What grade of fuel should I use for my car?

Question

I have a 2006 Volvo XC90. The manual says that the recommended fuel is 91 octane, and the minimum is 87. I found a forum thread with people recommending various things, but the consensus seems to be that while 91 is best, 87 is acceptable (and one person even claims they use 85).

At my gas pump I have the choice of 87, 89 and 91 octane fuel, thus running the gamut of possible fuel types I could consider.

Since higher grade fuel is substantially more expensive, I would like to know what benefit I am getting for the higher cost. I was only able to obtain vague suggestions that it leads to "poor performance". Ideally, I would just try a bunch of them and use what works, but I am inexperienced and so wouldn't know how to judge the performance of the car.

If I use low grade fuel such as 87 or even 86 or 85 in spite of the recommended 91, what negative consequences will there be?

• Will I have lower MPG? How much lower? (here fuel choice comes down to a simple calculation based on expected mileage and gas price)
• Will the car have less power (ie. drive slower or not pull as much weight)? (I don't really care since my needs are well below spec, so I'd use the cheapest fuel if this were the only concern)
• Will the engine accumulate more wear due to lower grade fuel? Will this wear result in inexpensive repairs, costly repairs, or irreparable damage? (unless the former, I would probably go with the premium fuel in this case)

As you can see there are multiple angles, and each one leads to different conclusions. As such I'm at a loss about how I should approach the decision. How do I know which fuel I am best off using?

Answer 1

Generally, you're best off using the fuel recommended by the manufacturer.

Octane rating is a measure of how resistant the fuel is to ignition. The higher the octane, the harder the fuel is to ignite. Fuel that is harder to ignite allows higher compression ratios and more agressive engine tuning.

If the fuel is too low octane, modern engines will detune themselves to prevent physical damage from preigintion, this affects both engine power and efficiency. How badly fuel efficiency is compromised depends a lot on how bad the fuel is. Often enough to completely wipe out any savings. Too low and the engine simply cannot run at all.

For very old engines (mostly carburettor designs) there is nothing to actively prevent preignition. Using too low a grade fuel can easily destroy these engines if preignition occurs.

A higher octane than the minimum necessary to run without detuning is just wasting money, as the engine cannot gain any benefit from the greater preignition resistance.