: Gas mileage dissapointment



Benjamin Simon
03-21-07, 03:06 AM
I have been driving my STS v8 around for about 2 weeks now and I am surprised my 99 STS got better gas mileage. Not that I purchased a lac to save money on gas, but I expected it to improve with 6 more years of engineering. My 99 only had a 4 speed tranny. I don't understand why a 5 speed does not get the same or better mileage. The 99 had 3.71 and my 05 has 3.42.
I am not talking anything incredible, maybe 2 mpg less on average, but it is noticeable.
Can anyone explain why with new tech and a 5 speed the mileage would decrease? Does the 05 weigh more? More drag with the 05? Maybe I am just really paying for that extra 20 hp...

cooncat
03-21-07, 06:51 AM
I think all Cadillacs don't score well in the gas mileage area. My V8 gets only 14 - 15 around town and that's compared to 20 in my 2001 Monte Carlo I traded in. I liken the gas mileage in the STS to the big SUV's. The worse thing is these cars require the premium, which I find to be a double ouch especially in the age of the high gas prices we are paying.

Onalaska
03-21-07, 07:18 AM
FWD vs. RWD The reason most manufactures went to FWD was for improved fuel economy.

Caroutisine
03-21-07, 07:33 AM
FWD vs. RWD The reason most manufactures went to FWD was for improved fuel economy.

Other things may come into play, but I agree that the change from FWD to RWD is the biggest factor.

Aurora5000
03-21-07, 10:24 AM
I get 20mpg combo city/hiway and use regular gas. 87 octane. Try it. No pre-ignition and No problems.

You might reset your avg. mpg indicator. You need to read the column back a few days titled:

"What Octane gas is best?"

rayainsw
03-21-07, 11:44 AM
The 4 speed automatic in the FWD ( 4T80 ) had a top \ fourth gear ratio of 0.68.
The newer 5 speed in the RWD STS ( 5L50 ) has a top \ fifth gear ratio of 0.75.
Meaning ( all else equal ) the newer RWD would turn about 10% more RPM.
And that would cost some additional fuel . . .

- Ray
With a GM 6L80, 2.56 final drive, sixth gear = 0.67
( OK, it’s in a new Corvette, but still –
I am thrilled with 1550 RPM at 70 MPH
and near 30 MPG at steady highway speeds )

Benjamin Simon
03-21-07, 11:46 AM
Yea, I used 87 in my 99, no problems at all.
I had just figured the northstar 6 years later would improve. It must be this RWD that is killing it. The 07 with the 6 speed seems to get the same as a Seville with a 4 speed.
Again, it is not horrible, but I expected an improvement. I get about 20 MPG where as my 99 got 22.5 or so.
I am really happy i did not get AWD...

IndyJeff
03-21-07, 05:42 PM
I have been driving my STS v8 around for about 2 weeks now and I am surprised my 99 STS got better gas mileage. Not that I purchased a lac to save money on gas, but I expected it to improve with 6 more years of engineering. My 99 only had a 4 speed tranny. I don't understand why a 5 speed does not get the same or better mileage. The 99 had 3.71 and my 05 has 3.42.
I am not talking anything incredible, maybe 2 mpg less on average, but it is noticeable.
Can anyone explain why with new tech and a 5 speed the mileage would decrease? Does the 05 weigh more? More drag with the 05? Maybe I am just really paying for that extra 20 hp...

The new Lexus LS460 (8 speed/380HP) is rated at 19 city/27 highway.

Onalaska
03-21-07, 06:42 PM
8 forward gears in an automatic, man that just blows my mind. Wonder want a rebuild on that tranny would cost?

Benjamin Simon
03-21-07, 07:22 PM
I wish the northstar ran on E85. From what I understand, it is not that difficult to make it happen.
An 8 speed would be really cool, but wow. I don't know alot about rebuilding a tranny, but that sounds hard.

big kahuna
03-21-07, 11:33 PM
In regards to the lexus with the 8 spd. transmission, remember the sts with the v8 has a rated 18 city/27 hwy. with 5 speed trans. I tried to really baby that gas pedal on the last few tanks, and the best I could get was a average of 18.7 combined city/hwy. Im sure the lexus is probably the same.

cooncat
03-22-07, 06:51 AM
If you look at the new EPA standards, the STS V8 is now rated for 15 around town. I get about 14 - 15 and I really baby it.

Aurora5000
03-22-07, 11:36 AM
The Lexus 460 runs on Premium gas with a comp. ratio of 11.8 to 1.

rayainsw
03-23-07, 01:38 PM
2005 & 2006 final drive ratios:

RWD: 2.73, 3.23, 3.42
AWD: 3.23

And top \ fifth gear = 0.75:1.

