View Full Version : Noticeably Better Gas Mileage..On 10 yr old+ Engine!?

Chuck C
08-08-04, 09:23 PM
Ever since I've had my car (since 1998), I've occasionally taken mental notes of the AVE MPG reading on the left side of info dash. It dips below 17 MPG in the winter and usually tops out at 18 MPG in the summer. This summer, the reading has been steady at about 19 MPG and just today, I hit 19.6...that, however, came after a 130 mile highway trip. Nevertheless, what kinda mileage do you guys get? Does anyone have an explanation for the better mileage? My thoughts:

-New Shell V-Power Fuel ACTUALLY WORKS
-Tuneup a month ago in which trans was flushed but spark plugs were not changed
-Coolant flushed 3 months ago
-Throttle body cleaned a year ago
-My foot is not as heavy (but not really :devil: )
-Computer is in error

08-08-04, 11:21 PM
I have a 150,000mi 96STS and a heavy foot. I get better fuel mileage. I find it difficult to get as low as 18. On trips with cruise at 78 the mileage eventually climbs up to 25.

With a short daily commute of two miles and 9 stop lights, to work which is mostly what I drive, the average mileage stays at 20 or higher.

After 130 miles of driving I would expect to see at least 22.

Night Wolf
08-08-04, 11:25 PM
well, it isn't a N*, but over the winter I was driving my '89 Olds 88 (3800 V6) I found out that town driving, with a heavy foot I got 21mpg... when I was easy I got 18mpg.... it seemed strange though. Tormorrow I get the Olds back ont he road, and it will be my daily driver, my '93 will get to relax (still on the road though) so I am going to do more millage comparisons.... I was getting 21/34mpg with the Olds though.... not too bad

long trips on the highway with cruise set is always good for engines,by having the engine at a constant rpm, not idel, but not redline, it allows the engine to clean itself out... carbon varnish and deposits from stop and go city driving... especially on older engines it is very noticeable...

08-08-04, 11:35 PM
I heard that cars get better mileage in the summer? Something about the gas slightly expanding or ? I know that I seem to get slightly worse mileage when it is -30 out, but at that time it seems to have more power because the air is dense in cold weather.

The owner's manual of most new cars recommend only 500 miles or so for the engine to properly break-in. However, I've read that 15,000 miles or so is more realistic, especially if an engine is babied, and at this time fuel efficiency might improve.

08-08-04, 11:40 PM
It really is more the winter gas than the cars. In NY they add ethanol and MTBE. Ethanol has very little energy per gallon. MTBE has almost no energy it is like filler except that smelling it causes several types of illnesses, and it is a major polutant. Why some genious has decided that we need this stuff in gasoline I will never know.


08-09-04, 06:52 AM
I am not getting good gas mileage. I am getting 14.9 around town. I have a 94. I need to change my shift solenoids so maybe that is it. Any ideas.

08-09-04, 02:49 PM
Majority of my driving is highway with just a splash of NW Chicagos suburbs. I get 24-25 during the summer and 21-22 during the winter. You will always drop during the winter. It takes longer to warm up, thus the PCM will run the injectors richer for a longer period of time.

08-10-04, 05:08 PM
I have a 92 Eldo with 136000 miles, when i drive it i'll get about 21.5 to 22. Now...when my wife drive's it it'll get around 18.5.lol Go figure!lol

08-11-04, 03:01 AM
You guys never mentioned how often you reset the AVG MPG reading.
That will make a big difference. If you never reset it then the average is over
a very long time.

I have 93 Eldo TC (110k). I do notice that my winter milage reading is lower than summer. I've heard that the winter gas formula is different than summer and that is part of the difference. Also, driving on snow or slush covered road wastes a lot of engine's energy.

My AVG MPG is quite low. It is about 16.4 right now. Per week, I drive about 80 miles on back roads and city. Speeds of 25-40mph with many stops and traffic lights (but no heavy traffic). Then, on the weekend I drive about 70 highway miles. Half of that is at 65-75mph, other half stop-and-go traffic. Terrain here is uneven (not flat) but since for most part I make round trips - it averages out.

So, my 16.4 AVG is over several months of this type of driving.

If I were to reset the AVG and just take reading over my single weekday back-road city trip, it woudl read around 15mpg. If I reset it before my highway trip (no traffic), it would be around 22mpg. If half of my highway trip was in traffic, then the trip would average out to around 18mpg.

Sounds like I'm geting lower milage than most of you guys...


08-11-04, 03:06 AM
I have a 92 Eldo with 136000 miles, when i drive it i'll get about 21.5 to 22. Now...when my wife drive's it it'll get around 18.5.lol Go figure!lol
Montano, I don't think 92 Eldos had a Northstar engines. You probably have a 4.5 or 4.9 engine. So, your milage would be different.


