Re: 156 MPG problem
The only way to accurately record your consumption is to top the tank at each fill and zero your trip, then check what's been used next time you fill up.
27 could be right but will depend on many things. Thermost is one possiblity, as mentioned.
Don't forget we're now in winter so the car wiill be running richer for longer, I always used to lose 1-2mpg with my 1.8 when the weather turned.
Tyre pressure will have a small impact but the primary impact will be from your driving style.
I used to get 29mpg from my Twinny, thrashing it all over the place....slightly less in winter.
So...heavier car means your mpg is about right. Our 2.0 Spider does about 30mpg and is driven on longer runs.