Fuel Mileage Regular vs Premium

[Texas Jim]

Well, I figured this trip was a good time to experiment with running regular in the DTS. Some of you have good luck using regular, so I thought I would give it a good try.

I used three tanks of regular in a row... almost 1000 miles.

Leaving home last Friday evening, filled up with Regular, about 50 miles n of home. I had between 1/4 and 1/2 tank of Premium at the time.

Mileage was 19.8. Hmmm..not TOO bad but not good either. I usually get 21.5 to 22.

Second tank of regular, I let it run down to below 1/4 before filling up again. 19.1

Third tank, again I let it run down to less than 1/4. 19 even.

Filled back up with premium (a BUNCH of times) ... mileage back up to my standard 21.5 to 22.5 with an occasional 23. Driving and speeds have been pretty much the same the whole trip, except for a 100 mile stretch yesterday when I ran it faster then usual. That tank is 21 even.

The mileage, I might be able to live with, but it seems to lose about 50 HP on regular.

It seems a little sluggish and don't JUMP when you step on it. Thats the part I don't like.

When you know what it CAN do, and then it don't DO THAT... looks like I will have to bite the bullet and keep putting Premium in it. Darn it, I was hoping for a different outcome.

I really do wish that it would run ok on Regular. It would save me a considerable amount of money, thus the reason for this experiment.

Maybe when (IF) I get the HPTuners stuff, I will try TUNING it for regular and see if that helps.

[jking220]

Ive been wanting to try this experiment myself. Ive always run regular in my 01 STS, and my milage aint great, maybe 17 or 18 (mostly backroad and city driving with some highway mixed in), but neither I nor my gf baby it. I can live with 18, but if I could get more for my money, Ide run premium, its still only 20 cents more in my area.

Now as for the performance issues, I may fill her up with premium tonight and start running some tanks through to see if I can feel any difference. I couldnt imagine my car feeling like it has 50 more horse, it already almost goes in the ditch when you punch it. My dad had a 99 STS that we always ran premium in, and I dont think it was as fast as my 01 on regular.

He just bought another 99, running premium, Ill have to line up next to him and see who's faster.

[Cadillac Jim]

The gas mileage figures of 19 and 21 may differ for different tests, because the percentage of alcohol in the different grades depends on what station you fill up at and where it is located. But, using that, if regular costs $3.62 a gallon but premium costs $4.00 a gallon, you pay the same 19 cents per mile, and you have the extra 50 hp. That's a no-brainer for me.

I think that your car automatically tunes for regular when you put it in and the knock sensor tells the PCM to re-learn. I doubt that a new chip will do much more than make the default tuning for regular gas rather than premium gas.

When my car was about six months old, I was asking my dealer repeatedly about performance chips, what was available, did they sell them, and what would be the effect on my warranty. The result is that they reprogrammed my PCM when I had it in for my next oil change, producing a noticeable difference in performance. The downsides are a perceptible hum at idle that I would not much notice, but a 35-year-old investment banker that had traded in his Mercedes would not like, and a few other things like that which mostly escape me at this late date, like a tendency to chirp the tires on take-off. Yes, your dealer can do it with a Tech II, and some of them know how. I've been running that way for 11 years and I like it. I think that this is not uncommon, once the dealer ascertains the nature and preferences of the owner, and that's why some ETC/STS cars are distinctly faster than others for no perceptible reason.

[gemmichelle11]

Texas Jim,

I suppose my CTS is much different than your DTS. I put premium gasoline in my CTS one day, and it hated it! The car would hardly start for me the first time every day until the tank was used up.

Also, I am very disappointed with the gas mileage of my CTS. It gets only 20 mpg, regardless of how I drive. I've driven a crossover SUV and a pickup truck that got 20 mpg. My previous car, a 2000 Chev. Impala V-8, consistently got 24 mpg in town, and more on the highway.

I am always asking why my CTS gets so little mileage, being a smaller car, just the size for a lady!

[Bruce Nunnally]

Now I think Texas Jim is quoting only highway mileage at 19 mpg or 22 mpg. When you say your 2003 CTS gets 20 mpg I assume you mean in mostly city driving or mixed city/highway?

I regularly get 25-26 mpg on the highway in my 2005 CTS 3.6L V6, so I would be surprised to hear that you get only 20 mpg in the 3.2L 2003 V6.

[gemmichelle11]

I drive a mix of highway and city miles with my 2003 CTS. With all my cars and trucks, I have kept track of the gas mileage. I suppose the 20 mpg is primarily reflective of the city miles I usually drive. However, it consistently gets 20 mpg per my calculation, and the electronic screen shows to be 21 mpg. It has never gotten more than 21 mpg, to my disappointment.

I drove a 2001 Chev. Avalanche V-8 that got the same mileage under the same driving conditions, same calculation. I'm not sure my CTS is operating to its optimum standard, to be honest with you. People are always in disbelief when I mention the mileage to them.

