If the road permits it i will stick to 70 mph and 80ish if i am overtaking someone and then back to 70mph.
I have seen someone with the same engine on a Facebook group saying he gets around 500 miles out a full tank but I've never been able to do this. His technique was to drive at ~ 56mph, behind a lorry, accelerate as less as possible and use the engine brake often.( i don't have the patience or will to do this, it's already a slow yet economical car, i might as well drive it normal, not try to squeeze as much mpg out of it)
I can safely do 360 miles, anything after that i am quite nervous i will ran out of petrol.