: STS N* v8 MPG

06-05-13, 05:07 PM
Hey I'm just curious what everyone else is getting. I have a 2005 with a v8 and I'm getting 11mpg average like 60/40 city/highway I know I shouldn't be expecting very high but still 11 is sorta embarassing haha. I also do not drive very aggressively like maybe once a week ill get it over 4000rpms otherwise nothing crazy.

06-05-13, 05:46 PM
I average about 20-21 with a 75/25 highway/city split...My car rarely moves outside of driving to work these days...If you are driving in a lot of stop and go traffic or encounter a lot of stop lights your mileage is being destroyed. The N* is a pig when not cruising down the freeway, pretty much a truck with 4 doors and a trunk at that point. :)

How many miles on the car? When were the spark plugs last changed? Do you run fuel system cleaner (Chevron Techron) through your fuel system on a fairly regular basis (I do one bottle every 5k)?

06-05-13, 05:52 PM
Depends on your rear end Final ratio, I had a 2005 with a 2.73 and got 16 and 27.
With the one I have now 2006, it has a 3.23 and 15 and 25.
Runs great,with a clean Air filter, and Mobile 1.
I used to have a HEAVY foot, but have calmed down a lot.
Happy Motoring !

06-05-13, 06:04 PM
I have a 2.73 ratio. Recently broke 40k miles on the odo. I have a big bottle of Lucas oil cleaner (etc) in the garage I will put some in next fill up. Not sure if the plugs were ever changed

06-05-13, 06:10 PM
That's worse than my V. Doesn't sound right. I get around 16, city/highway mix, with occasional hard driving.

Hey I'm just curious what everyone else is getting. I have a 2005 with a v8 and I'm getting 11mpg average like 60/40 city/highway I know I shouldn't be expecting very high but still 11 is sorta embarassing haha. I also do not drive very aggressively like maybe once a week ill get it over 4000rpms otherwise nothing crazy.

06-05-13, 06:37 PM
Yeah I wish I could get at least get into the 15 range. But with only 40k miles I can't think my plugs are shot... I changed oil and put new filter in before reset and I'm still getting 11.. I have a volant air intake too which shouldn't hurt my mileage

06-05-13, 06:37 PM
Awd v8s are worse than vs.

06-05-13, 07:00 PM
That seems like a lot worse. Probably should be in the 15-16 range at least.

06-05-13, 08:36 PM

If you've got a RWD N* w' 2.73, something does seem wrong. Crappy gas (ethanol) could contribute to your situation but 20 mpg hwy is NOT ambitious at all. Clean the throttle-body & change the air filter if it hasn't been done recently. I assume any idiot lights have received an appropriate response.

To get the best mileage, keep the rpm's at 2k or under.

06-05-13, 09:03 PM
I have brand new air filter in cleaned throttle body about 2 months ago and am relatively gentle on the throttle. Jason has lead me to believe it might be the fuel filter. I do have 40k miles and I am willing to be it's still the original 7 year old fuel filter on there.

06-05-13, 09:31 PM
40,000 miles isn't much on a fuel filter and if it's performing well, that doesn't seem likely. Later models don't even consider it a regular service item.

But it's your car. It's in froint of the right rear wheel and takes a special tool.

IMHO, Get your codes read by a good Cadillac dealer before spending any other money.

06-05-13, 09:45 PM
Since no one else said it, have you checked your intake manifold bolts?

06-05-13, 10:28 PM
^^^ Very common and throws no codes.

06-05-13, 11:44 PM
When I'm in NYC stop and go traffic ill get down to single digits mpg. On the interstate I do about 22mpg. Around here in Fayetteville it's a combination with some retardedly long lights and I really ring this baby out (love to hear the supercharger and feel it pull) and I average around 16mpg. When I used to drive in auto mode I used to get 15 but I noticed the transmission (which is different from yours) was reluctant to shift to 6th so I started driving in manual mode and improved mileage. The v runs the same ratio as the awd by the way, just more power and going to 2 less wheels. And more weight. You're low on mpgs. Not normal. Everything these guys are recommending sounds good. Check codes clean tb, all that.

06-06-13, 06:29 AM
Have no codes first thing I did check. I'm going to check manifold bolts though.

06-06-13, 08:37 AM
Why would you even remotely consider dumping snake oil into your engine ?? ("oil cleaner" ? ? ? ?)

Open the oil fill cap and look in there with a strong flashlight. See anything that would make you want to dump in some extra goop that costs money and therefore drives up the cost of mileage - just as non-essential gasoline additives do. What does your (GM) owner's manual say about using additives ?

Engine oils, straight out of the bottle, already have everything needed to keep your engine clean and happy. Let others buy the junk that puts stickers on the sides of Dale Earnhardt's car.

