Iridium spark plugs

I have a 1999 GMC Sierra 1/2 ton 4/4 with a 4.8 liter engine. At 75,000 miles I wanted to put in new plugs and wires. I was told that the best thing was to go with Delco Iridium plugs, but after installing the plugs and wires I lost just over 2 miles to the gallon, has anybody else had the same problem?

Did you regap them? They are suppose to be factory set to .040 and they recommend that you leave them at that gap.

Does it feel like there is a miss at idle?

//2000CAYukon

Did you regap them? They are suppose to be factory set to .040 and they recommend that you leave them at that gap.

Does it feel like there is a miss at idle?

//2000CAYukon
I did not regap them,I left them at factory of .040