Is there really a 20 cent difference where you are??
Yeah it’s always like that, right now regular is about $2.89, mid-grade is $2.99, and premium is $3.09.
I have a Passat (1997, supposedly the worst year for them) and I use the mid-grade. My manual says something like “for best performance, this fuel is recommended” but I don’t think it says “required.” Everything is so freaking expensive to fix on this car that I am not going to risk it. My husband, an over-all car guy but not a licensed mechanic, says it makes a difference in the way the car runs, and the price difference is not big enough to make a big deal about it. Either I spend $30 a week on gas or $33.
It so happens I drive a 2004 Passat, and wondered the same thing, so last year I cooked up a little test. I drove for three weeks on 87 octane, three weeks on 89 octane, three weeks on 93 octane, and three weeks on a blend of 89 and 93 octane.
I drove my regular route from home to my son’s daycare to work, then directly home. All city driving about 10 miles each way, but with lots of hill, so I do a lot of coasting. I tried to avoid using they car on weekends of errands, but was not totally successful. Using the car’s trip computer I recorded the following results.
87 Octane:
Miles: 338.4
Average speed: 24.8
MPG: 30.4
89 Octane:
Miles: 334.8
Average speed: 25.2
MPG: 31.8
93 Octane
Miles: 342.1
Average speed: 25.0
MPG: 32.2
Blend:
Miles: 339.4
Average speed: 24.9
MPG: 33.4
Like I said, I tend to coast when I can. I also turn off the engine at traffic light that I know last longer than a minute I’m sure the differences would be more pronounced if I engaged the engine more often.
My conclusion is that while I didn’t notice much difference in performance, my car’s computer noticed a difference in fuel consumption.
:rolleyes: While you’re at it, why not just use the same fluid for power steering, power brakes, and engine lubrication?
This is a poor idea; what you’re saving in fuel you’re losing in bearing wear (not to mention the loads on your starter, alternator, and battery which will reduce their lives). The lubrication of the engine bearings depends upon a thin film of pressurized oil, which essentially isolates the crankshaft from the bearings under all but heavy loads. The pressurization is due to the movement of the shaft itself, due to the viscosity of the fluid (i.e. when it spins it drags along a layer of oil with it). Every time you stop the engine, this layer is squeezed out, and when you restart the engine, the bearings are dry, causing a brief moment of direct contact friction and accompanying wear. Unless you’re going to be sitting around idling for more than a few minutes, it’s really better just to leave the engine running; most car engines running at idle only sip a very small amount of fuel, so the real savings you get from shutting the engine off for a minute isn’t much.
The fact that your blend of 90 and 93 got the highest fuel efficiency (and all other numbers differ only by a percent or two) suggest that it doesn’t really make much difference in your car. I would still go by what the owner’s manual says to use, and would tend toward what is recommended. It’s true that modern fuel systems will detect detonation and adjust compression accordingly, but this doesn’t make it a healthy state to operate in; as Rick states, even after adjustment detonation is possible, as is damage to the cat. Both of those are expensive things to replace. It’s your car, but I tend to place more confidence in the engineers who designed it than casual experiments which don’t control or observe all conceivable consequences.
Stranger
Very interesting, Maus Magill. Of course you’re not done yet, we hope you also have the dollar figures to calculate whether lower octane gas saved you any money. 
Just out of curiosity, how much gas would this save? I assume not a heck of a whole lot. Plus wouldn’t the savings be outweighed by the safety of not having a car turned off in the middle of a street?
Perhaps several of you missed the posts where Volkswagen (the people that built the engine, and who know more about it than you do) say that premium fuel is required.
Here I found a copy of a Passat owner’s manual allow me to qoute:
“You vill use premium fuel, and you vill like it. Ve have tested different fuels, and ve know what vorks, and vhat does not vork. Only a schienhunt vould try to get by on regular. If you try and use regular in shis fine Sherman automobile, you vill have to speak with the [del]Gestapo[/del] our consoomer affairs department. Ve will also need to see your papers. Papers, ve must see your papers!”
seems pretty clear to me. 
I don’t know if I’m feeding a troll, but of course I wouldn’t use the same fluid for power steering, brakes, and engine lubrication. Let’s compare apples to apples.
I would try a cheaper grade of fuel and see how things went. There are many variables involved; the car, the fuel, the driver, the environment the car is driven in. Just because someone along the line at VW says use premium fuel doesn’t mean the car couldn’t possibly be driven efficiently with regular fuel.
