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Well I filled up and came home. Drove 142 miles exactly. I went through just about a half tank of gas. Im one notch over half. This doesnt work out too well. Thats a 23 gallon tank. Thats about 11 gallons of gas. So I got 12-14mpg on the highway? I did between 80-85mph with cruise control on and AC running all the way. Is this normal?
Gio
 

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I'm going through a lot of the same stuff on my '93. Seems like low maintenance does it - I'm still cleaning up from the previous owner.

For example, the timing is several degrees off, that'll do it. I need to fix that. The fuel filter was clogged which makes the ECM play games. The air filter was clogged so it was gagging the engine. My best bet says the catalytic converter on my car needs to be replaced, and the injectors need to be cleaned. The ignition is all new but the guy who installed it must not have known you need to set timing. Transmission flush helped my car a great deal as well. I need to change the rear differential soon as well. How long has it been since you changed the O2 sensor?

12-14mpg for the 350TBI is absurdly low. Make sure the routine maintenance is up to spec, though I think it sounds like you have a more mechanical issue such as O2 sensor or timing (or both maybe!)

Oh by the way - the fuel gauge LIES from the factory! Keep track of how much you put in at each point on the scale. For example, 3/4 tank on the gauge means I'm down about 10 gallons and the gauge accelerates from that point as I use fuel! I hear this is intentional GM design to make the people who habitually fill their tanks think they are getting better mileage. Keep an eye on this and only measure mileage by filling the tank.
 

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Well it seems to have been maintained well. Its a low mile (87K) car.
My tranny fluid looks very clean, no burnt smell etc. Maybe I should change it anyway? Oil was changed. Air cleaner looks good. Maybe I need a tune up? Should I change out the fuel filter? I had a mechanic check the car out before I bought it. He found no issues. I dont know much about cars but Im learning. LOL! How can I check the timing? It idles smooth and seems to drive fine.
Thanks for any help.

Gio
 

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The fuel economy numbers posted on car stickers are tested on certain lower speeds, but not 85 MPH, A/C drags more fuel.
 

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^^I dunno where you heard that but 2 times as much fuel for 85 vs 60 seems like a bit much. It could be accurate for a car like a Fury or my 75, something with the aerodynamics of a brick (although my 75 only drops from 12mpg at 65 to 8mpg at 100.)

But the principle is correct, start by slowing down.
 

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With 11 gallons to go 142 miles I would be looking for a gas leak first.

Then address fuel press regulator, plugs, wires, cap/rotor, timing, tire pressure, ensure oil is fresh and correct weight, rear end lube correct weight, front end alignment, etc. These are not in any particular order.

My benchmark is 1976 Olds 98 (note 5000#+ car, aerodynamics of a 2 story building), 2.73 gears, 455 Olds, Q-Jet, THM400, no OD, HR78-15's (P235 70 R15's are fairly close) is 12 city 18 highway. The Delta 88 was same, but the Delta 88 got a 350 std. And was 13 city and 17 highway. so the 455 was the economy engine..... Feels good to say that....
 

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certainly start by slowing down the epa tests are at 55 no more than that because of the speed limit and they usually don't have the a/c on

I would suggest keeping the a/c on because on a recent show of mythbusters above 55 its more beneficial to use a/c than to deal with wind drag or turbulence from the windows being down.

below 55 its better for the windows to be down

What I do though just use the econ setting if its there that works out fine with my tests.

Try at most to go 65 there can be a good difference and try to go at a steady speed.

Culprits of bad mileage can be air filter oxygen sensors and egr valves.

Unless your car has some bad missing issues the above items usually don't give off bad feels but they will show up in low mpg's.

Fill it up with gas the older the car gets the more inaccurate the gauge is there is a good chance the tank is either 2 gallons above ur estimate or even 2 gallons below your estimate.

my gauge is very accurate I was actually surpised with my 94 I have seen that at exactly 3/4 full it is only low 4 gallons or 4 gallons used.After this at about 1/2 tank it's about 9-10 gallons used.

On other cars that are wacky this can vary a lot and even if I filled up on the same area of the gauge the gallons would vary across 4-5 gallons in difference.
 

