I have a 2017 Mustang now too but I got the last V6. The V6 was the only engine in the lineup that runs on regular gas. The Ecoboost and GT only run on Premium fuel which costs so much more. I get 30 miles to the gallon with 300 horse power.
I had a '13 V6 and really liked it - got remarried and traded it in on an Outback (mistake) but now have a '19 Ecoboost with the 10 speed auto. If you keep it under 75, it will get 32-33mpg and I have seen 38mpg on long stretches with no downshifting. I use regular gas.
“The Ecoboost and GT only run on Premium fuel which costs so much more”
Only run or only run at full output? The Ecoboost motor in my Fiesta ST happily adjusts down to 87 octane without complaint and certainly without damage; I generally put mid octane in it unless I want to have some fun — in that case 92 octane with two gallons of E-85 mixed in. Will reflash to take full advantage of latter when 3/36000 expires.