Usually you will notice an increase in braking efficiency: When switched on, the photons being emitted cause a force equal in magnitude, but in the opposite direction (Newton). Hence, helping to slow you down. With enough lights, you can actually stop without using the brakes. Syndicate can probably attest to this with his HID's.
I didn't note any difference with all the shrockworks stuff, Bumpers, sliders, skids. My test was a Trip from Cleveland Ohio to Parris Island Sc, factory X 20 21 MPG, So I added all the Shrockworks stuff on and did it again, same difference. ODD as it may seam. 2nd trip I had no Dog No Wife and that was it, so, All the Shrockworks stuff must weigh as much as my wife and my dog! No jokes about my wife, she is so thin when she puts on a fur coat people think she's a pipe cleaner. I found more of a difference with the Tires on than anything (285 75 16 BFG AT's) The lights and roof rack don't seem to matter much either, I went 1850 Miles one way to Moab with a Full load, Bikes on the rear, Roof Rack, Lights, Gigantic roof bag, totally shrocked and got 17 MPG going 70MPH most of the way or better. So, do what ya want the largest difference in MPG will be noted when you put on large tires. The rest is minimal.
17mpg? :cheers: you should be damn proud! I've been texcking my mileage since I bought my x around 18k miles ago and I'm just over 14mpg. And I'm still runnin the 265s. What gives? Guess maybe I'm a little hard on the skinny pedal.