Gears & Axles - changing gears what should i expect

i am changing my gears out of 2001 z2 automatic from 2.73 to 3.42's. I have a trex cam and stage 3 heads with 1 7/8 longtube headers or y-pipe to a 3 inch cutout, ls6 intake 80mm tb, 36lb injectors and slp lid. what kind of difference in these gears will it make?

you'll be faster! quicker out the hole, go through the gears faster, higher cruising rpms, less gas mileage on the highway, speedometer will be off unless you can change that in the computer.