Performance can vary car to car dramatically. When we took delivery of our 06 fiesta 1.4 tdci it was much livelier than the garage demonstrator which the salesman commented on. The 2 1.6tdci Fusions that followed were much more economical than the 1.4 tdci by a remarkable 5-10mpg.
The first 07 Fusion was great and recorded up to 61mpg (checked tankful to tankful) but didn't achieve that until it reached about 25,000 miles.
The 58 plate fusion that was traded in on Friday latterly achieved around 60mpg average but only reached that level of economy after 24,000 miles. That car was also much quieter than the earlier but identical car. I did 30 years in the police and would say that the carsi drove there although they were not tuned any differently seemed to be quicker than the civilian versons. They were of course thrashed and driven hard every day and were always running hot. Some of the Fords had test equipment fitted. I used to regularly drive an unmarked escort mk3 which had electronic ABS fitted. Those of you who know about Mk3 escorts will know that electronic ABS was never fitted to production Mk3 escorts.
We also had unleaded cylinder heads fited to Mk3 Escorts for test purposes in order to check them out over extended mileage before they were put on production vehicles - they cracked at about 50000 miles!
Going back to the question on economy the only way to check it accurately is tankful to tankful and to do it over several tankfuls to get a true picture of every day driving economy.
57 Titanium 1.6 110 DPF, X Sport pack, panoramic roof,Sony 6 CD changer with voice control bluetooth.