[Aka Nameless]

I bought a '99 Cadillac Deville D'Elegance about 3 weeks ago and I see so many people on these forums saying that they are getting 20-30 mpg in their car. Highest MPG i've seen in my car is 17 mpg, and thats the day I drove it off the lot, its been slowly dropping, its at 14.8 mpg now...

You are probably thinking that I race around town like a maniac, but thats not the case, I have been driving my baby very slow and rarely punching it to make a yellow light.

Just curious to know if anyone has any ideas on what could be the problem?

Car has no codes except for my beloved B2116 code. Which is nothing serious.

 

[ewill3rd]

It may be normal. Keep an eye on things like the oil level and the tire pressures. As temperatures drop, so do tire pressures. Low tires increase the cars "rolling resistance" and low oil (which is not uncommon for a N*) causes increased friction and higher temps in the engine which rob it of efficient power production.

Also don't forget that in the winter time most people have a tendency to warm their cars up. This burns fuel at a nice rate of 0 mpg, which will quickly drop your average. Be prepared to see lower numbers in town because of extended idling also.
There was another thread, the one about acetone in a N*, I posted a recent TSB from GM on tips for better gas mileage. If you can't find it let me know and I'll post it again.
(or maybe someone could link it?)

 

[Ranger]

I think most of the 20 - 30 MPG reports are at least partial highway driving. You don't mention what your doing. Also check the FPR (Tech Tips section).

 

[snowface]

I get 28 mpg on straight highway driving. In city driving (lots of stop and go but on 50 mph roads) I get between 19-22 mpg. I have a 1998 Deville. Found I get better gas mileage here in Phoenix than I did in Cleveland, Ohio. During the winter there, my gas mileage was much lower.

 

[ewill3rd]

BTW, when the weather changes, gasoline refieries reformulate the fuels for the local weather conditions. (I just rembered that)
They change lots of stuff to control things like Volatility and Reed Vapor Pressure (RVP), things that I used to know all about but have long since forgotten....

That can also slightly affect your fuel mileage. That and some of the things mentioned so far can take big bites out of the numbers.
Altitude and weather play a role as well.

 

[Aka Nameless]

Well... i'de say 99% of my driving is city driving. Someone mentioned extended idleing, A lot of times I will accelerate to a little over the speed limit then let off the gas totally and letting it coast just cause I was trying to get higher gas mileage. Maybe that is part of the problem?

 

[Aka Nameless]

Another thing is, Here in san diego gas is like $2.60 for premium, and i'm hearing lot of people say they have like $3.50 + premium where they live, maybe i'm getting lower quality gas in SD even though its 91 octane. I've gotten gas from the 7-11 a block from my house every time since I bought my car. Or maybe I should check my air filter, etc. I'm lazy.

 

[ewill3rd]

Slow, even acceleration is the key there. You burn more gas on acceleration than you do cruising. Zipping up to speed is where your mileage is going. You'd have to drive like an old man with a hat to really squeak those MPGs out but try to be less aggressive getting up to speed and I bet you'll see a difference.

 

[Ranger]

The air filter will not affect milage on a computer controled fuel injected car the way it would on a carburated engine.

 

[Richard S]

I have a 99 deville with 73,000 miles, and get 22 in town and 34+ on the road. I drive smoothly and try not to do jack rabbit starts or emergency type stops, rather anticipating the traffic and do rolling stops until I have to use the brakes. It takes as much energy to stop a car as it does to acelerate it.

Richard S

 

[phathead785]

My 96 deville gets like 24 highway (70 cruise) and maybe like 16 city- with average starts and stops. My tires, oil, fpr and everything seem to be ok. It has 112 miles, and I use 87 octane gas. Maybe thats my problem?

 

[MWA]

98 Eldo 23/city, 30/highway w/periodic WOT and premium fuel @ 90kmiles.

 

[MachX]

Ehm, if you are concerned about mileage, why did you purchase a high performance vehicle? Lincolns run on regular and get somewhat better mileage.

 

[CTSV556]

Well... i'de say 99% of my driving is city driving. Someone mentioned extended idleing, A lot of times I will accelerate to a little over the speed limit then let off the gas totally and letting it coast just cause I was trying to get higher gas mileage. Maybe that is part of the problem?