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My theory is that you see some improvement in slowing down, but honestly, it isn't the speed per se, but the time it takes to get there that is most damaging to mpg.

Say if you drive 75 mph, and you accelerate at 7.5 mph/sec, it will take you 10 seconds to get there. But if you accelerate at 7.5 mph/sec and only go to 65, you get there a little over a second shorter time, and that time is significant in the amount of fuel burned. Same with coasting. If you see a stop sign up ahead, and coast 1 mile to get to it, vs staying at 65 right up to 100 feet from it, you have 5180 feet of no fuel being burned vs nearly a mile of fuel being burned still, just to heat it up in the brakes.

I insist that there is little fuel economy savings at 65 vs 75, as a highway trip would show. If you get up on the highway for say 200 miles, nonstop pm cruise control, at 65 vs 75, I would you would not see much, if any, mpg difference. I speculate that you would see around .2 to .3 mpg difference if you are in hilly or windy terrain, not the 10% or more the politicians and so called self professed experts tell us. Now, if you are driving city/highway, and going from 0-75 often, yes, this will have more significant impact on fuel economy. But constant speed on the highway will not affect it as drastically as we are led to believe.

More depends on how much you are on the loud pedal than not. If you can be gentle (say no less than 9 inches vacuum under acceleration) under acceleration, vs heavy throttle starts to keep up with that Ferrari, you will do much more to improve your mileage. I also allow only the first 20 feet or so in first the plan for more acceleration in second and third. Traffic supporting me driving like this, I don't impede traffic flow, and will not, that ticks me off.

My experience with my 3.42's, mech fan and all, is 70 mph I get 23 mpg. Which is EPA rating for my car on the highway. Even going 75 I don't see much drop if any in my average mpg readings (and you know I track it carefully).

Look at fueleconomy.gov for the garage, you can enter in your own values there, even have it calc it for you and track it. There is only 2 Fleetwoods in there (as of last month), mine and another. I have over 50+ tanks of fuel in there with a solid 19 mpg AVERAGE for ALL driving since Feb 2005. Over 20K miles.

The resolution on fuel economy for less than 1 tank is poor, don't judge it by that. Take several full tank (try to fill to same point in tank) readings and average all the miles divided by the gallons. This is more accurate, vs a 100 mile trip. You will then see tanks like 12 mpg. But likely the next one might be 20. Average the 2, and it will be more representative oif reality.

Fuel economy is tough to characterize accurately. There are a ton of variables. Best we can do is shoot for the highest resolution that we can get, which is long term accurate tracking of conditions and type of fuel used, driving style (note it, if you were throttle happy that day beating that Hemi Ram again) or driving like a grandma, temp, weather, where you got fuel, is it ethanol or not (this DOES has a deterimental effect on mileage, between .4-.8 mpg less AVERAGE, I have a LOT of data to prove this, avoid if possible, and badger your politicians to stop advocating it, it is making a fuel economy loss for all Americans to the tune of millions more gallons of fuel needed a year, just for political gains!). So accurate tracking of these things will give you a better understanding of what affects mileage and what does not.
 

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N0DIH said:
I insist that there is little fuel economy savings at 65 vs 75, as a highway trip would show. If you get up on the highway for say 200 miles, nonstop pm cruise control, at 65 vs 75, I would you would not see much, if any, mpg difference. I speculate that you would see around .2 to .3 mpg difference if you are in hilly or windy terrain, not the 10% or more the politicians and so called self professed experts tell us. Now, if you are driving city/highway, and going from 0-75 often, yes, this will have more significant impact on fuel economy. But constant speed on the highway will not affect it as drastically as we are led to believe.
I would tend to agree with you that mashing it from the stoplight really affects mileage... but my MPG readout disagees with yout 65 vs 75 theory. I've tested it on a very flat road around here using my cruise and I definately saw about 2.7 mpg difference. I drove half way there at 65, then the rest of the way at 75; then on the way back I drove at 65, then 75. So the road and air temp was the same between 65 and 75, the only variable was the speed.

Keep in mind this is my ETC, so it's going to be a bit different in a Fleetwood, but the principle is the same.
 

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do a compression test and check your vacuum with a gauge...u could have a internal problem that makes the ecm poor out more fuel while u are just crusing along....
 
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