Lower MPG after replacing spark plugs

Any time in recent memory I've bought plugs they were properly gapped. I'd speculate the auto manufacturers work with the plug manufacturers, are not gonna spec a plug and then stipulate an oddball gap. And the manufacturing process seems to include a careful and accurate pre-gapping. Some even caution against trying to adjust gap.

I don't think it's been mentioned, torque spec (from the Repair Manual) is 15 ft/lbs, with dry threads. It's not easy to find that spec: there's a section on sparkplugs, but the torque for the plugs is off in another section, lumped in with the valve cover removal instruction.

 

Did anyone find the solution? I just did the change from Denso Iridium to NGK Laser Iridium, and noticed my MPG drop 6 MPG.

 

How do you calculate your MPG?

 

Just did the same thing (replace coils and plugs) and my mpg has dropped about the same amount. I'm just using the indicated numbers from the display to calculate the mpg. I didn't think it really mattered how it was calculated since it was a direct comparison using the same measurement method. I re-did the torque to 15ft lbs and it improved a couple of mpg. I had the plugs overtorqued on the initial install. Is there a reset process for the computer that will reset everything to default? It seems that a reset would better allow the car to establish a new baseline for engine management and therefore reset the mpg to "factory" outputs. I got 52-56 mpg from the day I rolled off of the lot back in Dec 2009. I saw one post that said to put all of the old stuff back in and see what happens. Very tempted to do that but it seems to completely defeat the purpose of "preventive maintenance". Starting to believe that the Prius doesn't really need or like preventive maintenance with all of the statements of high milage vehicles (>200-300K miles) that are still running the original coils and plugs.