My '91 yields about 2 mpg less on 87 versus 91, and gets better mpg on 92. YMMV. but for mine's, it matters.

-- Kane ... who gets better numbers from Unocal and Chevron too.
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While I would never deliberately mislead anyone, take into consideration that any information and advice provided was at no cost to you.

I have the 16v DOHC, and I can notice a significant difference in power and smoothness in the engine depending on the octane level, so I do not put anything but the highest octane in it (91 or 92 R + M/2). Volvo recommends 91 or better R + M/2 octane and a minimum of 86. In the turbo models, they strongly recommend against anything under 91.
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Manual tran 16v Wagon... Booyah!

When reading your owners manuals, be sure you are matching the correct "octane" numbers with the fueling system used in your country. In the US, we use (R+M)/2 to determine our "octane" rating, while the European countries use true Octane numbers (wich are slightly different from ours). This may be where some of the confusion lies. If you post what octane you use in your car, be sure to post your country and/or Octane system (true octane or R+M/2).

FYI, it would be adviseable to check your knock sensor for proper operation every once in a while (every timing belt change or so). You can watch the sensor output by hooking up an oscilloscope to the knock sensor and gently hitting the engine block with a hammer. If you knock sensor is non-functional, you ignition system should automatically retard the ignition a huge amount (for safety), and you'll be wasting fuel and have poor acceleration.

I'm eagerly awaiting the arrival of a replacement knock sensor for my 780 (B280). The PRV-6 feels very underpowered. If you need a knock sensor for a 4-cylinder (B230), they show up from time to time on eBay (new and used). There were a few that ended last week for less than $5 each.

God bless,
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 255k miles.
'88 Black 780, PRV-6, 148k miles.

My '91 744T runs fine on 89 octane "plus", but the '94 944T pings on anything other than 93 octane "super". No mods to either car.
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Mike W., six Volvos (four that run), Largo, FL

I've always used the lowest octane gas for my wagon (in sig). I've never noticed any pinging and I think - but am not sure - 87 is recommended in manual.
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Norm Cook; Vancouver BC; 1989 745T 206,000KM

Mine runs fine on 87, which what it was designed to run on. There is much confussion about what octane ratings mean. The best explaination is from http://science.howstuffworks.com/gasoline3.htm . Check it out.
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Paul NW Indiana '89 740 Turbo 110,000

My 87 runs fine on 89 at stock boost levels. The APC system that goes to 14.5 psi hates the 89 and will not let me go over 7.5 psi.