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Gas mileage question
Old 02-20-2013, 07:57 AM   #1
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Gas mileage question

On Sunday past we drove up to Orlando and back, about 190 miles each way. A few days before I filled the tank at Costco. My car uses premium gas, and gets very constant highway mileage of 30 - 31 mpg. It also calculates and displays current mpg.

On the trip up I noticed the mpg was low, so I reset the mileage calculator. It then showed 26 mpg over more than 100 miles of pure highway and a near constant speed of around 70 mph. Very strange. I did not refill until the tank was empty, and then tracked the mileage for the return leg. Back to the usual 30 - 31 MPG.

My conclusion is the gas I bought from Costco was really regular. I verified that my purchase was sold as premium. I plan to complain to them and the local regulatory agency, but before I do I wanted to ask if there would be another cause for this.
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Old 02-20-2013, 08:08 AM   #2
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As I understand it, using regular vs. premium gas would have virtually zero impact on mileage. Chances are you had a pretty good headwind on your trip to Orlando. Wind and driving speed can have significant impacts to fuel mileage.

Myth: Premium gas gives you more miles per gallon than regular gas
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Old 02-20-2013, 08:10 AM   #3
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Knowing you, I suspect you've already considered the factors I know of:
  • Elevation changes (not likely in FL)
  • Temperature changes, especially with hybrids (amazing how much better our mpg's are in summer vs winter)
  • Weight changes (cargo and/or # of passengers)
  • Average speed (sounds like you held pretty constant)
  • Excessive idling (traffic, waiting for an accident?)
  • Headwinds/tailwinds (if they're strong enough)
  • With/without AC
Best of luck. I suspect you'll have an uphill battle with Costco since they can point to all the known variables above to discount your findings.
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Old 02-20-2013, 08:13 AM   #4
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Originally Posted by REWahoo View Post
As I understand it, using regular vs. premium gas would have virtually zero impact on mileage. Chances are you had a pretty good headwind on your trip to Orlando. Wind and driving speed can have significant impacts to fuel mileage.
My understanding also ethanol % can impact fuel mileage.
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Old 02-20-2013, 08:15 AM   #5
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Tire pressure? (just adding one more variable that can affect gas mileage).
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Old 02-20-2013, 08:20 AM   #6
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In an engine designed for for premium fuel the ignition timing is optimized for that fuel.

In the wonderful world of computer controls, knock sensors are monitored. The regular fuel would cause the knock sensors to detect pinging. The computer programmed to to monitor knock sensors, seeing the the pinging will retard ignition. Thus less power. Then to maintain the speed, the 'puter will add fuel via longer injector pulses. Thus more fuel for the the same speed, less mileage.
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Old 02-20-2013, 08:20 AM   #7
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IMHO, there is no one who would take your complaint seriously, way too many variables.
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Old 02-20-2013, 08:26 AM   #8
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From what I've read, using regular gas instead of premium might drop your mileage but not more than about .5 MPG. I can't imagine it could reach 4 MPG.
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Old 02-20-2013, 08:27 AM   #9
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The manufacturer spec requires 91 octane fuel. Tires are at spec and new. Wind conditions were similar each way. Highway speed was pretty constant each way. Return trip had about 150 more lbs. All other driving conditions were similar.

I have driven > 50K highway miles on this car and never seen less than 29 mpg avg when highway conditions were favorable and constant. This drive was 26 all the way, and I reset it at least 4 times.

The avg mpg went from 26 to 30 as soon as I refilled the tank.

The differential cost of gasoline would be about $3-4. I'm not asking or expecting a refund. The gas station may not be aware of this, and they should know.

Quote:
Originally Posted by ls99 View Post
In an engine designed for for premium fuel the ignition timing is optimized for that fuel.

In the wonderful world of computer controls, knock sensors are monitored. The regular fuel would cause the knock sensors to detect pinging. The computer programmed to to monitor knock sensors, seeing the the pinging will retard ignition. Thus less power. Then to maintain the speed, the 'puter will add fuel via longer injector pulses. Thus more fuel for the the same speed, less mileage.
This is why I think it could be the gas. Coincidental that the difference in $ per gallon is similar to the difference in mpg.
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Old 02-20-2013, 08:27 AM   #10
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I have noticed ethanol content will affect my milage. But not as much as you noted.
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Old 02-20-2013, 08:49 AM   #11
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I have noticed ethanol content will affect my mileage.
Of the fuel, or the driver?
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Old 02-20-2013, 09:07 AM   #12
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Maybe time for a control test. Get on empty, put a few gallons of regular in and retest. See what the mileage is on gas known to be regular grade.

This article from Popular Mechanics reported one of their test cars got significant difference in mileage. One of the key things seems to be looking to if your car "requires" or is " recommended" for premium.

