: Fantastic gas mileage on highway trip - 2008 DTS



rebel7254
06-24-13, 12:17 PM
My car got 33 MPG on my trip to and back from Alabama this weekend. I was by myself and not carrying a heavy load or anything, but I still think that's pretty remarkable.

I know part of the reason for the great mileage is the fact that I put ethanol-free 93 octane gas in it. This makes a noticeable difference just driving around home, getting me 20 MPG instead of 17 MPG that I get with 10% ethanol 87 octane. But 33 MPG on the highway is a huge jump that I did not expect.

I also kept it at 65 MPH on the interstates, which helped I'm sure. I like that speed not only for the better fuel mileage but because it's just more relaxed since you don't have to worry about passing nearly as much.

I know there's debates about how much advantage you get by putting the ethanol free and/or premium gas in these engines, but I can attest at least in my own experience that it makes a significant difference. Yes, the price per gallon is going to be more, but I've done the math and in my area getting 20 MPG instead of 17 MPG makes up for the price difference. The cost per mile ends up being about the same.

Superjim
06-24-13, 08:28 PM
The only way mine would ever get 33 mpg is if I was going downhill all the way... LOL

Submariner409
06-24-13, 08:54 PM
rebel, You need to contact Cadillac and the EPA - you'll be an instant millionaire, considering the CAFE figures for your car are:

EPA - City = 15 mpg; Combined = 18 mpg; Highway = 23 mpg.

That you were able to see a 33% increase in highway mileage in this car is truly revolutionary.

I drove to Charleston, SC on Shell 87 E10 over Thanksgiving. I drove back using Shell 87 straight gas. No mileage difference - none. None. (and running Shell 93 makes so little fuel use difference it isn't worth mentioning - but, yes, it adds considerably to the mileage cost.)

SCDTS
07-01-13, 10:20 AM
Just some thoughts and comments. Did you calculate this mileage or take it off the dash computer?
All the Cadillacs I am familiar with have had very accurate computer readouts - gallons used to pump readouts gives accurate mpg (2 Chevrolet computers give less used than pump fill-up and high mpg on computer vs actual/calculated). I have found on trips from SC to West TN using Hi test and my foot to control the cruise I have gotten 24- 26(best for trip one way - about the same both ways), but not as great as yours.
Congratulations. Keep it up

StuJac
07-02-13, 10:01 AM
Yeah, I'm sorry but I don't believe it. The entire trip had to be downhill with a strong wind. For my area, there is no better test than the AC Expressway and the best I've ever been able to do is 26 and that's cruising at 55 mph, no wind, no traffic, perfectly straight road.

Superjim
07-02-13, 11:00 AM
The best mine has EVER got is 24... and that's driving REAL EASY...
Normal mileage for my car... on the highway... is 20/21....

StuJac
07-02-13, 11:15 AM
Meanwhile, in my daily trip back and forth to the office (4 miles each way, all back roads) I average 13.5 mpg which kinda sucks.

Superjim
07-02-13, 01:33 PM
Meanwhile, in my daily trip back and forth to the office (4 miles each way, all back roads) I average 13.5 mpg which kinda sucks.

I get 16/19 here in town...(as long as I don't STAND ON IT very much)... but most of my driving is on the freeway... :)

StuJac
07-02-13, 01:37 PM
That's the difference, the freeway. I'm on the freeway on Saturday's, on the way to dinner. Every other month to Atlantic City and that's about it.

stormnn
07-03-13, 07:14 PM
Just did a 80 mile round trip up north on 50 mph roads. Went thru a couple of towns. DIC said 8.7 liters per 100 km. which is 27.5 mpg U.S. gal.
I have not checked fill up to fill up as I like the DIC numbers.
Norm

hazcaddy
07-03-13, 07:42 PM
Norm you just nailed the issue: it's the axle ratio that makes the difference between you and Superjim. I too get great mileage out of town especially on the flat bits - up to 34 mpg (Imperial) and that is because we both have the Lux III, not the Performance model like Jim.