2007:
[ now ONLY ]
RWD: 3.42
AWD: 3.23

And top \ sixth gear = 0.67:1 .

[ source = GM, including:
http://media.gm.com/us/cadillac/en/product_services/r_cars/r_c_STS/index.html

I am on record here & elsewhere as believing that the RWD STS EPA highway numbers are higher than they “ought to be” – and many have posted disappointment in their actual highway fuel economy. I am not suggesting a “conspiracy theory” here – just that, GM \ Caddy had somehow figured out a way to post higher EPA numbers than reality would suggest was valid & obtainable in customers’ real world driving. And I believe that it likely had something to do with the exceptionally wide final drive ratio spread between the ( 2005 + 2006? ) 1SE, 1SF & 1SG versions of the V8 STS.

The 1SE had the 2.73 final drive. The 1SF = 3.23 & the 1SG = 3.42. All other factors being roughly equal ( tire rolling radius, etc. ) this would suggest that the 1SF or 1SG needs to turn 18% or 25% higher RPM at any steady cruising speed in top \ fifth gear. I have seen numbers elsewhere suggesting that increasing RPM ( say by selecting a lower gear ) in a vehicle maintaining the same steady speed will increase fuel used by something like 62.5% of the increase in RPM. 40% more RPM at the same steady speed = 25% more fuel used, for example.

So – if we assume [ postulate ] that the STS in 1SE form, with the 2.73 final drive actually could achieve 26 MPG in cruising at some highway speed ( say 65 MPH ) then the 1SF would likely achieve only 23 MPG under exactly the same conditions – and the 1SG approx. 22 MPG at that same speed.

Particularly with the relatively small ( 17.5g ) fuel tank in the RWD STS, these differences seem significant – to me.

For comparison, my current GM car ( and I did look verrrrrry seriously at an STS V8 – twice ) is rated at 27 MPG highway. In over 6,000 miles, I have averaged over 22 in all my driving – including much commuting mileage – often in heavy traffic, in & out of MidTown HotLanta. ( This is significantly better than any of the 4 previous 8 cylinder sports sedans I have had. ) And I can easily exceed that 27 MPG rating, on any long run – even cruising at well over the highest test speed the EPA has used. ( That’s be 65 MPH, through the 2007 MY. )

Being able to average over 28 MPG ( on 2 consecutive tanks, driving to Florida and back ) means that with an 18 gallon tank, and keeping at least 1 gallon ‘in reserve’ I have a real world cruising range of something like 475 miles – on any primarily highway trip. Versus over 100 miles less - more like 362 for the STS V8 1SG, assuming 22 MPG under the same conditions. And where the STS V8 theoretical range, if 26 could actually be achieved, would be over 425.

YMMV.
- Ray
P. S. – My current GM car is a 2007 Corvette Coupe with six speed automatic trans.

And I am definitely NOT suggesting that comparing a 2 seat GT car to a 4 DR like the STS is reasonable or ‘fair’ – I am only suggesting that the real world fuel mileage of the STS V8s seems never to have lived up to the expectations one could reasonably draw from the EPA ratings – where there are many Forum posts confirming that the EPA highway rating on the Corvette ( manual or automatic trans. ) can easily be exceeded in real world driving.

Benjamin Simon
03-23-07, 03:27 PM
Thanks for the very detailed thoughts.
I love RWD, but wow, my 99 STS with 3.71 does just as well as 2.73 in the 1SE.
Since I do so much city driving, it will appear I will not notice any change in mileage. Thus far it seems to be the same. about 17 - 16.5 MPG, all city.

Davidstan
03-23-07, 03:42 PM
Amazing but i am getting 14 in all city driving and thought it would be worse. Getting 19 or so hwy. The 6 speed tranny helps i agree.

Caroutisine
03-23-07, 07:49 PM
Interesting...my 2005 Corvette got incredible gas mileage. I couldn't believe 400HP could get the kind of mileage it did. And when I did get on it, it was hard to believe that it was only 400HP. The STS does seem to use a lot of fuel. The small gas tank dosen't help either.

Skibanker
03-23-07, 07:55 PM
My 2006 V6 AWD gets 19 in the city on regular gas. I guess I should be happy with that according to what I see here. : - )

Curious George
03-25-07, 04:31 PM
I'm glad to read that I'm not alone. I have an '06 Northstar STS with 1SG and PHP. I've driven it 28,000 in 13 months. I learned early that there are two ways to achieve the EPA 26 MPG highway fuel consumption rating: driving downhill or at a steady 50 MPH on level ground with no wind. The 17 MPG city rating must be a product of eating hallucinogenic mushrooms--13 MPG is more like my experience.

All EPA ratings are optimistic fiction because they are derived from laboratory emissions testing. And if the STS's ratings are ridiculous, hybrid ratings (Prius, Escape, etc.) are so absurd they verge on the sublime. The EPA has adopted new methodology intended to produce more realistic numbers. I believe that it will apply from the 2008 model year.

