Car Fuel Consumption Anomoly

kevhed72

This week I filled up my VW GLI w 93 grade, which I usually due because of mfg. recommendation. The gas station was out of 87!and 89 because of the massive fuel leak in Alabama into some river...which is terrible....but not the main point of my story. Normally I will have to refill once to get me through an entire 5 days of driving to work. Right now....my VW has a solid quarter tank left in her. Plus, two days this week I drove waaaay too fast to get to work during rush hours to arrive on time, which kills fuel mileage.
Any ideas on why / how this happened this week. I do also remember fueling up Sunday with the pump on the low speed because I heard somewhere this stirs up less particulate matter in the main station tanks to be pumped into your car.
Any thoughts....I am baffled....my car had about 76k on her.

motorhead43026

An anomaly I would say. Octane is the fuels resistance to detonation or what is commonly referred to as pinging. Anyway, if you car ran fine on the lower octane and did not detonate use it and save yourself money.

kevhed72

Whatever it was I'm hoping to replicate it....I'm going to refuel the same way later today and see what happens. I would brush it off except those 2 days I was flying which really burns up fuel....

Jim Shearer

Gasoline is a much more complex product than it used to be. Lots of different additives for a variety of reasons. Like detergents to keep fuel injectors from plugging up. And each refinery has its own special blend.

I keep close track of gas milage, and note that different brands can yield as much as 10% difference. (Although I have also occasionally gotten a bad tank of gas that dropped milage by 25%.) That fill-up may have been either a different brand of gas (sometimes suppliers make swaps due to availability) or it was just from a different refinery run by the same company.

Cheers, Jim

Jim Shearer

Forgot to mention: gas blends also change due to seasonal variations and from one region of the country to another, and this also affects milage.

heiney9

If your GLI is a turbo (you don't mention year, but I assume it is) then use premium all the time. You will get slightly better mileage as the computer doesn't have to compensate for the lower octane gas since it's a high compression engine.

I have a GTi w/turbo and ALWAYS have run premium as the manufacturer recommends. There is a reason they recommend it, high compression, turbo, direct injection are all a factors for the recommendation by Volkswagen.

That being said, it shouldn't be a huge gain in mileage, 2-3 mpg on average seems to be what the norm is.

As a very rudimentary explanation, you will make slightly more power with premium as the computer won't have to **** the timing and fuel parameters as much. When using lower octane fuel that's how the engine compensates to avoid pre-detonation which is that knocking or pinging sound one hears.


The above explanation is for Volkswagens (2.0 direct injection, turbo motors) as I have owned them my entire life. It may or may not be applicable in whole or part to any other specific model car.

Using premium will yield slightly better mileage, whether it's the entire reason for what you are experiencing; I don't know.

Keep using premium and see if it continues.

H9

P.s. I am not looking for any arguments from car guru's etc. What I have stated above is my experience and many other owners experience's with the said car in question as it pertains to using the factory recommended premium fuel vs regular octane fuel.

P.s.s Vanilla blanks out the word R-E-T-A-R-D. W-T-F?

heiney9

Jim Shearer wrote: »

Forgot to mention: gas blends also change due to seasonal variations and from one region of the country to another, and this also affects milage.

This is correct and here in the Midwest some stations are already changing over to the winter blend of fuel.

heiney9

Take into account weather conditions have an effect too, especially on turbo motors. The computer is constantly compensating for hundreds and hundreds of variances to give the best efficiency.

If you chip the car, you will get even better mileage, doesn't make sense to make more power and get better mileage. It's all in the tuning of the fuel trim and timing maps. Of course you can't be flooring it all the time using that extra power......lol

So taking into account the advantage of premium fuel, weather, traffic conditions and everything Jim Shearer mentions, and it's possible.

The best way is to track your mileage. Firstly you have the on board computer (which is pretty accurate over the long term but it's still an average over time) and you can do the math at fill up. That's the only way to know for sure what MPG you're getting and how much variance you are experiencing.

