Forgot to say it's a UK model, right hand drive

Hi everyone, I've recently bought a 2006 Volvo S40 diesel, 1.6 L and the average mpg I got is about 37-38 mpg. I wonder if the usage display is set to show US mpg or IMPERIAL mpg, as a 37-38 us mpg would mean 44-45 imperial mpg, much closer to the specifications. Or is there a way to change the mpg ...