Superjim
07-03-13, 07:55 PM
Norm you just nailed the issue: it's the axle ratio that makes the difference between you and Superjim. I too get great mileage out of town especially on the flat bits - up to 34 mpg (Imperial) and that is because we both have the Lux III, not the Performance model like Jim.

Maybe a combination of axle ratio and my big foot... LOL LOL
Also remember that freeway speeds here in Fort Worth ( not counting rush hour) are usually around 75mph and interstate speeds in Texas (once you get out of town) run around 85..
That's not a good way to get good mileage... :) :)

Submariner409
07-04-13, 12:33 PM
IF these later "DTS" cars have 2 different final drive ratios - 3.11:1 economy; 3.71:1 performance - then there is a 2 - 4 mpg difference in highway gas mileage.

It is not uncommon to see 26 - 28 mpg open highway in the 3.11 cars, 24 or so in the 3.71 cars. In town or near suburban it takes gas to accelerate 4,000 pounds and the final drive ratio factors less into that type of mileage.

rebel7254
07-06-13, 10:45 PM
The trip computer has a gallons used measurement, which I reset at fill-up. I also reset the "Trip A". Then when I'm at the station to fill up, I take the miles on Trip A and divide by the gallons used. Finally I compare the gallons used with how much the pump says I put back in the tank to fill it up. It is always within a half-gallon.

I understand the doubters, but it is what it is.

I'm looking at my receipt right now from where I filled up in the town I was leaving. I put 12.61 gallons in the tank. According to Google maps, the route I took home was 438 miles. I did not have to stop to fill up again.

438 / 12.61 = 34.73 mpg

By the time I filled up again, I had driven probably 30 or 40 miles "normally", so that could have taken it back down to the 33 figure.

Y'all don't have to believe me, that's fine. Doesn't bother me a bit. I'm taking my wife and son to Myrtle Beach in September. I'll report back here on what kind of mpg I get. I don't expect to do as well because:

a) I'll be hauling significantly more weight.
b) We'll have to climb the mountains in I-26 in Northeast TN and Western NC.
c) We might be traveling with my in-laws (separate vehicles) and my father-in-law won't want to go 65 mph.

----------


rebel, You need to contact Cadillac and the EPA - you'll be an instant millionaire, considering the CAFE figures for your car are:

EPA - City = 15 mpg; Combined = 18 mpg; Highway = 23 mpg.

That you were able to see a 33% increase in highway mileage in this car is truly revolutionary.



Why would it make me a millionaire? You think they'd give me a million dollars to study my car or something?

MoistCabbage
07-06-13, 10:55 PM
You don't always end up with the same amount if fuel when you "fill up", and you don't always fill up with the same amount of fuel remaining, so that's not an accurate way to track mileage at all. Google Maps distances are also not always accurate.

The MPG readout on the DIC uses the exact distance driven, and the exact amount of fuel that is injected to reach its number, THAT is accurate.

rebel7254
07-06-13, 11:31 PM
The MPG readout on the DIC uses the exact distance driven, and the exact amount of fuel that is injected to reach its number, THAT is accurate.

Why would the MPG on the DIC be accurate, but not the gallons used? The computer has to know the fuel usage to figure out mpg. Why would it be using a different counter for gallons consumed than what it shows on the DIC? And why would it be using a different distance counter than what shows on the DIC for Trip A if I reset it at fill-up?

As for questioning Google maps, I just loaded the same route in MapQuest and it came up with 445 miles.

Unfortunately by now I can't remember exactly what my Trip A mileage was when I reached my house. I do know that it was in excess of 450 when I filled up the next day though.

Like I said, I'm not going to get bent out of shape over some people on a forum not believing me. The main goal of this thread was just to share my experience with everyone else, and if folks think I'm making it up, I couldn't care less.

