UKPassats

Hi everyone, just a quick question regarding my 2001 1.8t estate. I understand with it being s big car and a petrol turbo, it isn't going to be the best on fuel. However it just seems to be terrible. Unless I drive around forty five mph, it will only do 34mpg at a push. I also have suspicions over the temp gauge. As it seems to go to ninety very quickly.
An example of fuel usage, I did maybe a hundred mile round trip, and this was an 'a' road, clear traffic averaging 75mph. It took half a tank of petrol. Is this seriously what can be expected?

Thanks for any advice!

Cheers Sean