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If I didn't double-check it and triple check it I wouldn't have believed it myself I have a 108000 mi DeVille Concours 1998. I just bought the car about a month-and-a-half ago and going through the service records of this single owner garage every might of its life car....I see it was very well-loved and everything that is mentioned in the forms that could possibly go wrong has been fixed or touched. I decided to go on a road trip this last weekend and to my astonishment I was getting 27 miles to the gallon running 72 miles an hour indicated which is probably a few miles an hour fast according to my GPS unbelievable and this is not using the info sister this is miles driven / gas used using GPS in a calculator. I repeated it throughout two tanks has anyone ever gotten this kind of mileage before?
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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19,254 Posts
If I didn't double-check it and triple check it I wouldn't have believed it myself I have a 108000 mi DeVille Concours 1998. I just bought the car about a month-and-a-half ago and going through the service records of this single owner garage every might of its life car....I see it was very well-loved and everything that is mentioned in the forms that could possibly go wrong has been fixed or touched. I decided to go on a road trip this last weekend and to my astonishment I was getting 27 miles to the gallon running 72 miles an hour indicated which is probably a few miles an hour fast according to my GPS unbelievable and this is not using the info sister this is miles driven / gas used using GPS in a calculator. I repeated it throughout two tanks has anyone ever gotten this kind of mileage before?
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27mpg on a Northstar powered car is nearly impossible -
MAYBE down hill with a generous tailwind -

my 1995 Deville with the 4.9 RELIGIOUSLY got 28.3 on the highway -
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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CF has posted, documented mileages of over 28 mpg - highway long term - on VIN Y Northstar Devilles and Eldorados.

hell, my 2002 STS gets 24.8 long term highway between here and Florida and back. On 87 octane, recommended for the engine.
 

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Discussion Starter #4
I realize it's unbelievable but that's what I was getting of course I was using 91 as indicated on the fill cap
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Yep - the pre-2000 and some post-2005 Northstars were calibrated for 91/93 octane. 2000 saw a slight decrease in compression ratio, a total combustion chamber redesign, and the change to different (roller/timing) cams which allowed the engines to make their rated horsepower on 87. The 2000 - 2005 PCM will not advance timing beyond set calibration in response to using higher octane fuels so the only "benefit" you get from 89 or 91/93 is more money out of your wallet.
 

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2004 Deville base
'05 SRX N*/AWD, Vetts/Birds/Trucks
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I don't know why not. My 2004 clocks 25-28mpg under 75mph pretty easily.
45-50mph/flat road on cruise 30-33 on the trip computer.

I have checked 'actual' gas fill against the computer 'fuel used' and it is within 1/2 gallon repeatedly, 14-17gal fills
I have checked the car comp against a scangauge, extremely close.

I have set the cruise on 60, zeroed the avg mileage, and seen 30-31 mpg for 20-30 miles, then off the fwy to home thru several lights/stopsigns.(5mi/35mph) Shut it down showing 23-27mpg many times.

0 fwy gets 17-19mpg, sometimes long warmups/idling, WOT's. Lowest I've seen around 16.. pisses me off.

Last O/C was M1 5-30,had conventional. When the gas gets back to 'summer blend' I'll be back at the 'egg under foot challenge'. haha
Many +80k mi/yr years for me, you get pretty good at squeezing, maint., and modding cars for high mpg.
 

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70 Deville 78 Seville 92 Deville 03 Deville
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It's fun to see how far you can go on a tank of gas while on a road trip. You should be getting close to 450 miles on a tank. I don't normally run a tank that empty but on a road trip I will do it once in a while to make it to the next town.
 

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2004 Deville
2004 Deville, Silver, Gone, but not forgotten, 1972 Eldorado, 1974 eldorado, 1975 CDV
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658 Posts
On strictly city driving, short trips around town and a short trip to and from work, my 04 gets between 8-10 mpg. Even at that i can still go 2 weeks on a full tank, but, next month were moving out to the styx and my car will be getting regular highway exercise, I may revisit this thread at that time.
Edit, should have been "mpg".
 

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2004 Deville
2004 Deville, Silver, Gone, but not forgotten, 1972 Eldorado, 1974 eldorado, 1975 CDV
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658 Posts
04 Deville, like i said all city driving all short trips, stop and go traffic, and when i get to work i might sit and idle for a few minutes with the heat going so i can finish my coffee in peace before i go in to work.
 

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White Diamond '03 DHS (with DTS floor shift)
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I assume you meant MPG vs GPM, hence my tongue in cheek response. That said, all city driving plus idling while you have coffee would explain the low fuel mileage.
 

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2004 Deville base
'05 SRX N*/AWD, Vetts/Birds/Trucks
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"my 04 gets between 8-10 mpg"
Ouch
Just filled up. It took 10gal, this car has 184k mi on it, all orig.
tripmeter-computer says:
16.7 mph average
16.9 mpg
Odometer/trip says 172 mi.
That's about as low as she goes, unless I'm working on it/idling/WOT blastsx5-10 times
This was all city, sitting running w/wife waiting heat on, several 10-15 minute stops, 3 weeks since last [email protected]
 

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If I didn't double-check it and triple check it I wouldn't have believed it myself I have a 108000 mi DeVille Concours 1998. I just bought the car about a month-and-a-half ago and going through the service records of this single owner garage every might of its life car....I see it was very well-loved and everything that is mentioned in the forms that could possibly go wrong has been fixed or touched. I decided to go on a road trip this last weekend and to my astonishment I was getting 27 miles to the gallon running 72 miles an hour indicated which is probably a few miles an hour fast according to my GPS unbelievable and this is not using the info sister this is miles driven / gas used using GPS in a calculator. I repeated it throughout two tanks has anyone ever gotten this kind of mileage before?
Going back and forth to myrtle Beach south Carolina I have gotten 28 MPG on my 2001 deville. That is when the wife is driving. I have a heavy foot. with just me driving it is 25 or 26 mpg.
 

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2004 Deville
2004 Deville, Silver, Gone, but not forgotten, 1972 Eldorado, 1974 eldorado, 1975 CDV
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658 Posts
Just had one of the o2 sensors changed today, (I've been getting an intermittent p0420 code.) On the way home, a 4.5 mile trip, the mpg ave went from 8.5 to 8.7, I realize it is only a small gain but might be worth keeping an eye on.
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
Joined
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19,254 Posts
Just had one of the o2 sensors changed today, (I've been getting an intermittent p0420 code.) On the way home, a 4.5 mile trip, the mpg ave went from 8.5 to 8.7, I realize it is only a small gain but might be worth keeping an eye on.
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P0420 points to a failed/failing catalytic converter -
 
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