MoistCabbage
07-06-13, 11:53 PM
Not trying to get anyone bent out of shape. My point is, if you want dead accurate MPG for any given trip, reset the AVG MPG readout right before you start driving, and look at it as soon as you arrive at your destination.

Superjim
07-07-13, 02:03 AM
I just went out and checked my GALLONS USED...
It is NOT in WHOLE gallons...
It shows 68.4 since the last time it was reset...

MoistCabbage
07-07-13, 02:38 AM
Post edited, but the AVG MPG readout is still the most accurate way to calculate average.

rebel7254
07-08-13, 12:38 AM
I'm looking at my receipt right now from where I filled up in the town I was leaving. I put 12.61 gallons in the tank. According to Google maps, the route I took home was 438 miles. I did not have to stop to fill up again.

438 / 12.61 = 34.73 mpg

Alright, well I have to admit this logic is flawed. The car has an 18 gallon tank, so the fact that I put 12.61 gallons in it before leaving doesn't mean I drove the 438 miles on that 12.61 gallons.

Like I said, I'll be more particular when figuring the MPG on the beach trip in September. I'll make sure to use the DIC's "Average Economy" readout in addition to the other methods. However, I don't have to wait until then to compare the difference between the 'Average Economy' readout in the DIC vs. taking 'Fuel Used' dvided by 'Trip A' miles. I'm going to fill up tomorrow. Before turning the engine on after filling up, I'm going to:

1) Reset the 'Fuel Used' to 0.0
2) Reset the 'Average Economy' to 0.0
3) Reset the 'Trip A' mile counter to 0.0

It will probably be another 10 days or so before I need to fill up again, at which time I will do the following:

1) Divide 'Fuel Used' by 'Trip A' mileage
2) Compare the result from #1 to the 'Average Economy' in the DIC

I'll report back here with my findings.

ronm01
07-13-13, 09:20 PM
Unless needed for other reasons (only occurred once so far), I always use "Trip A" to track total miles on each tank of gas before resetting it. When filling the tank again, I calculate MPG directly from the gas pump total gallons to fill the tank divided into the miles traveled based on total miles in Trip A. This is a fool-proof way to calculate the most accurate MPG. After figuring, I reset Trip A for the next calculation of MPG at the next fill-up.

MoistCabbage
07-13-13, 10:24 PM
There's no way to tell precisely how much fuel is left when you start to fill up, and there's no way to tell precisely how much fuel is in the tank when the pump stops. The AVG MPG readout is still more accurate.

ronm01
07-13-13, 11:28 PM
There's no way to tell precisely how much fuel is left when you start to fill up, and there's no way to tell precisely how much fuel is in the tank when the pump stops. The AVG MPG readout is still more accurate.

Don't know if you intended this for me or another poster. I do know that the mileage traveled via the "Trip A" is directly from the odometer, so is precisely how many miles traveled since last fill-up. Refilling the tank tells me how many gallons purchased. Divide number of gallons purchased into miles traveled from Trip A and you arrive at the most accurate MPG. My "Fuel Used" displayed on the DIC is never in sync with the gallons to refill. How could you possibly dispute the hard math that is self-evident to calculate MPG based upon gallons used? Has nothing to do with in-car DIC calculations. Has everything to do with number of gallons purchased divided into miles traveled based upon odometer reading. Simple math. I understand your point, but will continue to believe my own calculation is superior to that on the DIC. Over time, this will give the more accurate account.

MoistCabbage
07-13-13, 11:40 PM
The trip meter is accurate, but gallons purchased doesn't tell you exactly how many gallons were used since you reset the trip meter, unless you're letting the tank run bone dry. As posted, there's always going to be a different amount of fuel left when you start filling up, and the pump doesn't always fill the tank to the same level.

It's splitting hairs, the traditional method is pretty accurate, I'm just saying the DIC readout is more accurate.

Go back in this thread to see how the DIC arrives at its number, and why it's more accurate.

ronm01
07-13-13, 11:48 PM
The trip meter is accurate, but gallons purchased doesn't tell you exactly how many gallons were used since you reset the trip meter, unless you're letting the tank run bone dry.