7 More Fuel-Sipping Myths Debunked: Mechanic's Diary - Popular Mechanics
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Old 02-20-2013, 09:27 AM   #13
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Are you calculating the mileage manually vis a vis I drove x miles and pumped y gallons? New cars shouldn't be filled up to the top of the fill tube due to emission controls that capture gas fumes, they can be damaged by raw gas getting into them. If you did not start with totally full and then totally refill you have an inaccurate number of gallons of gas. Maybe your odometer is off and you think you are traveling x miles but really some % of x and if > than actual again an inaccurate calculation. A lot of variables. IMO today we can record the number of gallons used over say 1,000 miles and consider that a good guesstimate.
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Old 02-20-2013, 09:34 AM   #14
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It is interesting, but I wouldn't go back to Costco and complain on this anecdotal evidence.
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Old 02-20-2013, 09:55 AM   #15
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Quote:
Originally Posted by rbmrtn View Post
Maybe time for a control test. Get on empty, put a few gallons of regular in and retest. See what the mileage is on gas known to be regular grade.

This article from Popular Mechanics reported one of their test cars got significant difference in mileage. One of the key things seems to be looking to if your car "requires" or is " recommended" for premium.

7 More Fuel-Sipping Myths Debunked: Mechanic's Diary - Popular Mechanics
Good idea.

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Originally Posted by veremchuka View Post
Are you calculating the mileage manually vis a vis I drove x miles and pumped y gallons? New cars shouldn't be filled up to the top of the fill tube due to emission controls that capture gas fumes, they can be damaged by raw gas getting into them. If you did not start with totally full and then totally refill you have an inaccurate number of gallons of gas. Maybe your odometer is off and you think you are traveling x miles but really some % of x and if > than actual again an inaccurate calculation. A lot of variables. IMO today we can record the number of gallons used over say 1,000 miles and consider that a good guesstimate.
The calculation is done by the car. I know the odometer is off by 1% because I've compared it with GPS mileage.

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It is interesting, but I wouldn't go back to Costco and complain on this anecdotal evidence.
Good point. That was really the purpose of the thread, and now I have doubts. Another round of regular gas with highway driving would help. That will happen in a month or so, I just need to remember.

Quote:
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I have noticed ethanol content will affect my milage. But not as much as you noted.
Somethng I've not given any thought to. How much does ethanol affect gas mileage?
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Old 02-20-2013, 09:57 AM   #16
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Originally Posted by ls99 View Post
In an engine designed for for premium fuel the ignition timing is optimized for that fuel.

In the wonderful world of computer controls, knock sensors are monitored. The regular fuel would cause the knock sensors to detect pinging. The computer programmed to to monitor knock sensors, seeing the the pinging will retard ignition. Thus less power. Then to maintain the speed, the 'puter will add fuel via longer injector pulses. Thus more fuel for the the same speed, less mileage.
Good point. I had a Chrysler 4.0L (minivan) the maker recommended mid-grade (89 octane) fuel. Tried regular (87) during several hwy trips & saw consistently 10-15% worse mpg than mid-grade. OTOH- I had another Chrysler (300M with 3.5L) that they also rec midgrade. In that car I never could detect a difference between 87 vs 89 fuel.

Re OP- I could understand that 87 fuel sure could give worse mpg in engine designed for 91. And that somehow the 91 tank got filled with 87 by mistake. However proving the latter would be tough so long after the incident.
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Old 02-20-2013, 10:08 AM   #17
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Here is a snippet from another post about premium gas, that may help:
If your car requires high-octane gasoline and you habitually use regular gas because the engine exhibits no sign of knock, youíre outsmarting yourself. Most modern, computer-controlled engines include a knock sensor that detects knock and retards the ignition timing, causing the spark plugs to fire slightly later in the cycle. This typically prevents abnormal combustion and knock, which allows vehicles specified for premium fuel to run on lower-grade gasoline if it is all thatís available. While this removes the immediate hazard, itís a bad idea to make a habit of running a vehicle on gasoline of lower-than-recommended octane. Retarding the spark causes a richer fuel/air mixture, which decreases fuel economy, increases emissions, causes the engine to run hotter, and reduces the longevity of both the engine itself and the catalytic converter. The money you save by pumping low-grade fuel into a car that demands higher octane is lost anyway, in decreased fuel economy and possibly gradual damage.
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Old 02-20-2013, 10:58 AM   #18
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How much does ethanol affect gas mileage?
A quick survey of articles -- How much does ethanol affect gas mileage? - Google Search -- suggest a difference of 10% (in 10% ethanol vs. 100% gasoline, but most gasoline today is 10% ethanol).

The difference you noticed (31 vs 26) is slightly more than 10%, but not a lot more, so if you used 10% ethanol vs. 100% gasoline, IMO you're in the ballpark. YMMV.

Tyro

PS: We had an engine light go on a couple weeks ago (one that we didn't recognize). Looking it up in the owner's manual, it said one reason for the light to go on is [crappy] gas. The mechanic suspected this was the reason as he could find no other, but said it would take another tank or so (for the crappy gas to work out?) for the light to go off after he reset it.
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Old 02-20-2013, 11:14 AM   #19
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you might want to consider water in the fuel. If you have ethanol the water actually mixes with the fuel. if regular gas then the fuel and water separate, but still I owned a convenience store and know that there is water in fuel and if they just got a load in their tanks then it might have mixed the water and fuel before separating again.
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Old 02-20-2013, 12:05 PM   #20
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Don't forget, YMMV.
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