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Cadillac STS/STS-V: Real World MPG

16 messages,  Last post on Oct 20, 2006 at 3:12 PM

You are in the Cadillac STS/STS-V Forum.

What is this discussion about? Cadillac STS, Cadillac STS-V, Fuel Efficiency (MPG), Sedan

Messages Page 2 of 2

#7 of 16 Re: Cadillac STS gas mileage [rayainsw] by dracula

Jun 20, 2005 (8:55 am)

Replying to: rayainsw (Jun 20, 2005 5:07 am)
I bought the 1SF (Luxury performance) with Adaptive cruise control and the sticker showed 26 mpg on the highway. I was not looking for the best gas economy; I did not want a Toyota; I wanted the best American car.
If I get 21 mpg, per your figures, that would be OK except that I am getting about 14 mpg on the highway. I filled up with 87 Octane gas and I will see how the second gas tank mpg figure comes out after the break-in period. I do not know what kind of gas the dealer put in the STS when I bought it. I guess I will have to use premium to get the best mileage but I would be interested in the trade-off between cheaper gas or better mileage.
 
Thanks for your responses.

#8 of 16 Re: Cadillac STS gas mileage [rayainsw] by coolguy

Jul 22, 2005 (5:45 am)

Replying to: rayainsw (Jun 20, 2005 5:07 am)
i'm thinking about getting the 2006 STS with V-8, and trying to understand the final drive ratio. (I wanted to study engineering years ago but 2nd semester calculus washed me out so I transferred to economics!) If I get the STS, I want to feel that Northstar V-8 acceleration, no doubt, so I guess that means the 1SF with 3.23 final drive. What does that mean. I remember from the old days the sports car folks bragged in their advertising about a 1:1 final drive in fifth gear (before they even thought of 6th gear). So how does this concept work in automatics?? Also, how much of a mileage penalty would you guess this would be?

#9 of 16 Re: Cadillac STS gas mileage [coolguy] by rayainsw

Jul 22, 2005 (6:20 am)

Replying to: coolguy (Jul 22, 2005 5:45 am)
“I want to feel that Northstar V-8 acceleration, no doubt, so I guess that means the 1SF with 3.23 final drive. What does that mean. I remember from the old days the sports car folks bragged in their advertising about a 1:1 final drive in fifth gear (before they even thought of 6th gear). So how does this concept work in automatics?? Also, how much of a mileage penalty would you guess this would be?”
 
Wouldn’t it be nice if GM actually provided comprehensive technical information for potential purchasers? (Rhetorical question.)
 
I don’t know if any changes were made for 2006, but in 2005s, here is the ratio information:
 
On a 1SE it's 2.73, and on a 1SF and it's 3.23, and on a 1SG and it's 3.42.
 
Here is what this means TO ME:
 
1SE = 2.73 / and 1SF = 3.23 = 18+% more rpm at any same cruise velocity. (Tire radius = essentially identical and additional equipment on 1SF/1SG-s would mean further detriment to fuel economy.) This means approx. 18% more fuel would be required to maintain the same mph. Assuming 26 mpg for the 1SE = a reasonable cruise mpg, then 1SF or 1SG could be expected to return something like 82% of 26 = 21.3. A significant difference, to me.
 
Now the difference in acceleration (‘liveliness’) I noticed on my 2 test drives last Fall (1SE driven first felt somewhat sluggish to me, 1SF acceleration felt much more like what I’d expected from 4.6L and 300+ HP/TQ) may be worth an incremental loss in real world MPG. To you.
 
I am just (still) surprised that Caddy has somehow found a legal (I presume) way to publish exactly the same EPA ratings for STS V8-s with such widely different final drive ratios.
 
[[Note: The V8 AWD version, with a couple hundred extra pounds, and (I believe) the same final drive ratio as the 1SF is rated at 22 mpg highway.]]
 
- Ray
Not an Engineer – and not playing one here . .

#10 of 16 Comment on axle ratios/fuel consumption by sls002

Jul 22, 2005 (7:24 am)

A test of the STS with 1SE package resulted in performance that was very close to that of the 1SG package. That is not to say that the higher performance gearing will not "feel" better. Running the engine at a higher speed does not result in equal increases in fuel consumption. To cruise at a particular speed requires a certain amount of power. Running the engine at a higher speed will increase the power output of the engine, so the throttle will not be as wide open with the higher performance gears at any one speed.
 
