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Old 09-16-2011, 04:39 PM   #1
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I was just wondering if soeone could give me an idea for increasing my highway mpg. I commute with my 88 bonne 120 miles everyday and the slowest high way speed is 65. But a fair amount of the way it is 75. And i generally do 80 then, however my car is doing something like 2900 rpm in overdrive at that speedso my mileage is like 7 mpg less than it should be, according to what the reviews say. I wasnt sure if i could do different gear ratios or a different connection from tranny to the wheels. Sorry i dont know much about how the power is sent to the front wheels in a fwd vehical.

Any suggestions are welcome.
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Old 09-16-2011, 04:53 PM   #2
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Slow down to 70. Increase your tire pressures, and make sure they are checked regularly. Make sure routine maintenance is done on a regular basis (oil changes, spark plugs, air filter, etc.).
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Old 09-16-2011, 06:06 PM   #3
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cruise control is your friend... on one of my trips up to chicago i made an effort to not use it on an entire tank, and my mpg suffered by about 8mpg. my theory is we subconsciously speed up going down hills and lay into the throttle more than whats needed going uphill.
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Old 09-16-2011, 06:46 PM   #4
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Couple of concerns here. First off, the RPMs are not what I would expect them to be. Even with a 3.33 FDR, you would not be turning more than 2400rpm at 80mph. GM gauges are not known for their accuracy, so it is possible it may be off, but if it is close, your torque converter may not be locking. Easiest way I can tell mine is locking is by maintaining a steady speed on the highway with throttle only, and while applying steady pressure on the throttle, slightly begin to drag the brakes a little bit. This should disengage the lockup, and you should notice the RPMs jump some, when you release the brake, it should settle back down as long as you have light to moderate throttle applied.

You will also want to check the other standard stuff as well, make sure you have a good tune up, oxygen sensors included (biggy there), and even some mods can do you some good as well (high flow cat, Cold Air Intake, etc)
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Old 09-16-2011, 06:53 PM   #5
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I was just crunching some numbers. Assuming 205/70r15 tires, and 2.84 FDR, 3rd gear at 80mph would be 2903rpm, exactly where you say it is running. Are you running the car in Overdrive? How is your transmission fluid looking? This could be transmission related as well.
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Old 09-16-2011, 06:59 PM   #6
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Yes i am running in overdrive. And i know for a fact if shifts into overdrive. But i noticed about 2 months ago that my transmission fluid was kinda dirty. I was gOnna chande it about a month ago, but then i broke my femur...so not really easy for me to work on it at the moment.
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Old 09-16-2011, 07:03 PM   #7
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Also i noticed awhile back that when i first start out for the day my car does a stuttering kind of thing. It'* like something is slipping but once i get through like 5 stoplights that stops and i dont have a problem after that. So could that be a hint that my transmission is going? Its got 154xxx miles on it.
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Old 09-16-2011, 07:32 PM   #8
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If somebody told me the transmission blew up after 23 years and 154k miles, I would say that it hung in there pretty well. I would get the fluid and filter changed, and hope for the best. How do you know for sure it is shifting in to overdrive? 2900rpm is a lot at cruise.
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Old 09-17-2011, 02:19 AM   #9
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Quote:
Originally Posted by rjolly87 View Post
If somebody told me the transmission blew up after 23 years and 154k miles, I would say that it hung in there pretty well. I would get the fluid and filter changed, and hope for the best. How do you know for sure it is shifting in to overdrive? 2900rpm is a lot at cruise.
My transmission'* up to about 140,000 miles, and it had the same problem when the fluid and filter were long overdue for a change. Get that done when you can, and your problems may go away.

Other than that, the front oxygen sensor is a biggie. I changed mine recently and got an extra forty miles per tank.

I can't speak for the '88, but 2900 RPM sounds close to what I have on the highway in my '97. GM gauges aren't known for accuracy, so bring a passenger with a scan tool if you can to get the true numbers.
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Old 09-17-2011, 02:39 AM   #10
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Alright thanks guys. Ill change my fluid and filter as soon as i can and just see what
Happens.
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