[Patrick7997]

:hmm:
I have another MPG Question...

1997 ETC, Pearl white with black cloth top, 73,000 miles.

I hear lots of people on here say they're getting 26 mpg...

I am getting 20 with regular gas, 23 with premium, mostly freeway, cruising at 73 to 75.

Premium is paying for itself & more, and the car runs better, so I have no problem with that.

I was told the cloth top will take away 2 or 3 mpg... is that true??? Drags more???

Car is running fantastic, no problems... Just curious about the mileage.

Thanks!

 

[Eldyfig]

I can see the theory behind suggesting the cloth top will make a difference in mpg. Granted there is a difference in the friction. However, I think it would be negligible.

 

[SteveThefolkie]

Hmmm .... interesting question regarding the cloth top and drag ... and not as simple as it may appear.
If you've got a really smooth shape moving through the air it is possible that you'll pull a mild vacuum in the "wake" of the vehicle as it moves which would tend to try to retard forward progress. This happens with really sleek objects like salt flat streamliners, motorcycle helmets and the like. So how to stop the parasitic drag? You put something in the airstream that will interrupt the boundary layer and create turbulence rather than a vacuum which eliminates the drag -
So -if your caddy is REALLY sleek it's possible that the cloth top is interrupting the laminar flow over the surface, breaking the adhesion and it's associated vacuum and improving your mileage - on the other hand it it protrudes far enough into the air stream it'll provide drag which will slow you down -
Anybody got a wind tunnel???

Steve

 

[Patrick7997]

The main thrust of my question is, does 23 seem a little low, for mostly highway miles, or is that about right????

 

[darkfuntime]

i get about 25-26 going 75 with my 94 etc seem a bit low to me did you check the tires,fuel filter,etc, etc.and i do use premium

 

[Eldyfig]

does 23 seem a little low, for mostly highway miles

For "mostly highway miles" that is about right.

Now, if you said "all highway miles" then it would be higher. On my recent excursion to Orlando from Jacksonville, on the highway I averaged over 25 mpg. As soon as I got off the expressway and into town, that mileage dropped due to stop and go driving.

 

[Patrick7997]

:cool2:
Tires are all around 32-33.

Fuel filter will be my next step.... I also have an air cleaner and a pcv that I will throw in tonight....

I was barely getting 20 when I first bought it, but a few of the "2nd gear Wide-Open Throttle" treatments brought it up to 23, and greatly improved the running of the car in general....

If I reset the average with the info button after entering the interstate, I can get 24.5 or so of 100% freeway, over the course of 32 miles, cruising at 75.

So, this is about right??

 

[Eldyfig]

Yes.

 

[Patrick7997]

Thank you. Appreciate that.

 

[EcSTSatic]

That seem low for hi-way IMO. I get 27-30 on the highway at 70-75 with the A/C and I'm not the only one.
Do you have any codes? Dead bodies in the trunk? Is your tranny shifting into OD? How many RPMs are you turning at 70mph?

 

[Patrick7997]

I keep the dead bodies in my Tahoe...:devil:

Sorry, joking. I have no codes. At 75, it is a hair under 2500 rpms. Tranny is not shifting. Just me riding in the car. Car runs fantastic. IN Wisconsin, we have State emissions testing, and I passed that this morning, so, I'm inclined to think nothing too serious is wrong with it, if anything at all....

Reading the threads on this website, I think plugs & wires might not be a bad idea either. Also, I see Accel makes ignition coils. Looking at my car, I see one has been replaced. Perhaps I should replace them all too while I'm at it....

I was gonna throw the air filter in, but got called in to work. I saw a post today that a K&N Air filter for a 97 Bonneville will fit a 97 ETC, which is what I have. I may try that instead.

 

[91TexasSeville]

I believe (for me anyway) the sweetspot is 72-73 miles per hour, cruise control ON. Mine seems to get the best mileage at that setting.

 

[MACTICULOUS]

The last road trip that I took from New York to Boston, MA, I averaged about 75-80 MPH (going with the flow on 84, any New Yorkers and CT drivers know what I mean) and I average 29 to just over 30 MPG. I have a a 99 SLS and I only use Mobil premium gas.
I don't know just how much that the cloth top would change the MPG. My father has a 2000 DeVille with a cloth top and on the highway, his mileage is about the same as mine. By the way, you said you have the ETC, right. My SLS has 275 horse, I think that your's has 300. That will decrease it a little too, just a though. In the long run, I dont think you're that far off. Good luck and let us know if there's any changes.
P.S. I don't keep the dead bodies in my trunk either, they'r ein my fathers, mwah ha ha. :lildevil:

 

[JC316]

2500RPM at 75 mph seems high to me. I know that the ETC uses the 3.71 gears, but my SLS runs 1800-2000 RPM at that speed. I get 26-27 on the highway with the A/C on and I run regular gas. Check your air filter.

 

[Eldyfig]

nevermind.

 

[Patrick7997]

:cool2:
Thanks for all your replies. This is a lot of good information.

I happened to see a friend of mine today, who kind of alluded to what Macticulous is saying, that the 300 horsepower cars will not get the big high mileage numbers that the 270 horse cars do.

He also said to do the air filter first, see what happens.

Thanks again for all the replies. I appreciate your interest.

I will keep you informed.

 

[sjwoodruff]

'98 STS w/117K.

50/50 driving highway and local yields 24.5 - 25.2 mpg on a full tank of 87 octane. No noticeable improvement with premium.

With cruise set on highway at 65-70, I returned 29.4 mpg on a recent 320 mile trip with light traffic and little construction to deal with.

I inflate tires to 38 psi cold - this noticeably helps my mpg.

 

[JC316]

The only difference in a 300hp N* and a 275HP is a slightly reworked intake. I cannot see how that would affect gas milage that much. I am inclined to believe that it is something like the air filter because you are getting much worse milage running regular. Also, there are plenty of people with ETC's and STS's that get 26-30 on the highway. Air filter, tire pressure, added weight, or bad wires and plugs would be my main checks.

 

[zonie77]

Gearing is different. 300hp has 3.71's, 275hp has 3.11. Higher rpm's are not conducive to high gas mileage. 25 or so is great for this fast a vehicle. 30 is even better though.