As far as the EPA rules go, the STS is one model/series, whereas the old Seville was two models (SLS and STS), so different numbers were published. The AWD STS has a different number. I think that it would be easy to rate each axle ratio that is available, but this does not seem to be done. The 2005 Corvette offered a high performance gears for both the automatic and manual transmissions, yet there is only one set of EPA numbers for each transmission...
 
I have a book on the 84 Corvette that shows the fuel consumption in different gears at various speeds. At 70 in third gear (1:1), the MPG is just under 20 (~19+), while in 4th (0.7:1), the MPG is just over 25 MPG. The engine is running 2000 RPMs in 4th70 MPH, and about 2800 in 3rd. The engine speed is more than 40% more, but fuel consumption is about 25% greater.

#11 of 16 My 2006 STS by coolguy

Apr 30, 2006 (6:35 am)

Just wanted to go on record to say that my 2006 STS is great. I got it in blackwith the black leather interior, and the luxury performance package. Sound system is five stars, performance and acceleration fantastic, etc. The engine doesn't even wake up until its at 80 mph, and is ride is smooth as your road surface. I have about 2500 miles on it now, mostly suburban driving, and overall mileage has crept up to 20.3 mpg. Its not as frugal on gas as I'd like, but I'll bet it will do a lot better on an upcoming interstate road trip. I think the engine is overkill for around town.

#12 of 16 2007 EPA MPG by rayainsw

Jul 20, 2006 (3:57 pm)

V8 w/6 speed manumatic = 17 / 27
2006 was \ is: 17 / 26
 
Oh, well.....
- Ray
Hoping for a bigger 'bump' in highway mpg
( sigh )

#13 of 16 Re: 2007 EPA MPG [rayainsw] by sls002

Jul 21, 2006 (10:41 am)

Replying to: rayainsw (Jul 20, 2006 3:57 pm)
Not knowing what the axle ratios are (at least I don't), it is hard to make any judgement on the results. However, after seeing what the six speed did for the Corvette, the results seem about what I would expect. They (EPA) did not list an AWD V8, nor are there any SRX listings yet. If the axle ratios are unchanged from last year, then the results are for the 2.73:1 ratio. The AWD results would be for the 3.23:1 ratio. The six speed should improve performance, even with no axle ratio change, although the 3.42:1 axle ratio was already too low to improve acceleration. I think with the new transmission, a 3.08:1 axle ratio would be closer to optimum.

#14 of 16 Re: My 2006 STS [coolguy] by jerrymcshane1

Jul 26, 2006 (9:33 am)

Replying to: coolguy (Apr 30, 2006 6:35 am)
It's to have a "beater" to run around town in and just keep your "ride" for more extended highway driving. I have a BLK/BLK 18" tire STS V-8 also. The car is powerful and very NIMBLE. I agree the STS is a GREAT car EXCEPT for the "murderous" depreciation factor.

#15 of 16 Gas mileage by bigmike5

Oct 20, 2006 (11:01 am)

OK, so a year has now passed, and if anyone can update now with how their STS is performing mpg-wise I would like to know. I am really close to pulling the trigger on an 06, and the preference is the V-8, for power, but if it is going to be a gas glutton I might opt for the V-6 since my wife will be getting the car anyway. Any updates would be helpful. I just hate to trade a solid 00 Eldo that gets 31 mpg on the highway, and has two years left on an extended warranty, for something that won't do near as well. Info on true mpg from a six or two would also be welcome. Thanks.

#16 of 16 Re: Gas mileage [bigmike5] by mnjasper

Oct 20, 2006 (3:12 pm)

Replying to: bigmike5 (Oct 20, 2006 11:01 am)
2005 1SG AWD. My mileage was terrible, than began improving after about 10 thousand miles. Me or the car? I don't know for sure. I get about 18.5-19 m.p.g. on mostly (70%) highway driving according to the car's mileage computer now. At first I was getting about 13.

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