Several times I've done test runs on level ground on my way to and from work.. Checking and restring my mpg each time... Sport mode makes no discernible change in MPG.

Then again I got the three pedal model.. So the auto trans shift points don't come into play.

However, it does make you want to full throttle it a bit more.

It stays on. Heck I can get a solid 38 mpg on the highway.. Sport mode or not.

I generally leave it on... Keeps the engine cooler and drops oil temps by a boatload. (At least for 2013).

And I like the pop and gurgle/backfire.

 

Interesting info on the mpg, my auto gets about 5mpg less if I leave it in sport but it may just be my right foot causing this.

I also notice my tail pipe soots up more in sport!