getting very less Miles per gallon for recently bought Audi A4 2015 pre certified
 

Hi, I bought a Audi A4 pre certfied just 10 days back. It was mentioned to me during the sales that miles per gallon is 21 in city and 31 in free way, but now I see I am getting only 16 in city and around 23 in free way. I used shell premium gas (91) twice to fill the tank. I drive at a speed of 65 to 70 on free way and I dont accelerate too fast. I drive in cruise mode in free way most of the tome. I drive at average speed in city. I called up Audi showroom from where I bought and they said they cannot help much on that as if there is any issue with the car , there are indicators in the car which would indicate if there were any issues. They say it might be cause of the way I drive but I don't agree cause I have been driving other cars before and I drive at average speed like most of them. What can be done to fixed the miles per gallon issue. Please help.

About the car it is Audi A4 Quattro , 39,000 miles , 2015.

Regards,
Avi

If i Take the care to audi service i s there any way they can find out if there is any issue with miles per gallon?