I think economy is just good driving.
If you are planning, there is very little need to brake just lift off early.
Mentally, put half a dozen fresh eggs on the passenger seat, now drive to keep them there.
Keep engine revs down, on most road cars max torque is around 2000 - 2500 RPM.
With today's computer managed engines most cars are comfortable at low RPM even in high gears, OK they won't accelerate quickly but when you need to, change down, that's what that stick thing is for.
Providing the road is clear, you can corner without slowing as much (no brakes) less fuel used to get back up to speed. (just keep throttle neutral not accelerating or slowing)
Waiting to overtake, when it is clear, change down, as soon as it's done back into high gear.
Brakes are for stopping, gears are not for slowing, worn brakes are cheap, worn gearboxes are expensive.
Open widows increase fuel consumption by about 15%, air con about 10 -12% all today's cars heating system can give you through ventilation (there are automatic vents at the back somewhere)
Roof racks cost have drag, about 15 - 20%.
So, plan, drive smoothly, stay off the brakes, remember a cars wind resistance (drag) is the cube of it's speed but don't compromise safety (free wheeling compromises safety, the car is out of control)