Volvo drivers: A gas and mileage question.

I was told to switch from regular unleaded gasoline to premium unleaded in order to improve the mileage my Volvo (S40 2002) gets. I still have an eighth of a tank of regular unleaded gas left. Will my car have problems if I fill the rest of the tank with premium unleaded gas? And most importantly, will I get better mileage if I use premium gas instead of regular?

Thanks for your responses!

Who told you to switch for better gas milage? Unless your car is supposed to use premium (check the owner’s manual) or you are in bad need of a tuneup, you will get better gas milage with regular. Premium gas has actually less available energy, but can be used at a higher compression ratio. Unless your Volvo is designed to use premium gas, stay away from it.

First of all, I second Telemark’s opinion that premium gas will just be a waste of money if your car’s manual doesn’t specifically call for it. Unless you’re planning to get rid of the car very soon, it would be better to get a tune-up.

Gasoline is a mixture of many different hydrocarbons. The different grades of gasoline just alter the proportions of some of them slightly. If your tank has a little 87 octane in it and you fill up with 93 octane, you might get a tank of 91.5 octane-equivalent gas, but it shouldn’t hurt anything.

My car “recommends” premium gas, but “allows” anything as low as 87 octane. I originally asked the consequences of this here somewhere, but did the experimenting myself. I used the onboard computer for determining mileage:

Premium (91 octane): 22.5 MPG average
Regular (87 octane): 20.5 MPG average

Let’s say the price difference is $0.20 cents (although I often see a larger difference!). Twenty gallons of premium gas cost $4.00 more than regular unleaded. So, Twenty gallons of regular get me 410 miles, while twenty gallons of premium get me 450 miles. The difference is 40 miles, which would have taken 1.95 gallons MORE of the cheap gas. If unleaded cost $2.05, it would be a break-even prospect. If unleaded costs less than that, then the premium is a waste of money. If the difference in premium is more that $0.20, then it’s an even bigger waste of money.

Well, I drive a 1980 Volvo station wagon (as I’ve mentioned before here) and get around 15 to 20 mpg. In winter I run it on regular, which works just fine, but I tend to put in the mixed octane (well, the stations mix the regular and the premium to get the plus, so that’s why I call it the mixed octane, you see) in the summer because in my experience it reduces knocking and pinging and the car runs better.

Oh, I know this one! I knew that having a job with Volvo as a technical instructor would come in handy on this board sooner or later. :slight_smile:
From the owner’s manual

Here is the deal, your engine is set up to produce excellent power with low emissions. To do both of these the computer controls the spark advance on each cylinder indivually. (the more spark advance the more power, up to a point) However if a cylinder starts to knock(too much advance) even before you can hear it, the computer will retard the advance on that cylinder until the knock goes away. if you were to run very, very poor fuel, the engine would run with so little spark advance you might think you were draging an anchor.

Here is what I suggest. Drive a couple of tanks with prem. See how the car runs. Drive a couple of tanks with mid-grade see how it runs. You allready have experience with reqular. make up your own mind. Also it does not hurt to mix fuels.
In my company cars I tend to run mid-grade or prem. depending on the model and my usage. When I have run reqular I tend to notice more sulfer smell from the exhaust.