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If haven't tuned up your car in a while, I would try that first. Depending on the condition of your ignition compoments, it may make a big difference. I would change out plugs, cap & rotor and perhaps the ignition wires as well. Stock wires go for around $65. If you've never replace the O2 sensors, that would a good candidate for replacement as well. On Turbos, you have a front and rear. They go for about $130 a piece.
FYI, there's been a lot of discussion on spark plug types and here's my experience. I've used both Volvo OEM and Bosch FR7DC (copper) spark plugs with good success on a '94 850T, '96 855T and '97 855GLT. The Bosch plugs are inexpensive and I usually just order them as part of a tune up kit from iPd which includes a Volvo cap & rotor (~$50). Some people have suggested using FR7DP (platnium) and I will probably try those next. If you have some money to throw around, you could also try the Nippon Denso Iridium plugs which run $12-$15 a piece and I believe the correct application is IK22, but double check that to be sure.
I did the plugs (FR7DC), cap, rotor and front O2 sensor on my friend's '97 855GLT last summer. Then a week or two later we drove 200 mostly highway miles driving between 60-65 mph and averaged 32 mpg! Quite an improvement from the 23-24 mpg highway that car was seeing.
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