2000 Pontiac Grand Prix Bad gas mileage

Tiny
RDWUESTEWALD
  • MEMBER
  • 2000 PONTIAC GRAND PRIX
  • 137,000 MILES
My car gets the gas mileage it should from full to about 3/4ths a tank from 3/4ths to half I get maybe 20 miles and then from half to a quarter I get 50 or so miles, now I've owned another grand prix same year and I was getting 200 miles to a half tank and another 200 to empty, I have replaced the fuel filter spark plugs and wires cleaned the mass airflow sensor there is no check engine light on, about 3000 miles ago I had a new transmission put in, I can't figure out why it gets such bad gas mileage could it be a bad oxygen sensor but its not showing?
Thursday, September 19th, 2013 AT 7:00 PM

7 Replies

Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
How exactly are you figuring gas mileage? Hopefully not with your gas gauge there not always right plus that's not how you figure miles per gallon.
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Thursday, September 19th, 2013 AT 7:11 PM
Tiny
RDWUESTEWALD
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  • 3 POSTS
I drive highway home and drive highway to work, I should be getting 400 plus miles to a tank these cars get 30 miles to a gallon interstate. I shouldn't be on empty when I hit 200 miles right?
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Thursday, September 19th, 2013 AT 7:14 PM
Tiny
SATURNTECH9
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From what your saying it doesn't make any sense to me why you would get worse mileage when your fuel level is lower.I hear what your saying but I have to fix gas gauges that read incorrectly all the time. The only way to truly figure the mileage is to run the tank down reset the trip odometer fill the tank till the pump shuts off. Then drive till the tank is way down then refill the tank and divide the trip odometer miles driven by the gallons of gas it took to refill the tank. That will give you your miles to the gallons. Doing it with the gas gauge is not acturate too many unknowns.
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Thursday, September 19th, 2013 AT 7:57 PM
Tiny
CHRISTINEW0430
  • MEMBER
  • 3 POSTS
Your questions doenst sounds strange at all to me, I have a 2003 pontiac grand prix gt, and mine does the same thing, full till 3/4 I get great mileage, and as soon as it hits 3/4 its just like it sucks it down at a rapid rate, I drive 16 miles to work one way, so from 3/4 by the time I get to work its halfway to a half a tank, and by the time I come home 16 miles ill have a half a tank. That at all does not seem right to me, I shouldnt not be burning that much gas driving 16 miles. When I bought the car it had 105k on it, it now has 124k, and I noticed this started to happen around 122k
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Friday, October 25th, 2013 AT 8:37 AM
Tiny
SATURNTECH9
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Again we can figure gas mileage from what are gas gauges read we don't know how many gallons used to divide by miles driven. Also the gas in the tank destroys the fuel sending units so they go bad all the time common problem.V
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Friday, October 25th, 2013 AT 9:39 AM
Tiny
RDWUESTEWALD
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  • 3 POSTS
Actually Saturn the issue is the top of the tank where its welded around the fuel pump rusted away so when I fill up the gas tank if pours out when too full and a bad head gasket possibly
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Saturday, October 26th, 2013 AT 6:58 PM
Tiny
SATURNTECH9
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If your leaking gas that would explain some bad fuel mileage mikeage. Why ddo you think your head gasketis bad?
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Saturday, October 26th, 2013 AT 7:11 PM

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