With stock 3.27 my gt turned roughly 2k in fifth at around 75mph. With 4.10s it turns around 2.3k at 75mph, or 2k at 65mph.
Cruise at 65-70mph and your mpg doesn't really change that much, if your car's tranny is like mine. I'm posting from my phone so idk which section this is in.
2016 Comp Orange GT (current)
Aurora - 2002 Boosted GT! (Sold, sadly)