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Cadillac STS/STS-V: Real World MPG

16 messages,  Last post on Oct 20, 2006 at 2:12 PM

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

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#2 of 16 STS gas mileage by bigmike5 Apr 28, 2005 (4:16 am)
Can anyone who drives one give a real world report on the mpg that the new STS is getting? We have a 00 Eldo [32K miles] and just finished a 2400 mile trip to Houston and back with some in and around mileage [300] included. Running on Regular, we got an average mpg for the entire trip of 29.3 mpg [generally not running faster than 65-70 mph on the highway]. It was actually up to 29.5 mpg at one point on the way home, but I had to do some [a lot of] lead foot exercises the last 140 miles to avoid all the trucks on I-65 from Nashville north in a pouring rain. The Northstar engine is great. I would be interested to see what the real mpg reports are on the new STS. Replies to this message: sls002 (May 04, 2005 3:06 pm)
#3 of 16 Re: STS gas mileage [bigmike5] by sls002 May 04, 2005 (3:06 pm)
Replying to: bigmike5 (Apr 28, 2005 4:16 am) I can't say what the STS does, with the VVT northstar. However, with the base axle ratio, 2.73:1, the overall gearing is very similar to the SLS or base Eldorado gearing of 3.11:1. With the 0.75 overdrive the gearing on the STS is 2.05:1 while the old FWD's is 2.11:1, so fuel consumption should be similar and the EPA rating is. With the high performance gears, the STS has on overall ratio of 2.56:1, which compares with the old FWD STS's 3.71 x 0.68 = 2.52:1 gearing. So fuel consumption should be similar, depending on what package you get.   I think that in another year or so a six speed automatic will be standard. I also think the packages will change in time too.
#4 of 16 Cadillac STS gas mileage by dracula Jun 18, 2005 (9:57 pm)
I bought the Cadillac STS V8 2005 about two weeks ago and my first tank fillup shows me a 12.9 mpg which is very low considering I did mostly highway driving. Is this a break in mileage number? The dealer told me to bring it in to look at the filter. Anybody else had this problem after the STS purchase? Replies to this message: baltoman (Jun 19, 2005 6:21 pm) rayainsw (Jun 20, 2005 4:07 am) dracula (Jun 20, 2005 7:55 am) coolguy (Jul 22, 2005 4:45 am) rayainsw (Jul 22, 2005 5:20 am)
#5 of 16 Re: Cadillac STS gas mileage [dracula] by baltoman Jun 19, 2005 (6:21 pm)
Replying to: dracula (Jun 18, 2005 9:57 pm) Most new STS users get poor gas mileage. You need to break it in, You also need to use premium gas with the V8. You can burn regular, but your mileage and power will suffer. Mileage on cars of similar power - like the Infiniti M45 - is about the same. Finally, most new STS users can't keep their foot off that power! And that really hurts mileage.   If mileage was the all important criteria, you would have been shopping Chevys or Toyotas. But I think you - like me - wanted upscale American power and luxury.   You've got a great car. Great looks. Great power. Great control and ride. Sit back, drive and enjoy.
#6 of 16 Re: Cadillac STS gas mileage [dracula] by rayainsw Jun 20, 2005 (4:07 am)
Replying to: dracula (Jun 18, 2005 9:57 pm) Depends on what V8 you bought. As you probably know, the 1SE has a rather dramatically lower final drive ratio than the 1SF or 1SG.   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 MPG being not the only reason I decided against a 1SF . . . Replies to this message: dracula (Jun 20, 2005 7:55 am) coolguy (Jul 22, 2005 4:45 am) rayainsw (Jul 22, 2005 5:20 am)
#7 of 16 Re: Cadillac STS gas mileage [rayainsw] by dracula Jun 20, 2005 (7:55 am)
Replying to: rayainsw (Jun 20, 2005 4:07 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 (4:45 am)
Replying to: rayainsw (Jun 20, 2005 4:07 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? Replies to this message: rayainsw (Jul 22, 2005 5:20 am)
#9 of 16 Re: Cadillac STS gas mileage [coolguy] by rayainsw Jul 22, 2005 (5:20 am)
Replying to: coolguy (Jul 22, 2005 4:45 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 (6:24 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.

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