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Why is my gas mileage so bad?


duke_winchester
12-02-2010, 07:25 PM
I have a 97 Grand Prix GTP with about 166,000 miles on it. Runs well, but do need to change out the supercharger coupler as I have the characteristic rattle coming from it. My concern lately is that my mileage seem to be pretty bad. I used to get above 30 mpg easily on the highway doing 80 mph and pretty well above 20 city, but now i am getting less than 18 city and well below 30 highway. I keep my tire pressures good, run a K and N filter, keep up maintenance (synthetic oil, platinum spark plugs, etc.), yet can't find a reason my gas mileage has markedly decreased. Is it just cuz the car has so many miles? Could the coupler have anything to do with it? Gas at 3.15 a gallon here in Florida for premium is killing me.

BNaylor
12-03-2010, 05:32 AM
Any mechanical issues like the noisy SC coupler could affect your fuel economy so best to get it repaired.

What condition are your tires in? How much less than 18 city and well below 30 highway?

tblake
12-04-2010, 09:49 AM
What are your RPM's after the car has warmed up and cruising at 60mph?

Not sure if the 4t60e has issues with the OD clutch shaft splines like the 4t65e does, but it may be something to check.

Also it could just be due to winter gas. I only get about 300 miles per tank right now. Due to the winter grade gas coupled with warm up times. I used to be able to get about 350 per tank.

BNaylor
12-04-2010, 11:40 AM
Not sure if the 4t60e has issues with the OD clutch shaft splines like the 4t65e does, but it may be something to check.



Tim,

The '97 GTP has a 4T65E-HD autotransaxle.

tblake
12-05-2010, 09:51 AM
Oh, ok, thanks for the heads up. So it was the 97 se's and gt's that had the 4t60e?

grandprixgtx00
12-05-2010, 12:13 PM
So it was the 97 se's and gt's that had the 4t60e?

early model 97's got the 60, late model 97's got the 65. My 97 SE had the 4t65

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