Best Fuel Management

I Took My Suzuki Grand Vitara Into The Dealership The Other Day For A Service. Prior To The Service, Fuel Consumption Was 550 Kms Per Tank Averaged Over 3 Years Of Use (since I Bought The Vehicle New). After The Service Fuel Consumption Was About 470 Kms Per Tank. Engine Power Is Up About 10% (this Is Welcome Of Course). The Tip Of The Exhaust Pipe Is Now Very Black And Sooty On The Outside Of The Pipe. It Was Perfectly Clean For The Entire Three Years Prior To The Service. I Have Asked The Dealership What They Did. They Say They Changed The Plugs Only. One Year Ago The Plugs Were Changed By Another Dealer And Made Absolutely No Difference At All. I Think The "we Changed The Plugs" Answer Is Lame. I Think They Modified The Engine Management System Software. Any Ideas Other Than This?