It's splitting hairs, the traditional method is pretty accurate, I'm just saying the DIC readout is more accurate.

Go back in this thread to see how the DIC arrives at its number, and why it's more accurate.

No thank you, my method suits my purpose over the long haul. Splitting hairs over MPG is not necessary for either of us, unless one of us is determined to have the last word or has a deep-seated need to prove a point. I defer to you, if you wish the last word. No offense taken or given. Have a great week ahead. :highfive:

MoistCabbage
07-14-13, 12:06 AM
:hmm:

??? Where'd that come from?

It has nothing to do with a need to prove a point or get the last word. I even said it was splitting hairs, and the traditional method was accurate.

Many people don't know how the readout on the dash is calculated (any car, not just Cadillacs), and/or think that the reading is altered to make the vehicle appear more fuel efficient, so they waste time and effort doing the math, only to arrive at a number that is less accurate than the number they could simply look at on the dash. I explained earlier in this thread, and elsewhere on these forums, how the number is reached, so that they can trust the number, and save time (if they choose to).


I understand your point, but will continue to believe my own calculation is superior to that on the DIC. Over time, this will give the more accurate account.Your post is a perfect example of why I explain it in the first place. You don't trust the number. I'm not trying to force anything on anyone, just offering facts for anyone looking.

ronm01
07-14-13, 01:04 AM
:hmm:

??? Where'd that come from?

It has nothing to do with a need to prove a point or get the last word. I even said it was splitting hairs, and the traditional method was accurate.

Many people don't know how the readout on the dash is calculated (any car, not just Cadillacs), and/or think that the reading is altered to make the vehicle appear more fuel efficient, so they waste time and effort doing the math, only to arrive at a number that is less accurate than the number they could simply look at on the dash. I explained earlier in this thread, and elsewhere on these forums, how the number is reached, so that they can trust the number, and save time (if they choose to).

Your post is a perfect example of why I explain it in the first place. You don't trust the number. I'm not trying to force anything on anyone, just offering facts for anyone looking.

Since you have asked, I will reply without trying to make it appear as though I want the last word. In response to your question, please accept my apology for my awkward and unnecessary end to my last post: "unless one of us is determined to have the last word or has a deep-seated need to prove a point." It s clearly evident to me that you wish to impart information concerning the accuracy of the DIC display. Thank you for drawing my unnecessary assumption to my attention. :bricks:

MoistCabbage
07-14-13, 01:12 AM
Since you have asked, I will reply without trying to make it appear as though I want the last word. In response to your question, please accept my apology for my awkward and unnecessary end to my last post: "unless one of us is determined to have the last word or has a deep-seated need to prove a point." It s clearly evident to me that you wish to impart information concerning the accuracy of the DIC display. Thank you for drawing my unnecessary assumption to my attention. :bricks:No hard feelings :thumbsup:

rebel7254
07-19-13, 10:22 AM
Alrighty folks, I decided to go ahead and fill up again today.

After I parked at the pump, my DIC readouts were as follows:

Trip A: 249.7 mi
Fuel Used: 13.4 gal
Average Economy: 18.5 mpg

As I mentioned previously, what I did before was take "Trip A" and divide it by "Fuel Used" in order to get mpg. Let's do that here:

249.7 mi / 13.4 gal = ~18.6 mpg

So the difference between the "Average Economy" and calculating mpg manually using the "Trip A" and "Fuel Used" is insignificant.

The meter on the pump said I bought 14 gallons. That's a notable discrepancy, but I do agree with MoistCabbage that the pump doesn't necessarily always fill the tank to the exact same level.

Like I said, I will post back here in September after the beach trip to let you know what kind of MPG I get.

Submariner409
07-19-13, 10:35 AM
Our fuel tanks have an air volume of about 1.5 gallons at the top of the tank to allow for fuel expansion and contraction with temperature - and that volume change is significant.