This could be a result of my driving habits, too. I drive with my foot in the floor, and hardly ever drive the speed limit. But, again, I've driven all my vehicles the same.

It dismays me even more to learn that my CTS should be getting better gas milege. I had wondered if others were the same as mine, or if the CTS was geared differently, even though it's a small car.

Caddyinfo, does your CTS burn premium fuel well?

Thank you for your response and discussion.

[gemmichelle11]

Hello, Texas Jim,

I just read your post about your road trip itinerary. It sounds so exciting that you and DW are seeing the sights in your Cadillac! I'm amazed how different the Cadillac models are in HP, mileage and performance.

My 2003 CTS prefers regular gas and gets, to me, low gas mileage for the conditions which I drive. I live in Texoma, so you can relate to my driving conditions. My CTS has no lack of horsepower. I like that it's really responsive and full of pep! I like to check out passing gear occasionally!

Have you noticed any mileage changes due to the weather or the terrain? What is your destination before you head back home?

Drive safely and have fun!

[Ranger]

FWIW, I drive MOSTLY highway and am averaging right about 24 MPG using regular. I think I got 26+ on the way home from Texas when I bought it. If you do a search for "Stats" you'll find time, speed, mileage etc.

[Bruce Nunnally]

Caddyinfo, does your CTS burn premium fuel well?

I use regular unleaded unless I am doing acceleration testing for the site. Certainly if your foot is on the floor that will have a large impact on your fuel economy. As long as you are enjoying the CTS then all good in my book.

[TDK]

Folks,

I don't know if you notice the octane rating in your respective areas, but around here we have 89 Premium, 87 Super and 85 Regular.

My youngest son says Regular in his area is all 87 octane.

I have less of an idea of what methonal does to octane, but I would guess there are considerable differances.

Either Caddy seems to get about 23-24 mpg at 82.5 average speed, (with a few runs up into the triples), regardless of fuel. However, on trips I do usually run premium to have "all of it" available for passing or whatever. It runs whatever will burn around town (which is very little).

Later,

[Cadillac Jim]

In NJ our octane ratings are (R+M)/2, which means the average of Research and Measured octane. I presume that Research octane is that which the gas has as it comes out of the blender at the refinery, but before the butane and such boil off somewhat. Our premium octane runs 91, 92, or 93 octane, with Sunoco offering 94 octane. With premium rated 89 octane, I suspect that your pump ratings are measured octane. Frankly I would rather see that.

[joeb]

GMS06, a 2000 impala would have a V6, not a V8? either a 3.4 or 3.8. not sure of the motors they had but I know they did not have a V8. i would expect a V6 to get mid-20's on the highway.

[gemmichelle11]

joeb, you're correct! I believe it was the 3.8. You jogged my memory. Also, I think it was a 2001, not a 2000. It got 24 in town and at least 26 on the highway. I traded it in on my 2003 CTS. I recall now that I thought the engines of my Impala and my CTS sounded very much alike. Thank you for the clarification.

[Ranger]

I have less of an idea of what methonal does to octane, but I would guess there are considerable differances.

I don't Ethanol think anyone is using Methanol anymore, but Ethanol increases the octane rating while lowering the BTU and thus the MPG.

[Texas Jim]

Ethanol increases the octane rating while lowering the BTU and thus the MPG.

You got THAT right. On my road trips, it makes a little over ONE MPG difference in my mileage. I now have over 45,000 miles on the DTS. The first DTS, had over 35,000 on it when it burned. I have used a lot of fuel in those 80,000 miles. Some of it with ethanol and some without ethanol.

The net savings to ME, by using ethanol, is minuscule. Sure, there is 10 percent less fuel used for the stuff that goes in my tank... but I burn MORE OF IT... so the net savings just ain't there. Plus my car runs so much better on fuel that does NOT have ethanol in it.

In my opinion, the whole ethanol thing is a big scam... boondoogle... fraud... and just about any other name you want to call it.

But thats just MY OPINION. I am not a fuel engineer... I am just a lowly computer tech.

[Texas Jim]

Folks,

I don't know if you notice the octane rating in your respective areas, but around here we have 89 Premium, 87 Super and 85 Regular.

Later,

Unless I am mistaken, the Shell station you sent me to, Premium was 91 octane.

I was able to make it to Salt Lake City on one tank... but JUST BARELY. Filled on on the south side of town... Murrey. Put 16.6 gallons in my 18 gallon tank. I usually never run it much below a quarter. Filled up again in Grand Junction and was in the Holiday inn in Denver at I-70 and I-25 at 8PM.

[Ranger]

In my opinion, the whole ethanol thing is a big scam... boondoogle... fraud... and just about any other name you want to call it.

Of coarse it is. Lobbyists from corn producing states. Nuff said.