after gearbox replacement fuel consumption doubled, why?

My berlingo van's 5th gear just stopped working, it went in but it was like in neutral. so long story short;
had a new gear box but now fuel consumption is sooooo bad.
Where once i put in £20 to travel up to L'pool & back (174 miles) and still have loads to run around in, now it takes £20 just to do the 87 miles there!
Could this have something to do with the gearbox being fitted wrong or something?
i'm confused and can't afford to run the van like this.
any ideas anyone?