https://www.motorverso.com/vw-golf-gti-mk6-review/
I just read this pleasant review of your model MK6 GTI. Looks like a fun, sporty little hatchback. Nice choice. I did notice the claimed combined MPG was stated to be around 38. That’s a good bit more than what you are seeing at 70mph. Perhaps the stated mileage was only considering lower highway speeds.
At just shy of 3000lbs, I’d think that little turbo 2.0 would offer you better numbers at 70 mph.
As a comparison, my 2016 Jeep Grand Cherokee SRT is 5200 lbs, all wheel drive, running a 475hp 6.2 hemi with wide performance tires. At 70mph I get between 25 and 26 mpg(while running in Eco mode (4cyl mode) CD is reportedly .371.
It’s absolutely dead factory stock. I would expect your GTI to get noticeably better mileage than my Jeep muscle car SUV.
Is it possible your car is out of tune somehow? Tired spark plugs, etc. It would be great if a minor tune up service could unleash hidden mpg for you.
George