Gas mileage dissapointment

[Benjamin Simon]

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]

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]

FWD vs. RWD The reason most manufactures went to FWD was for improved fuel economy.

[Caroutisine]

Originally Posted by Onalaska

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]

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]

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]

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]

Originally Posted by Benjamin Simon

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]

8 forward gears in an automatic, man that just blows my mind. Wonder want a rebuild on that tranny would cost?

[Benjamin Simon]

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]

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]

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]

The Lexus 460 runs on Premium gas with a comp. ratio of 11.8 to 1.

[rayainsw]

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]

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.