Cylinder de-activation would improve fuel economy on the highway, but I fear that Cadillac's humiliating "V8-6-4" experience precludes that fix, at least until the Division's institutional memory has evaporated.

Benjamin Simon
03-25-07, 04:36 PM
Division's institutional memory has evaporated.

I really enjoyed your note. :)

Kadonny
03-29-07, 07:31 AM
You V8 guys got it good, our V's are considerably worse. I average 12......but I just can't keep my foot off the gas, I know my problem :thumbsup:

corvette00
03-29-07, 08:43 AM
My 07 1SG, V8 AWD is currently getting 14.7 in the city. I only have 1,500 miles on the vehicle. My 05 V8 RWD averaged 16.5 per gallon. It seems Cadillac could do something to improve the fuel economy short of dumping the Nothstar (which I understanding is coming). However, I was aware of the poor fuel economy when I purchased the vehicle and I had to have the V8. Therefore, no complaints on my end regarding fuel economy.

dan1166
04-04-07, 06:24 PM
I currently own a 2006 STS V-6, and my mileage is a disappointment. I am getting 22 MPG on the highway.

My first Seville was a 1990 V-8, 4.6L and I did get 27 MPG on the highway. This was not the Northstar engine.

Now can someone tell me why 17 years later, the mileage is worse. The technology and research that was put into the Northstar, seems to be a waste of money.

Surely, I can afford the gas, even at todays prices, but what bothers me is that Cadillac is going nowhere with these fancy engines and all those fancy valve trains. Is this what they call the Mark of Excellance?

Dan, the STS Man

dhemrick
04-04-07, 09:44 PM
I am getting right at 15 around town in my STS-V which I think is pretty good considering I am heavy footed and this is SUCH a powerhouse of a car.

It seems counterintuitive, but the amazing thing to me is that I recently got between 24 and 25 MPG on a purely highway run up I 85 between Atlanta and Charlotte at a steady 80 MPH and no traffic backups. To me, that was very impressive for such a large car with a very powerful engine. When I averaged a lower speed (around 65 or 70) on another similar leg of the same trip, I barely made 22.

My old 98 STS was very similar in the fact that it got better gas mileage at 80 than it did at 65.

Onalaska
04-05-07, 07:17 AM
I have a 2005 1SF with a 3.23 rear axle and 5 speed auto and average 18with everyday local driving and 22 on the highway at 75 Mph. We don't live in a city so the local mileage not include any stop and go driving. These are actual numbers from fuel used and miles dirven, not from the DIC which shows about 1 MPG better consistantly.

beanjapan
05-09-07, 12:44 AM
I think it all depends on how your driving the thing.

w/ a heavy foot, i get roughly 15 mpg in the city. but if i ***** foot it, i can get better than 21 or 22 mpg in the city. As for highway, riding on i-75 between detroit and flint, i average just better than 27 mpg doing 78mph. If i cruise a little slower and drive it a little nicer, i've gotten pretty close to 30 mpg (some drafting during heavy traffic)

beanjapan
05-09-07, 12:45 AM
I think it all depends on how your driving the thing.

w/ a heavy foot, i get roughly 15 mpg in the city. but if i ***** foot it, i can get better than 21 or 22 mpg in the city. As for highway, riding on i-75 between detroit and flint, i average just better than 27 mpg doing 78mph. If i cruise a little slower and drive it a little nicer, i've gotten pretty close to 30 mpg (some drafting during heavy traffic)

Benjamin Simon
05-09-07, 10:14 AM
You must have the v6. If I am going 55, I can get 25mpg...

evaccaro
05-14-07, 11:03 PM
I currently own a 2006 STS V-6, and my mileage is a disappointment. I am getting 22 MPG on the highway.

My first Seville was a 1990 V-8, 4.6L and I did get 27 MPG on the highway. This was not the Northstar engine.

Now can someone tell me why 17 years later, the mileage is worse. The technology and research that was put into the Northstar, seems to be a waste of money.

Surely, I can afford the gas, even at todays prices, but what bothers me is that Cadillac is going nowhere with these fancy engines and all those fancy valve trains. Is this what they call the Mark of Excellance?

Dan, the STS Man

Your 1990 Seville had about 200 HP, was geared at about 2.73, and went 0-60 in about 8 to 8.5 seconds (pretty much average for the day). At 60 MPH the engine was probably turning no more than 1700RPM. Your 2006 V6 has 255 HP, is geared at 3.42, and hits 60 in 6.9 seconds. At 60 MPH it is turning over 2000RPM and the car is heavier. Today's STS is more oriented towards performance and acceleration. More power, faster acceleration, more mass requires more fuel. Add the fact that the massive tires on today's STS have a bit more rolling resistance than the 70 aspect tires on the 1990's vintage and you a have another contributor.