08-11-04, 07:11 PM
I am getting very consistant 20.9 MPG with 75% hwy 65-70 mph and the rest in town. I reset the avg mpg at every fill up and am using 87 octane costco gas. I also love the sound of WOT and am not shy about doing it getting on the hwy.

08-14-04, 03:00 AM
I was averaging about 19.5 mpg over a couple months of driving on my 99 STS. I drive it pretty hard and frequently do the carbon cleaning at WOT. Anyways, I installed a K@N filter about two months ago and reset the trip. I am currently getting 21.9 mpg. Pretty damn good IMO. Can't wait to see what type of mpg I'm getting after I install a 3in catback with 2.5 flowmaster 50 series.

08-23-04, 11:41 PM
I used to get 25+ mpg on the highway in my 1994 seville.

then i changed out the engine earlier this year and since then i'm lucky if i can get 20mpg on the hghway.

everything new (sparkplugs, filters, hoses fluids etc. . . . .) during swap.

they engine i got was supposed to be less than <2000 miles from ex-dealer with interesting history.

my mechanic said lit looked newer, and not from a pressure wash, for what that's worth.

It is a 1997 model engine, but I wasnt aware of any material design changes that should have effected gas mileage adversely.


08-24-04, 11:40 PM
my 2 cents.........

94 sls, 97,000 miles. i use, and always have used, the highest octane possible and have recently stuck with V-power from shell. i also use Mobile-1 synthetic oil (which i havent changed in 9000mi !) and i still get 27 mpg on the highway and sometimes more.

although, my suspension is kinda shaky :hmm:

08-25-04, 12:12 AM
my 2 cents.........

94 sls, 97,000 miles. i use, and always have used, the highest octane possible and have recently stuck with V-power from shell. i also use Mobile-1 synthetic oil (which i havent changed in 9000mi !) and i still get 27 mpg on the highway and sometimes more.

although, my suspension is kinda shaky :hmm:

27 Avg. mpg ?! On a highway ?

How fast do you drive? Is the terrain perfectly flat? No traffic to speak of (which would make you slow down and spead up)?

Mine is in low 20s on a good day... :annoyed:


08-25-04, 01:09 AM
you guys are sad lol ;) Im averageing a little over 18 in town and 27 flat on the highway. On the highway its usually driven 5mph over the speed limit. probably an average of 65-70mph. And I have the N*300 only the 275's should get that milage. I think the 300 is rated at like 17/25 now.

09-06-04, 04:26 AM
On a recent trip to Mount Vernon and back from Lynnwood (50 miles one way) I reset AVG MPG while on the freeway doing 70-75 mph. My Eldo 94 with 114K miles did 34.6 on a downhill stretch of the road, and averaged 26-28 during the whole trip.
After returning to the small town stop-and-go driving the AVG MPG indicator gradually went down to 18.1 and stayed there.
I am driving on low octane Costco gas.

09-14-04, 02:32 PM
I get almost 26 when I get on the interstate and set my curise to 70-79 and around base when I'm short trippin w/ stop lights I get between 16-18

09-14-04, 04:53 PM
I have a 98 Deville and get 25+ going back and forth to work on regular gas. This summer I went from Florida to Virginia filled up across Florida State line in Ga and put 93 octane in her and filled up again in Danville Va 28.4 miles to the gallon. You just have to love that. :bouncy:

09-14-04, 09:00 PM
14.8!! hell YEAAHH!! no really, what can i do ? How do i clean the TB when i cant even look past the two piece palte

09-14-04, 11:42 PM
98 Concours: 18 in town 27 on the highway with three large adults and a weeks worth of luggage in the trunk with or without A/C. My 98 Mazda 626 only got 19 in town and 29 on the highway . N* are very miserly considering the 300 HP and 3.71 gear ratio. I'm very impressed

09-15-04, 12:39 AM
The variety of responses here is TRULY amazing...makes you wonder if there's that much variability engine to engine...
My 94 Eldo (275 hp) was getting the high 16s around town before the head gaskets were done in July and since then about 18...I usually buy Shell and my mileage actually seems a bit lower since they brought out the V-power (?) product so I threw 10 gallons of 89 in as an experiment. No knocking; a slightly noticeable loss in oomph; a few tenths of a bump in mpg...you folks getting 26 on the road must be turning it off on the downhills!

09-19-04, 03:14 AM
Avergage MPG around town is about 16.2... in summer with A/C running, 22-24 on the highway running 75MPH. The MPG goes up a tad in the fall /early winter without running the A/C or heat depending on my driving habits ;)

09-19-04, 07:24 AM
I get typically 22 mpg on my '97 STS... on a weekly basis... and around 25-26 on my '96 SLS (which my wife is now driving.)

The STS has a higher engine rev typically plus the additional engine tweaks to get the extra 25 HP out... which account for the difference I think. I burn premium gas in both.

09-20-04, 09:47 PM
i get what it says it should, 16 in the city and 25 on the highway, but too bad when around town and i gas it alot i get sub 14s but on the highways its reall high like mid 20's