Hey members.
first I would apologize for my English. I live in Denmark and english is my third language. So I would like to apologize in advance.

I bought my second Mustang a couple of months ago, and it has been acting a little weird lately.
it's a 2014 3.7V6 Mustang

First thing is, and maybe it got something to do with the second thing.
When I press down my speeder, when I release my foot, it downshifts in 3 steps, ........ ...... ... like I tried to "draw" with the spots...

The second thing is that after I installed a Magnaflow axle back exhaust, and only that.
my idle is ticking a little bit higher, and my car went from a normal miles per gallon. To something that a humvee edition 1 uses .. Dont know what it is in miles since we use kilometers. But the normal is 8-9km per liter, and have been doing this for the 2 months I have owned the car up till the day I changed the axle back and it's down to 1-2 km per liter.
no engine light has come up, or anything .

So do maybe one of you have an idea ?
the thing is, in Denmark we have so little mechanics that know anything about American cars, so the ones we have charge 120$ an hour just to find the problem.