Contrary to common belief, coasting will get you lower mileage than if you left it in gear. When coasting your car does roll for longer because it doesn't have the engine friction slowing it down but the engine is using gas to stay idle. When in gear the engine friction is slowing the car down more but while doing so the engine uses no fuel at all as the speed of the car is enough the keep the engine above idle rpm.

 

[72186]

I get around 32mpg on the highway with 93 octane fuel with octane booster (>100 octane with alcohol injection on ) and around 18-19 in the city. I mostly attribute my good gas mileage to the condition of my engine, my supersonic spark plugs and my higher then usual octane gas.

 

[Ranger]

Higher octane does not help gas mileage or produce more power. Octane is just a spark knock suppressant. Nothing more.

Supersonic spark plugs? Whatever they are, is not likely to be doing much either. The spark plug serves one purpose and that is to ignite the A/F mixture. It works or it doesn't.

 

[irishbear]

:want::want:That is total B.S. I have been trying regular for the last 10 tanks and I am getting 24 highway and 13 city. I was using Premium 93 and was getting 29 highway and 16 city. This was after I had changed all my fluids and coolant and started from new. While saving money on the price of regular I am going to go back to premium. I can not believe there is no difference. I have tried it myself and recorded the mileage from 10 tanks each and the extra mileage is truly there. I use Nitrogen filled tires as well.

 

[Ranger]

http://www.bajajusa.com/High Octane.htm

"There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs. The only benefit is increased profits to the oil companies that have cleverly convinced some of the public that their new "Super-Duper, Premium-High-Test, Clean-Burning, Used-By-Famous-Racing-Types-All-Around-The-World, Extra-Detergent-Laden-Keep-Your-Pipes-Clean, Extra-High-Octane" fuel is your engines’ best friend. The swami is telling you the truth, don’t listen to that talking cartoon car.

The swami hears people insisting that they got better mileage, better acceleration, and less dental plaque by switching to a high-octane fuel. The swami reminds these people that in every pharmacy is a special miracle pill that is often prescribed by doctors, it works wonders because people believe that it works wonders; it’s called a "placebo". The swami warns: never confuse faith with physics!"



http://74.6.239.67/search/cache?ei=...+rating+ratings&d=DWViCELURppG&icp=1&.intl=us
Octane Myths
• High octane gasoline improves mileage.
In general, if your car is designed to run on 87 octane gasoline, high octane gasoline will not improve
mileage. If switching to high octane gasoline does improve mileage, you might find that your engine, or its
control systems, need repair.
• High octane gasoline gives quicker starting.
No, it doesn’t.
• High octane gasoline increases power.
If your car is designed to run on 87 octane gasoline, you shouldn’t notice any more power on high octane
gasoline. Again, if it does make a noticeable difference, your engine, or the engine’s electronic control
systems, may need repair.
• High octane gasoline has been refined more – it is just a better product.
Additional refining steps are used to increase the octane; however, these additional steps do not necessarily
make the gasoline a “better” product for all engines. They just yield a different blend of hydrocarbons that
burn more slowly. The additional steps also increase the price.

http://www.handymanwire.com/articles/octane.html
The facts on High Octane Gasoline
Do you buy a high octane gasoline for your car because you want to improve its performance? If so, you should note: the recommended gasoline for most cars is regular octane. In fact, in most cases, using a higher octane gasoline than your owner’s manual recommends offers absolutely no benefit. It won’t make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner’s manual.

Do your own searches. These are but a few.

 

[aeronca36606]

Wile i agree with the statement "There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs", There can be a benefit to not using a fuel with an Octane rating that is too low. Many of todays higher performance engines may have there octane requirements increase over time due to carbon buildup raising compression slightly, Hot spots developing within the combustion chamber, Worn plugs,etc. This can cause pre -ignition. Many of todays engine management systems will detect this and retard the timing to compensate. This can decrease fuel mileage. When a higher octane is used, Higher mileage can be seen. This is not because premium has any more energy but that the engine is running closer peak efficiencies.

41 MPG Highway (The best trip tested)
30 City (Town really)
85 HP 1985 Coupe Deville V6 Diesel
Just don't try to pass! But it runs on B100

 

[Ranger]

All understood and agreed with, but it won't make the kind of difference that :want: is talking about.