I've heard 70km/h elsewhere, but the only personal data I have is I used to drive a 2002 5-speed manual VW Golf Mk IV, the 2L petrol version, and that had a fue

I've heard 70km/h elsewhere, but the only personal data I have is I used to drive a 2002 5-speed manual VW Golf Mk IV, the 2L petrol version, and that had a fuel economy display on it plus a nice chart of economy per gear in the owner's manual. The book similarly said best would be at around 70km/h (~45mph) in 5th gear, and I confirmed that was true in my own tests. The 6-speed GTI was charted in the owner's manual as well and its best economy was shown as a little faster, in its extra 6th gear.