpg, doing probably 80-90% highway driving. When I first got it, i put plugs, wires, crank sensor, fuel filter and a o2 sensor on it, NGK plugs, delco wires, delco o2.sensor.

the car runs just fine, I don't romp on it everywhere usually 70mph on the interstate.synthetic oil changes every 5k, regular air filter changes.

To be honest it is probably due for a tune up, it has been 60,000 miles and 2 years since I changed anything on it. But IMHO it should be fine right now, as this average fuel economy has stayed the same the entire time. to date the BEST fuel economy I ever got from it was 26mpg on a drive to Toledo, OH with the cruise set around 70 the whole trip.

I believe that the car has the 3.06 gear ratio. I don't know what I should expect out of it, but I think it is capable of more then 23mpg.
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Old 12-09-2009, 04:53 PM   #2
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Put some injector cleaner in, check for vacuum leaks, check the operation of the TCC solenoid switch, check for a plugged cat. The mileage your getting is not far out of norm. One thing I found that killed my mileage was that really great 10% ethinal fuel. I dropped a few miles per gal.
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Old 12-09-2009, 06:51 PM   #3
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Originally Posted by White93z34 View Post
I believe that the car has the 3.06 gear ratio. I don't know what I should expect out of it, but I think it is capable of more then 23mpg.
You have to remember that the winter blend gas is out now and with the colder temps your not really going to see much more than what your getting now....

Now if this was May or June with the better blend gas than yes you should be seeing higher than 23 more like 26-29 if you drive her nice...

My wife and I have gotten up to 33 on a long distance road trip driving in summer

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Old 12-10-2009, 12:26 AM   #4
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TCC defiantly works. I don't think it is the season affecting it, but this is what the car has gotten on average for around 2 years now. To date I've never seen higher then 26mpg ever in this car.

I guess the car is still whiten its EPA ratings of 17/26mpg with a average of 20mpg. I guess I'd just been spoiled of the regular 25mpg-while-romping on it that I regularly pull out of my z34 which has worse EPA standards then the Bonneville that I drive nice does.
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Old 12-10-2009, 08:10 PM   #5
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my bonne gets about 19mpg but i tromp on it a lot. my dads 05 lesabre gets like 23mpg average driving regurlarley, if i really really babby it i can get about 33 average, thats no hard acceleration and driving like 65 while drafting behind a semi truck.
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Old 12-10-2009, 09:13 PM   #6
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my bonne gets about 19mpg but i tromp on it a lot. my dads 05 lesabre gets like 23mpg average driving regurlarley, if i really really babby it i can get about 33 average, thats no hard acceleration and driving like 65 while drafting behind a semi truck.
ya know us truckers hate when ya do that
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Old 12-12-2009, 06:55 PM   #7
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Did you change the air filter and does the mass airflow hose fit tight?
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Old 12-12-2009, 07:20 PM   #8
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That reminds me.. I haven't put gas in the car for over 2 months, I'm still running summer blend. Wonder if that'* why it was having a rough time..
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Old 12-12-2009, 08:07 PM   #9
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ya know us truckers hate when ya do that
X2! Nothing like having some yahoo playing peekaboo in your mirrors.

Anyhoo. Seems like you covered everything else except for tire pressure. What are your tires pumped up to?
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