I've been running 100% bio diesel for the last few weeks. Disregarding the first couple of fills to get the proportion of petrodiesel right down, I was getting 26-28mpg in my '86 90 with a 200tdi. Tyres are oldish Colway 235/85x16 ATs, 32 front, 35 rear. That mileage was a combination of motorway at around 50mph, and some twisty A-roads at 40 or so. Then I switched back to petrodiesel, and got 36mpg, same A-road driving, but 55-60 on the motorway.
Once the prices were compared (92p for bio, £1.22 for petro), the cost is virtually the same at 15p per mile. The difference is that the motor feels more powerful with petrodiesel - it was easier to get up a bit of speed to pass lorries, for example. That, and the fact that the nearest bio place to me is a 60-odd mile round trip, means that petrodiesel is the better option.