Volkswagens are notorious (as I'm sure other cars are too) for seeming like the first half of the tank burns faster than the 2nd half. It seems for me anyway that the first half is gone much quicker than the 2nd half.

H9

rpf65

I drive a 2011, F150, with the 5.0 liter engine. It's designed for E-85, and I tried that crap once. Bad performance and mileage. That being said, I put Valero gas in my tank whenever possible.

I've tried 5 or 6 different brands, and even some of the no-name stations, and have discovered that I get between 17 and 18 mile per gallon with Valero. Every other brand, with the exception of Exxon gets me 14 to 15 miles per gallon. I get about 16 with Exxon.

Doing the math, it's actually cheaper for me to pay the premium price for Valero, about 10 cent more per gallon than other/no-name brands on average. You may want to try different brands rather than a lower octane rating.

Probably the best example of using a lower octane ratin than the manufacturer recommends is lawn mowers. Cousin called me when she bought a new lawn mower, and asked if I could look at it because it was hard to start.

Asked her what kind of gas she used.

She told me the brand, and I asked what octane level.

She said the regular.

Told her that was the problem, almost all small engine manufacturers recommend at least 87 octane or higher.

She changed to the higher octane, and the issues went away.

Personally don't think there is an issue to occasionally use a lower octane rating than the manufacturer recommends, but I wouldn't make it a habit. The same with using a higher octane rating than manufacturers recommend. There are a lot of reasons manufacturers recommend different octane ratings for gas, just as there are reasons for the oil viscosity they recommend.

txcoastal1

Around here they rate 87/89/93

Emlyn

If the vehicle is a Jetta GLI, it seems it has a 14.5 gallon tank? I haven't owned a VW, but have noticed with Audi that their fuel gauges do not really leave much of a reserve in the tank when it registers empty and the last quarter tank drops quick. With Infiniti and Jeep vehicles with 24 gallon tanks, there was always between 3-4 gallons left in the tank when on empty.

Based on average fuel economy ratings for a GLI, one should get between 348 and 464 miles on a tank. As Heiney9 says above, if you are really interested in fuel economy obtained you'll have to do the math at fill up for a few weeks to see what the real fuel economy is each week. When I cared about fuel consumption, I used to get a receipt and write the mileage down on it at fill up then do the same thing the next fill up and calculate. I suspect there may have been a switch away from summer fuel. But, I also suspect there wasn't really a whole lot of difference in fuel consumption per mile driven in this case.

heiney9

In Canada what rating do you use? RON or AKI. There is MON also. but I think that's just used to calculate AKI. (Ron + Mon)/2 = AKI

RON is research octane number
MON is motor octane number
AKI is anit-knock index

Typically we have 87/89/93 AKI. But some stations have 95 AKI and some stations sell racing fuel which is 98 or 100 AKI.

Years ago Shell stations in the US had 95 AKI premium. Now (atleast here in the Midwest) the highest Shell sells is 93 AKI.

H9

agfrost

heiney9 wrote: »

P.s.s Vanilla blanks out the word R-E-T-A-R-D. W-T-F?

You used the word in the correct manner, which almost never happens on the internet! Vanilla works very hard to prevent incendiary language. :eyeroll:

ZLTFUL

My Jeep is VERY finicky for mileage dependent upon which octane gas I use.

I ran some tests over nearly identical 300 mile loops using a tank of 87 octane E10 and 93 octane no ethanol blend.

With the 87, over the course of the loop, 80% 2 lane rural highway, 15% 4 lane super highway and 5% stop and go, I averaged 59mph and got 25mpg.

With the 93, over the course of the same loop but with 1 section of 45mph construction zone of 15 miles, same percentages of highway, I averaged 58mph and got 29mpg.

Temps were within a few degrees and humidity and barometric pressures were almost identical.

But, most of my day to day driving is stop and go and the differences in mileage did not make the extra cost worth while. If I spent my time driving constant highway speeds, I would spend the extra for the higher octane but in the normal stop and go driving, the 87 octane E10 is considerably cheaper.

lightman1

And we all know what happens when you drive "your" interstate speed, Ryan.
Zoooommmmm!!!! Are those blue lights behind me???!!

