Check your brakes and make sure they're not binding. I had this
problem on mine soon after I bought it SH, the discs were badly worn and the front right pads were binding.
Only noticed it because of the loss of power, rising engine oil temp and poor mpg when driving along (although the smell of burning brakes may have helped to give it away

)
If its a new car then this probably wont be the case. Although I've noticed that in my car the performance and mpg will differ from day to day and will change slightly with air temp, humidity and wind speed/direction. Of course the speed you travel at and the weight of your right foot will always make for the biggest changes.
I find its most efficient around 55-60mph getting an av. of 64.2mpg on very long journeys and about 57mpg on shorter ones. Around town and in rush hour traffic you can be talking anywhere from 35-52

.