This isn’t “apples to apples”, and it’s not just a matter of efficiency; by the time you “see how it went” you may have done irreparable (or at least very expensive) damage to your engine and exhaust system. If the manual says “Use premium fuel” then you should use premium fuel. To reinforce this, they don’t just stick it on an obscure page in the manual, but also print it on the instrument console adjacent to the fuel guage and typically on a placard inside the fuel access door. Do you think that they’re going to all this trouble because it is optional?
Will they put me in “The Cooler” if I don’t cooperate? 'Cause I’ve always wanted to be a Cooler King.
Stranger
You must have the “bad cop” version of the manual…mine just says that if I’m a good boy and follow the fuel recommendations in the manual for the life of the car VW will send me a free case of frozen Schnitzel 
And, just to reiterate, I am fully convinced by this thread (and googling) that there are two kinds of premium fuel cars: those that suggest it, and those that require it – and that I am in group two.
I’m not going to experiment with my baby.
Obviously it is optional because I can put what I damn well please in my cars.
High octane is not optional if you want to get the horsepower figures the manufacturer quotes you. They require the high octane to get the maximum performance out of the engine.
If you’re willing to accept less performance a modern car’s engine can cope with lower octane. As long as your engine’s anti-knock system does it’s job, you’re fine.
Let’s go to Wikipedia: Octane rating - Wikipedia
Furthermore:
So yes, premium gas is necessary to get all 300 horsepower out of your Mustang GT, but if you drive conservatively I submit that you may very well save money and do no damage by using lower octane fuel.
I live at over 4500’, and have to use premium gas in my 2003 Vibe GT. The octane rating at this altitude for premium gas is 91, which is, IIRC from trips “downhill”, lower for the same type of gas at a lower altitude. So if you drive up in the mountains, the octane is adjusted already by the gasoline companies.
control-z, you seem to have this forum confused with IMHO. Your willingness to risk engine damage doesn’t actually mean that the VW Passat doesn’t require high octane fuel.
The people who built the engine, VW, are the ONLY people who have the detailed information necessary to determine if that particular engine requires a particular octane rating, and have issued a statement on the topic. They have said high octane is required, the requirement has a legitimate basis in IC engine theory, I question how anyone outside of a disgruntled VW engineer can suggest they are wrong.
And you’re also free to use mineral oil in your engine lubrication. But don’t go crying when you have to do a rebuild after a few thousand miles. Wikipedia is not a credible source for authoritative information; your car manual is. If you decide to run nonpremium fuel in an engine where the manufacturer mandates the use thereof, it’s your choice, of course, but to claim that the only difference is that the engine will run at lower efficiency ignores both the experience offered by an automotive technician trainer (who stated that anti-knock provisions may not be enough under load to prevent detonation) and the long history of emprical knowledge from the manufacturer of the engine.
Stranger
On a side note, follow all OEM oil reccomendations super-close.
That hot little engine loves to sludge motor oil.
Let’s do a little math here shall we?
Using Maus Magill’s numbers from upthread on the same model car there is about a 2 mpg improvement using premium. On a gas tank of just over 15 gallons that works out to saving about one gallon per tank.
Regular gas is right at $3.50 / gallon here in LA. The extra cost of premium is 0.20/ gallon. Looks to me like it is about 0.50 cents per tank more expensive to burn regular. :smack:
When you add to that the people that know the most about the engine say that premium is required, it seems to me that anyone that is arguing for running regular in this particular model car is stepping over a crumpled $10 bill to pick up a shinny penny. :dubious:
My pedantic nitpicks:
-Compression is determined by engine design and condition. A higher compression engine may require a higher octane rating fuel, but just putting high octane fuel in the tank does not change compression.
-At lower air density, ignition needs to be advanced, not retarded, for best efficiency, and cars have been doing that since at least the 1960’s (Vacuum advance we called it, and we LIKED it!)
I didn’t intend to imply otherwise; using high octane fuel in a naturally aspirated low compression engine gets you nothing. In a high compression (turbocharged or supercharged) engine, however, it’s a necessity. Modern fuel management systems have a good deal of intelligence built into them to optimize the fuel mix for efficient combusion. Trying to second guess this by using something other than the recommended fuel composition is like buying an HP RPN calculator and then trying to write a program to make work like a normal algebraic calculator.
Stranger