Nightfall

Here in Colorado our choices are 85/87/91

tonyb

Nightfall wrote: »

Here in Colorado our choices are 85/87/91

Those are your choices for Pot, not gas.

BlueFox

These gas mileage figures are just placebo effects. You need to do a highly controlled A/B blind test to determine your mileage.

mhardy6647

BlueFox wrote: »

These gas mileage figures are just placebo effects. You need to do a highly controlled A/B blind test to determine your mileage.

Jeepers, sounds like you're talkin' about cables or somethin'

mhardy6647

In some seriousness -- the phenomenon is the anecdotal observation leading to a perceived correlation.

The classic example is the oft-believed occurrence of more births on the full moon. Presumably, obstetrics personnel would notice, on a busy night, that the moon was full. And an anecdotal correlation was (heh) born. There is, in fact, no correlation -- much less cause and effect.

There are also spurious correlations.

For examples, see, e.g.
http://tylervigen.com/spurious-correlations

kevhed72

While I have no mileage data to back up my observations....my fuel gauge finally hit a quarter of a tank today. I actually thought perhaps the gauge was broke and worried I was going to run out of gas somewhere. I was sure to fill up at the same station and same pump, on low speed, to see what happens with this tank. Perhaps a drop in the humidity has an effect on turbocharged engines....

mikeyb128

As a vw/Audi tech of 12 years, I highly recommend using at the very least 91 as recommended by vw. Not sure why you would put anything less in your GLI, assuming it's a 2 liter turbo. I have a 2012 golf R, a gti and a corrado, they all recommend minimum 91 octane in the fuel door.

kevhed72

Golf R......nice. Thats the GTI on steroids, right?

ken brydson

When I'm close to E, I fill up. Nuff said...

F1nut

The speed at which you pump has got jack to do with stirring up stuff in the under ground tank. That happens when the tanker delivers a load, but there are filters that capture the junk.

tonyb

F1nut wrote: »

The speed at which you pump has got jack to do with stirring up stuff in the under ground tank. That happens when the tanker delivers a load, but there are filters that capture the junk.

I think he means stirring up stuff in the cars tank Jess. Over time junk sits in the bottom of the tank and if your the type to wait until the light goes on warning you to fill up, your sucking in some of that junk possibly. Gas filters catch it, but what happens when that fuel filter starts to get clogged ? Less mileage maybe ?

I always fill up at a quarter tank to avoid that scenario, but the wife will drive it to the last drop. A losing battle.....

mhardy6647

I love the thought of a hot electric pump in a tank full of a mixture of gasoline, gas vapor and air, don't you

It really is a wonder that cars don't blow up left and right, every day (even without intervention by the Mob)!

charley95

nbrowser wrote: »

tonyb wrote: »

I always fill up at a quarter tank to avoid that scenario,

Also keeps the pump itself submerged in cooling gasoline extending the life of the pump itself. We always tell customers of this when they've had a failure and we usually get the truth easily, most times when we replace said pump in a near empty tank.

Well said! Most people don't realize the effects of continuing to run their tank near empty that shortens the life of the pump. I never go below a 1/4 tank and most times fill up at 1/2 a tank.

motorhead43026

I guess I am lucky, I run till my fuel light comes on plus 45-50 miles never had a fuel pump failure. I would assume as long as it had fuel to pump it will stay cool. I hear most men play fuel gauge roulette....lol. It drives wives and girlfriends crazy.

mikeyb128

kevhed72 wrote: »

Golf R......nice. Thats the GTI on steroids, right?

Actually not a GTI but a golf on steroids yes. And highly tune-able.

F1nut

Tony, no he meant the station tanks.

I do also remember fueling up Sunday with the pump on the low speed because I heard somewhere this stirs up less particulate matter in the main station tanks to be pumped into your car.