I recently purchased my used car, why is my car so bad on gas


Asked by Feb 08, 2007 at 05:18 AM about the 1998 Pontiac Bonneville 4 Dr SSEi Supercharged Sedan

Question type: Maintenance & Repair

16 Answers


Hello, There may be a number of reasons for this. First, Bonnevilles are not the best on gas. They average 22mpg on combined city and highway, according to fueleconomy.gov. Next, it's an older car, so the other owners may not have taken great care of it. I would recommend getting a tune-up. If you can't do that, then fill your tank with high-octane gas to clear out the lines. I hope this helps! CarGrrl

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Best Answer

Well what is your definition of bad on gas? You should get about 18-22 in the city depending on your driving style, i get about 20, and you should get about 26-29 on the highway, I get about 27.

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very important that you use premium fuel. The computer retards the timing if it detects premature detonation that is caused by using regular unleaded in a supercharged engine. I owned a gtp with the same engine and averaged 18-20 around town. The Bonneville is a heavier car so I would expect closer to 18. I used regular unleded gas in my gtp once by accident (wife put gas in), and I got about 8 miles to the gallon.


Yeah, idk if you have the manual with it, but the SE takes regular and the SSEi requires 91octane or higher...


Cause its a big s'charged V8. What did you expect?

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tune up and keep your foot out of it

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it's a v6, and should get 25 mpg (I have the same engine in a 97 grand prix)


Ha, my assumption about american cars with V8's overtook my willingness to think before i speak


using regular gas in it? that will make the computer go nutty when that big ass supercharger winds up try using premium or getting a car that isnt a supercharged v8


1. its supercharged 2.its a V8 3.its supercharged. u gotta use premium gas too. using regular on a supercharged or turbocharged car can decrease mpg..


Another thing you may want to try is a K and N air filter. I put one in my Jeep Liberty and with a fresh oil change, I noticed a significant change in gas mileage.


If you are using your a/c it will rob your gas mileage also.


Swap a turbo on and keep out of boost should solve half of the problem..........but the pricing? i dunno.......


because its a piece of garbage.

I have a 2001 Bonneville supercharged and you can simply have it running for a split second and the Overflow container boil what could cause that

Phantom1213, are you sure it is boiling or are you getting bubbling from a blown head gasket?. It's really impossible for an engine to over heat that fast. Here is a bit of info about nearly every 3800 liter V6 ever built since about the mid 90's. The factory failed to do a proper job of engineering the front exhaust manifold on these engines. Number 5 cyl. is subjected to excess heat from back pressure on account of this issue. The head can crack between the valve seats and also you can loose seal of the head gasket from the heat. In the most serious cases these engines will stick the piston in the cyl. and cause it to break which will completely destroy the engine block ! The best way to address this problem is by installing what is referred to as a power log manifold made by a company called ZZPerformance. It eliminates the back pressure on number 5 cyl, if and only you have taken care of the problem of compression leaks into the radiator if this is what is causing the over flow tank to bubble over.

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