For some homework, Google "top tier gasoline".

06-06-13, 09:16 AM
When I was younger I had a 97 Altima and I bought a can of some oil additive. I happened to be taking my car in for an oil change so I gave the mechanic the can and asked him to add it in and he basically told me I was wasting my time. The stuff doesn't do anything. Ever since then I've been convinced that stuff is all a scam.

06-06-13, 07:09 PM
Reset the avg mpg 50 miles ago now I'm down to 10.2-10.3 c'mon :headroll:

06-06-13, 07:36 PM
Don't believe the DIC. What is your calculated mpg?

06-06-13, 08:27 PM
:yeah: I have found mine to be a bit generous at times and sometimes not so generous when calculating my own MPG after filling up.

Fill her up, reset trip, reset AVG MPG, drive 100m or so and top her off again...Check the DIC's math vs. real math.

06-06-13, 08:58 PM
I'm going to do that tomorrow morning as I filled up an hour ago. How many gallons are our tanks 20 or 18? I had little less than 1/8 and it took 15gallons


Could putting 10w oil instead of 5w cause the engine to work harder and give worse mpg? I may have put 10w now that I think back but I'm not certain -____-

06-06-13, 10:03 PM
IIRC 17.6 gallons is the fuel capacity. I'd have to consult the manual to confirm.

06-06-13, 10:43 PM
If you put in more than 17, you may be getting cheated by the gas station. My 2006 manual says 17.4 and my 2007 manual says 17.5. I've run it low, but don't remember ever putting in more than 17 gallons. (But I don't 'top off'!)

06-07-13, 02:07 PM
05 1SG (rear wheel drive), with the 3.43 gears, I get about 20 in 75/25 highway/city driving +- 2 (ish).

06-08-13, 11:14 AM
06 AWD N*, 120 K, I get 21 hwy at 85 mph with the cruise set. I can avg 18 normal driving, drops to about 15 when enjoying sport mode.

Plugs were changed at 80K, original fuel filter

06-08-13, 12:12 PM
3RD-That's outstanding! What tires, size & pressures?

06-09-13, 01:46 PM
My 75% highway/25% city commute, which is ultra-predictable Fort Worth traffic, I'm deep into the throttle a couple of times per tank, 87 octane from Costco, same pump, same station every single time, and I get about 20.5-20.6, depending. On light timing and A/C use.

I track all my MPG, by hand, for all vehicles (save for my wife's current one), since my first 72cc Honda scooter back in 1984, and I can say you should be getting much better MPG.

I have the 3.23 rear, on Continental Extreme DWS tires, 38 PSI front, 31 PSI rear. Typical highway speeds are 60-65 MPH, gentle acceleration, backing off early for traffic/lights (I used to drive a tour bus for a living, you get very good at being gentle so as not to have any members of a bachelorette party spill their drinks on either themselves...or you), just 'saving' the car/tires/fuel for fun times.

I've been down in the 17's, but not much above 21.0. I've not taken it on any road trips, though.

06-09-13, 08:59 PM
^^^^why 38psi in front and 31psi rear? I always put 32-35psi all around.

06-09-13, 09:44 PM
Even that's above our tire tags.

06-09-13, 10:56 PM
Recommended inflation pressures have so many variables, I don't consider pretty much any of them, save for weight carrying, useful.

Tires, how they're driven, temperature, ride quality, handling expectations, wear expectations, noise, etc.

I experiment with tire pressure until I find what **I** like, and which performs as I expect, taking my expectations into account.

I run 38 front, 31 rear because the last set, fronts were excessively worn on both inner and outer edges, decent amount of tread in the middle, yet alignment was fine, and rears wore excessively in the middle, though the PO ran the rears at about 36 PSI.

edit for grammar

06-09-13, 11:07 PM
I don't necessarily disagree.

06-09-13, 11:24 PM
That's basically how I do my air. If it wears too much on the edges which I hate I just add more air. Although I've moved to nitrogen to avoid variables such as outside temp and speed as much as possible. Keeps the PSIs more constant instead of it fluctuating.

06-10-13, 12:25 AM
^^^I have read a few posts about nitrogen in the past couple yrs. I might just pull the trigger and try it

06-10-13, 06:45 AM
I don't necessarily disagree.

I understand, it's just I've had people question my logic for, well, a few decades, now.

It's really noticeable on a two-wheeler, when 2PSI can make a very obvious change in pretty much everything.

06-10-13, 04:40 PM
Could putting 10w oil instead of 5w cause the engine to work harder and give worse mpg?

No - 5W-XX flows a bit better at cold start - the final number: -30 is the warmed-up running engine oil viscosity figure.