B737-300 FLT CREW FAILS TO ADVISE ATC OF REDUCED CRUISE SPD.

Narrative

WE LEFT THE GATE 10 MINS LATE AND THE FMC STILL SHOWED AN EARLY ARR IN SJC. WE WERE GOING TO UNDERFLY THE LEG BY 10 MINS OR MORE; SO WE THOUGHT WE WOULD TAKE THE OPPORTUNITY TO SAVE SOME FUEL AND PULLED THE PWR BACK TO CRUISE AT .70 MACH PER THE FOM. TRANSITIONED THROUGH ZAB AIRSPACE WITH NO COMMENTS. WE WERE ON ABOUT OUR THIRD ZLA FREQ AND THE CTLR ASKED OUR SPD. I REPLIED .70 MACH. HE THEN ASKED IF WE HAD A SPD ASSIGNMENT. I SAID WE DID NOT AND THAT WE WERE GOING TO BE ON TIME IN SJC AND WERE TRYING TO SAVE THE COMPANY FUEL. HE THEN ASKED IF WE HAD TOLD ANYONE THAT WE WOULD BE CRUISING AT MACH .70. I SAID NO. HE TOLD US TO DSND TO FL280. I RELIED THAT WE WERE ABLE TO GIVE HIM ANY SPD HE NEEDED. HE SAID; 'WE'LL JUST WORK AROUND YOU; BUT WE'LL BE IN TOUCH.' THIS WAS A SURPRISE TO THE CAPT AND MYSELF BECAUSE WE BOTH THOUGHT THE NOTIFICATION TO ATC ABOUT SPD CHANGES WAS MORE THAN +/-5%. WE HAD BEEN USING FMC ECONOMY SPD AS THE BENCHMARK INSTEAD OF TAS FROM THE FLT PLAN. WE ALSO HAD BOTH FORGOTTEN ABOUT THE ADDITIONAL REQUIREMENT OF +/-5% OR 10 KTS. WE ARRIVED IN SJC RIGHT ON TIME; WHICH MEANT WE DID IN FACT UNDERFLY THE LEG BY 10 MINS AND UNDERBURNED BY ABOUT 400 LBS. THIS INCIDENT HAS MADE ME REALIZE THAT ATC CAN NOT READ OUR MINDS. IF WE ARE TRYING TO SAVE TIME; FUEL; AND THUS MONEY FOR THE COMPANY; WE SHOULD SIMPLE INFORM ATC OF WHAT WE ARE DOING. AS PNF; I SHOULD HAVE BEEN MORE VIGILANT WITH MY REQUIRED RADIO CALLS AND PUT ATC IN THE LOOP. WE WOULD STILL HAVE ACHIEVED OUR GOAL AND NOT CAUSED A PROB FOR ATC.