how many miles do you have on your spark so far?
you might have to keep on driving your chevy spark a good 500-1000 miles before you get to see the real MPG's it's rated for. usually this is the case with new cars.
might be best if you keep us posted on your MPG's as you add more miles to the vehicle.
Welcome to the forum! Don't forget to break the spark in and the first oil chsnge is the most important. Be sure to come back here and post your milage for each tank and state your driving style. How much did you end up paing out the doof for ths Spark ls?