Understand that certain plugs can last that long and may be unnecessary to change earlier but the OP is complaining about sluggish acceleration. If he has had a sudden drop in MPG it may be prudent to change them now. And, we don't know how long the OP has had the car or if different plugs were installed in it.

Suggestion: If X cars have a different maintenance procedure than non-X perhaps X5world could post it up as a sticky for us new owners.

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'03 X5 3.0, '98 M3 vert, '02 Audi TT, '99 323i