When sharing cars, I've always gotten better mileage than even other people who drive for good mileage. And I do that while maintaining a constant (rather high) speed, so long as the road isn't so hilly that' I'd need the brake to do so. Provided you're not so relaxed about time that you're prepared to coast down to below 40 mph, the4 two biggest thing you can do to reduce fuel consumption are coasting when you know you'll have to slow down anyway and, despite all the semi-nonsense about avoiding jackrabbit starts, accelerate up to the speed where it's safe to upshift as rapidly as you can without making the engine ping. You want to spend as little time as possible in low gears, because they're inherently wasteful.