On slower roads
It's not always so easy to cruise at a steady speed on slower roads, but you can still save a packet. Just because you see a white sign with a diagonal black slash through it doesn't mean you have to hit 60mph.
Stay at 40mph, slot your car into as high a gear as possible and you'll save around a quarter in fuel bills.
That's before you consider the extra braking and accelerating you'd have to do for tight corners from higher speeds.