Illustration dating duct tape mpg
Blocking the center is probably worse - air would end up banging around in front of the truck and maybe enter thru the remaining open vents anyway.
Better check to see what's behind the open locations - radiator has to get air, and depending on where you are maybe the a/c doesn't need it in January. I'd suggest something stronger than saran wrap though.
My reasoning is this way you have a smooth front surface, better aero than the segmented recess thing you'd get with cover panels behind the grill.
Also better to have the edges blocked and let the air flow away to the sides and hood.
The first step is to buy a small notebook and keep it, along with a pen, in the door panel of your car.
You really won't need much opening to get enough air in, especially if it's cold outside.
Trucks have an oversized cooling system compared with a car to begin with as well.
Trip: The trip meter is usually in the same area as the odometer. You should lock the gas pump handle on and let it shut off automatically. Each gas pump will shut off at a slightly different level, but there is no other way to verify that you have a full tank of gas. This will explain any unusually high or low readings that you get.
Later, you will be able to average the miles-per-gallon readings you get to find the lifetime average. After you set up your notebook, get a full tank of gas and drive.