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I just got this deville with 125K on it; I always thought that checking your fuel economy was a good way to tell how the car is running. Anyway, I live in Buffalo, and its in the 30's right now so its cold. I do mostly city driving, with an occasional 5 mile or so highway stint, but Id say 90 percent of my driving is city. Anyway, my computer said 17.2 mpg, but its wrong, its gotta be. I did calculated myself by how much gas I had to take to refill as opposed to how far I drove and it took 151 miles on 10.8 gallons (about 14 mpg). Is this right? Remmebr, my computer one said 17.2 for those of you who go by that; I just want to make sure shes running [email protected]!
 

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It's colder here (0 F) and I'm getting 18-20mpg city. Summer, light-footed I get 20-25 city, highway upwards to 35mpg, depending on how many hills and the wind. I would say your computer could be out, but then I don't know if higher mileage vehicles get less mpg. What's your tune-up situation?
 

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Discussion Starter #3
are you going by the computer or your own calculations, because my computer says I get 17.2. I figured 14. The car runs very smooth, but I am planning on getting new spark plugs and wires soon. Anyone else?
 

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RAMAIR57 said:
are you going by the computer or your own calculations, because my computer says I get 17.2. I figured 14. The car runs very smooth, but I am planning on getting new spark plugs and wires soon. Anyone else?
So far, that's just what the computer is saying, I haven't worked it out manually yet. I assume it's correct because my 1991 4000 pound Grand Marquis 5.0 got 18-20 city and 32 highway. Not bad for a big boat with new Bosche plats, Accel wires, dist., new fuel filter, etc. There was another thread that discussed the Caddy computers could be set from the factory to "overoptimistic." Either way, these 4.9's are awsome on gas mileage. My buddy just bought a 1999 Deville with 70,000 kms, and he could not believe he got 32 mpg on the trip to B.C.
 

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1995 Sedan De Ville (4.9 L)
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I posted the similar thread on MPG somewhere. I do experience
the same probem. My computer is also optimistic, I don't know why.

What I thought is low fuel pressure. But I don't know how the computer
calculate the MPG exactly, I can't be really sure about it. I thought
the computer calculates MPG from injectors open time by assuming
the fuel pressure is right. My thought is if the pressure is low, then
the injectors inject lesser amount of fuel that that with right fuel
pressure while open time is the same... My fuel pressure is 40 psi
@ idle, which is at the low edge of the right range (40-50 psi, I think).

Or maybe GM set up the computer that way to make us feel better...
 

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2006 STS V
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I get 19.5 mpg in mostly city (30-50 mph) driving in my 1992 STS with the 4.9. It has a fresh tune-up.
 

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2006 STS V
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RAMAIR,
I had new spark plugs, spark plug wires, distributor cap, rotor, and fuel filter, and fuel pressure check. This gave me a baseline for knowing the status of these items.

Funny was that my mileage went from 20.5 mpg to 19.5 mpg after the tune up. Very strange.
 

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2005 CTS-V, 1994 Infiniti Q45
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12-14, all city, redline daily.... Kinda like a death wish for my car!! :)
 

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I get 9-10 mpg.. some one please help me. I aint rich. Is it real hard to change the plugs and wires? I know knightwolf said I'd better have small hands. But I wil have dave comeover get the job done in half the time hopeully:bouncy:
 

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carnerd said:
I get 9-10 mpg.. some one please help me. I aint rich. Is it real hard to change the plugs and wires? I know knightwolf said I'd better have small hands. But I wil have dave comeover get the job done in half the time hopeully:bouncy:
Plugs and wires aren't hard, just awkward. You'll be doing the back half by feel. You need a plug socket, short extension, decent universal and a socket wrench. If you also have a short handled socket wrench it would be helpful. Just make sure you get the threads started right before you start tightening.
You can do this.
Bob
 

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Thanks for clearing that up for me.. I knew I could do it.. Its the time it would take to do it. I go to tech school for automechanics so I just didnt want to start something That i wouldnt finish in an hour or two.
 

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carnerd said:
Thanks for clearing that up for me.. I knew I could do it.. Its the time it would take to do it. I go to tech school for automechanics so I just didnt want to start something That i wouldnt finish in an hour or two.
You can get this in an hour. Plugs and wires. Just do them 1 at a time. And watch that you don't accidently knock anything loose. Things are just a bit close together.
Bob
 

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1985 Cadillac Sedan DeVille (83k miles)
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I get around 15 mpg in town and 22 mpg on the freeway going 75 mph. The car has a little over 76k miles and hasn't had a tune-up in several years. I'll be taking care of that by the end of this month and will post changes in results if there are any. Enjoy your Cadillac!
 

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Stop looking at the 'Instant Average', it's exactly that !

1. Fill your tank up and reset trip meter.
2. Drive until you have 2-3 Gallons left.
3. Fill 'er up, Again.
4. Divide miles on trip meter by how many gallons you just bought.

Yesterday I did this 311 miles 14.7 gallons = 21.15 MPG
The Fuel Center said 20.4 MPG

It's absolutely accurate.

P.S. WOT from a stoplight I get 0-2 MPG and going down a hill I get 70 MPG (These cars would get awesome mileage if the world was all hills ;)
 

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93 & 88 Allante, 88 Eldorado
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I get 23 around town and 30 on the highway. I drive mine real easy though. I accelerate slower than most people and let off on the gas much earlier than most drivers when I see a red light ahead. Driving styles make a big difference but it pays off over here in Southern California where gas prices are now $2.30 per gallon.
 
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