I've driven various diesels for a good few years now. I hired a Moneo 2l estate once and for an easy 55mpg from it by driving between 50-65mph on the route between Salisbury and Exeter, mainly dual carriageway at times of low traffic. My Mazda 6 2.2l estate delivers 42mpg urban and uptown 60mpg on long runs if I hold it to slightly under 70mph on fast roads and just under the limits on other roads. However my past Disco's rarely did better than the book 27mpg, often worse, and my VW 2l Transporter camper van does about 27 urban and no better than about 33 on longer A road journeys. Weight, tyres, driving style especially momentum management, are all factors. I generally use gears to slow down as much as possible even on automatics, letting speed decay naturally, if I can.