How long has this been going on and is there anything different you're doing in that time?
The reason I ask is because we have a VW Tiguan bluemotion (2wd) and took it away on a driving trip through France for 2 weeks. We were hoping for an average of over 40mpg but in the end we only got 34mpg and mostly going at 60-80 mph.
It was soon obvious that AC on full time, heavy loads and bikes on the back had a major effect obviously!
So have you got a boot load of stuff, new roof rack, ac on with windows down, low tyre pressure, crap cheap fuel etc?
92-96 305 1.9 GTXi (sold)
96-00 Fiesta Mk1 Ghia (sold) Uni car with rev counter and faux wood dash
00-01 Clio Williams 2 (dead) A1 until I met a tree at 50mph
01-02 206 Gti (sold) a bad Gti & chipping it did nothing
02-04 GTV 3.0 V6 (sold) cracking engine & a beauty
04-06 Clio 172 Cup (sold) loved it but 95k miles it had to go
06-10 S2000 (sold) RWD roof down fun but baby forces sale
10-11 Clio 197 F1 Cup (sold) too much fun to write about
11- 159sw Lusso JTDm (now) Daddy cool