In general you would think that was true, but with cleverer delivery of fuel, you often get both performance and economy gains. The two engines in question also moved from a cast iron block to a magnesium block so the whole car should be lighter as a result.

 

Weight difference is 20kg (45 lbs). Over 1650 or so for total weight, I doubt you'd notice. Some options would add more weight than that, never mind passengers.

 

Be aware of the effects of the cold weather? Are both figures recent? Have both mpg trips been reset and covered similar mileages in similar weather conditions?

My 520d mpg has dropped off quite dramatically as the colder weather has hit, particularly when being used only for short trips. Avg was around 44mpg, it's now running at 36!

Stu.