That air volume is one of the factors that clicks off Pump A at so much fuel backflow and Pump B at another. The only way to totally fill a fuel tank (nowadays) to the max is to slowly trickle in that last 3/4 to 1 gallon of gas.

The DIC indication of fuel used is very, very accurate, especially for quickie one-time fuel use looks. If you were to keep paper records over 5,000 miles or so you would then have a pretty good calculation of your average fuel consumption.

I can fill the STS and spend a week in and around DC and wind up with an average 13 mpg - one "tank" of gas. I can then fill up and head down I-95 for SC and get 24.8 mpg on that tank............... but the average for the two tanks is 18.9 mpg. It takes a LOT of long term highway driving to pull that long term average DIC figure up into the 20's. I never reset my average fuel mileage figure - I do reset trip miles and fuel used readouts. Right now, mostly rural driving, the DIC says 19.3 mpg long term average.

mgbumpus
07-21-13, 11:02 PM
I get a consistent 26-27 mpg on the interstate ave.@70mph. Tires are @ 33PSI cold. My best to date was 29.1 MPG from Duluth, Mn. to Janesville, Wi. (360 mls @ 72mph Ave.) BP-93 100% gasoline. It was a very hot day - Over 100 Deg last July. Locally I lose 10% if I use E10. Who knows what I would get get if I played nice @ 55 mph or 65 mph.

----------

I get a consistent 26-27 mpg on the interstate ave.@70mph. Tires are @ 33PSI cold. My best to date was 29.1 MPG from Duluth, Mn. to Janesville, Wi. (360 mls @ 72mph Ave.) BP-93 100% gasoline. It was a very hot day - Over 100 Deg last July. Locally I lose 10% if I use E10. Who knows what I would get get if I played nice @ 55 mph or 65 mph.

kman779
07-22-13, 12:54 AM
Wish I could get 33mpg. The best I get is around 19 in town and 28 on the interstate doing 65-68mph which is better than quite a few of the others posting here. I havent given premoum gas a try yet so maybe I will try it and see if I can get even better mileage.

tjwhite721
08-04-13, 11:50 PM
ALL,

The NORTHSTAR engine is made to be driven HARD! The faster you drive the better the engine performs and the longer it lasts. You guys can drive 55 or 65 mph and have all that carbon build up in your engine if you want. I personally average between 80 and 90 (MPH) when I drive on the highway.

There is no better feeling when you drive the DTS at 80+mph, with the windows down, feeling the smoothness of the car. The faster you drive the smoother it rides :-)

I average 20mpg driving 80+ mph. I would rather drive fast and average 20mpg, than drive slow and hope to get 25+mpg.

Submariner409
08-05-13, 09:50 PM
.............:yawn:

MoistCabbage
08-06-13, 01:13 AM
Going 20 MPH over the speed limit isn't going to do much for keeping carbon buildup down. Do some reading in the Technical Archives.

Chuckj19
08-18-13, 09:19 AM
Our fuel tanks have an air volume of about 1.5 gallons at the top of the tank to allow for fuel expansion and contraction with temperature - and that volume change is significant.

That air volume is one of the factors that clicks off Pump A at so much fuel backflow and Pump B at another. The only way to totally fill a fuel tank (nowadays) to the max is to slowly trickle in that last 3/4 to 1 gallon of gas.

The DIC indication of fuel used is very, very accurate, especially for quickie one-time fuel use looks. If you were to keep paper records over 5,000 miles or so you would then have a pretty good calculation of your average fuel consumption.

I can fill the STS and spend a week in and around DC and wind up with an average 13 mpg - one "tank" of gas. I can then fill up and head down I-95 for SC and get 24.8 mpg on that tank............... but the average for the two tanks is 18.9 mpg. It takes a LOT of long term highway driving to pull that long term average DIC figure up into the 20's. I never reset my average fuel mileage figure - I do reset trip miles and fuel used readouts. Right now, mostly rural driving, the DIC says 19.3 mpg long term average.

