Wind/drag does make a massive difference, as does how long you are driving for after the car has warmed up etc.
I used to commute 52miles each way and was getting 58-60mpg, no
problem.
Now I commute 30 miles each way and am getting more like 55mpg (although there are a lot of roadworks at the moment).
When I have the bike on a rack on the roof, I can get anywhere between 46 and 49mpg.
Last week I drove 50 odd miles in moving, but busy roads and was probably sat at around 60mph for most of my journey, rather than 70mph. My fuel consumption when I got home was 65.4mpg.
I'd expect out of the 100tdi at least 55mpg on a run and realistically nearer 60mpg.