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Agree with ard...
run a couple of full tanks, in your normal driving routes, do the arith, then see what your mpg is, assuming you 'knew' the usual 'average' before the plug change.
Imo, current plug technology combined with spot on engine monitor electronics, EFI et al, allows for a long time/lots of miles before a set of new plugs will offer any benefit.
Maybe the genius tech left something undone or screwed something up during the plug change.
GL, mD
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