He never lied..

rebel7254
09-18-13, 10:55 PM
I'm back.

Believe it or not I actually forgot about this on the way down to the beach (it was a crazy morning and day), but I did remember to reset all fuel related DIC readings right after I filled up in Myrtle Beach. When I got home, I took pictures:

http://imageshack.us/scaled/thumb/826/59dx.jpg (http://imageshack.us/photo/my-images/826/59dx.jpg/)
http://imageshack.us/scaled/thumb/513/kz1f.jpg (http://imageshack.us/photo/my-images/513/kz1f.jpg/)
http://imageshack.us/scaled/thumb/32/jym9.jpg (http://imageshack.us/photo/my-images/32/jym9.jpg/)

As I anticipated, having the car loaded down with the extra passenger and luggage along with having to traverse the mountains brought down the MPG. But, I'm still quite pleased with 28.5 MPG all things considered. If you take the mileage and divide by the fuel usage manually, it actually works out to be slightly higher @ 28.8 MPG. Either way, as I said, I'm rather pleased and it's something I never expected with this car.

Submariner409
09-19-13, 09:26 AM
Your 2008 "DTS" - what is the 8th character of the VIN ?

I don't think we ever determined if your car has a 3.71 or 3.11 final drive - and most of the Devilles (base, DHS) with the 3.11 see 27, 28, 29 mpg on the highway, long term, with ease.

The acronym "DTS" for the new replacements to the Deville line is misleading because originally DTS meant the performance model, had the 3.71 final drive, and were lucky (like the STS) to get 25 mpg open highway.

Glad the trip went well - We're headed from here to Myrtle Beach on 2 October - 4 day submarine crew reunion. For me, it's a 9 hour drive.

Play with this for a while - but, as I posted before, using straight gas or E10 in either the car or trucks makes no difference in mileage for me -

http://pure-gas.org/index.jsp?stateprov=MD (http://pure-gas.org/index.jsp?stateprov=MD)

rebel7254
09-19-13, 09:40 AM
Submariner, I never claimed to have the performance model. It definitely has the 3.11 final drive. The 8th VIN character is "Y".

So with that in mind I guess my hope for becoming an instant millionaire is out the window? :D

Submariner409
09-19-13, 10:11 AM
No problem - ALL the VIN Y cars will beat a VIN 9 car in the gas mileage department.......... but the original 300hp/3.71 DTS association dies hard.

There was a mileage thread in Seville a few years ago, with spreadsheet figures - for an ESC on a long trip .......... that one got just about 30 mpg, including mountains, with 2 people and some luggage.

Superjim
09-19-13, 01:05 PM
I took a 500 mile trip over the past weekend...
VIN 9
Got about 20mpg...

big greg
09-20-13, 05:17 PM
No problem - ALL the VIN Y cars will beat a VIN 9 car in the gas mileage department.......... but the original 300hp/3.71 DTS association dies hard.

There was a mileage thread in Seville a few years ago, with spreadsheet figures - for an ESC on a long trip .......... that one got just about 30 mpg, including mountains, with 2 people and some luggage.

My 01 seville would always see 31-33 mpg cruising 65 mph. I loved it. Still gets about 29ish. 130k that car still rides and runs awesome

jazoo
09-21-13, 01:27 AM
Believe it or not I actually forgot about this on the way down to the beach (it was a crazy morning and day), but I did ........ home, I took pictures. ................As I anticipated, having the car loaded down with the extra passenger and luggage along with having to traverse the mountains brought down the MPG. But, I'm still quite pleased with 28.5 MPG all things considered. ...... slightly higher @ 28.8 MPG. Either way, as I said, I'm rather pleased and it's something I never expected with this car.

rebel7254, Congratulations, that you were consistently getting ~28.5 MPG with your style of driving and where & how you were filling up the tank. We are never going to come to a solid conclusion on this issue as there are too many variables associated with this subject matter.

Anyway, I applaud your persistency.