At the end of the day, these aren't ecoboxes.

Benjamin Simon
05-15-07, 12:36 AM
I was just comparing it to my 99 STS with 3.71s. I would get 24.5 - 25 going 75 mph.
6 years later, in my 05 STS with 3.42s I get 22 mpg going 75 mph.
I knew MPG were going to go down when I purchased it, but I just don't understand why. The only explanation I get is RWD is not as efficient as FWD.

To be clear, the MPG does not really bother me. I just like having the best. My brother's E550 has something like 380 HP and gets the same gas mileage.

dkozloski
05-15-07, 12:09 PM
FWD vs. RWD The reason most manufactures went to FWD was for improved fuel economy.
Manufacturers went to FWD because it makes for a cheaper transaxle package, floor is flat with no transmission and driveshaft tunnel, and assembly is easier.

z06bigbird
05-15-07, 12:40 PM
Disappointing. My 97 SDV got 28 mpg at 80 or so. As good as my LeSabres.

rchern
05-20-07, 08:18 PM
I had a 05 CTS and on the highway it got 27 MPG. I got rid of it and got a STS with the same drivetrain and gearing and I'm lucky to get 24 MPG on the highway. Although the STS is about 300 lbs. heavier I've checked the mileage in the CTS when I had a passenger and luggage (which had to be good for 200-250 lbs.) in the car and I still got about 27 MPG. The dealer says that it's because of ethanol in the gas, but we've had ethanol in the gas for years.

Benjamin Simon
05-20-07, 08:34 PM
You must have the V6 STS.

evaccaro
05-25-07, 07:22 PM
I had a 05 CTS and on the highway it got 27 MPG. I got rid of it and got a STS with the same drivetrain and gearing and I'm lucky to get 24 MPG on the highway. Although the STS is about 300 lbs. heavier I've checked the mileage in the CTS when I had a passenger and luggage (which had to be good for 200-250 lbs.) in the car and I still got about 27 MPG. The dealer says that it's because of ethanol in the gas, but we've had ethanol in the gas for years.

The dealer is smoking dope and wants to share it with you! Don't bother, it will just give you a headache. Take a look at the frontal cross section that the STS presents to the wind and compare it to the CTS. It probably has 25% more frontal area. That will add drag and cut fuel mileage - especially at speed. It gets worse the faster you go.

As I said before, economy was not high on the list of performance goals for GM when it came to the STS. Given the performance envelope, the STS numbers are respectable.

Benjamin Simon
05-26-07, 11:45 PM
I am on a road trip right now. I did a 100 mile segment where I was only going 62 mph on mostly flat road and I was getting 26.1 MPG! I have the 1SG with the 3.42s. I was very impressed.
During a different segment, I was going 82 mpg and only getting 21.9 mpg.
I suppose the higher speeds really kill the mpg. My 99 STS seemed to do much better at 80 mph. Oh well...

malatu
05-27-07, 03:27 PM
I just heard a public announcement on the radio sponsored by AAA. It stated for every 5 mph you drive over 60 mph is equal to spending 20 cents more per gallon of gas compared to driving at 60 mpg. I don't know how true this is but I'm sure someone could crunch the numbers and confirm.

evaccaro
05-27-07, 03:58 PM
I just heard a public announcement on the radio sponsored by AAA. It stated for every 5 mph you drive over 60 mph is equal to spending 20 cents more per gallon of gas compared to driving at 60 mpg. I don't know how true this is but I'm sure someone could crunch the numbers and confirm.

It depends on the price of gas, your car's specific fuel consumption at speed, most efficient RPM for the engine (usually around the peak torque), gearing, terrain, weather (big difference between hot and cold), even head or tail winds (sorry to get so technical about this but I was trained as a pilot in the military. We get pretty serious about this stuff when it's the difference between getting home or not...)

Anyway, the statement is probably based on the general fleet average economy of cars and SUVs on the road today with a price of gas somewhere around $3.00 per gallon. I just love broad generalizations with contrived statistics to back them up.

This all sounds like a way to persuade the general public to keep speeds down. Have you noticed that average speeds on clear interstates have crept up to about 70 MPH or better regardless of the posted speed and futile enforcement efforts? Try to maintain the posted limit on most major highways and YOU become a safety hazard as everyone changes lanes to pass you. I've had a state trooper flip me the bird when I saw he was tailing me and locked on to 55. He couldn't stand it for more than about a mile and a half. He blew by me at about 75.

ARTSBEST
05-29-07, 10:07 PM
A State Trooper flipped you off? What State was this in? Ain't no way I'd allow that to go unreported.