I made a crude grill block for the lower grill of my Yaris sedan. It's made of cardboard and it's purpose is for testing to see if it's worth making a nicer permanent one.
I did 4 runs each on 2 courses, total of 8 runs. The grill block was alternately installed or removed between each run, a true A-B-A-B test. I chose high speeds so aerodynamic drag changes would be emphasized. The first course was 2.1 miles long with a slight downhill, run at 65 mph. The second course was 1.7 miles long with a slight uphill, run at 75 mph. For each run the cruise control was set at exactly the target speed and the scangauge II reset button was pressed just as the car passed a roadside marker. The scangauge II current mpg was read as the car passed the finish line roadside marker.
Conditions were cold (-5C) night time with no felt wind or traffic.
I tried my best to do everything the same for each run.
The effect of the grill block was measurable but there was quite a bit of difference between the runs. It looks like the improvement is somewhere between 2.5 and 4.9%. That is higher than I was expecting. Also it was unexpected that the higher speed run would show less difference. I think that came from the slight uphill on course 2, as more fuel was used to overcome the hill, the difference due to aerodynamics was a smaller percent of the total. To get a more accurate number, more runs would need to be made, however the wasted gas to do this would counteract the gains made by the grill block in the first place.
If I assume a 3% fuel economy improvement over 100,000 miles, $2.50/gallon gas cost, and 43 mpg average, that comes out to $170 savings. Looks like a winner to me. I'll be making a permanent (and nicer looking) grill block soon.
When I did my grille block test, I saw about a 3% improvement, and at lower speeds. That said, my grille openings are inexplicably gigantic compared to your car, and closing them probably makes a larger difference.
As for the differences you saw between runs, one question I'd raise is: was the car fully warmed up for the first test?
I don't just mean coolant temp. In winter conditions, it can take the transaxle, bearings & tires quite a while to reach stable operating temps, and your mileage will also not stabilize until that point. That could explain the larger difference in the first A-B pair.
That long warm-up period is why I don't attempt much winter testing.
EDIT: Which is not to say it can't be done, just that it takes longer if you have to drive until the car is fully warm before you start testing stuff.
Metro is right about winter testing, I just blocked off my lower part of the grill a week ago, I did some testing but I didn't see any gains. My car seems to take almost a half hour of driving before its completely warmed up.
Also you have to be aware of your Intake air temp. If you shut off a car in the cold and let it sit a few minutes and then go drive the intake temps are way up. it takes 5to 10 miles of driving for the intake temps to even out.
You'd get better results if you blocked the upper rather than the lower grill opening, especially if your car doesn't have underbody panelling. In the absence of underbody panels under your engine compartment, all of your cooling exhaust air is going to end up under your car, adding to the drag turbulence down there. Some of the air that currently isn't entering your blocked lower grill is going to end up under your car anyway. But if you block your upper grill, all of that air is going to end up going over your car, where it creates less drag.
If you look at many sports cars (example: 1992 Camaro, and also on the 92-95 Civics), you will see that they place their grill opening very low on the front of the car just for those reasons mentioned above. On most cars, that upper grill intake is dictated by the automotive styling department, since many people don't think a car "looks right" unless it has a grill opening at the upper front of the hood. That look is a holdover from the cars of the 30's where the entire front the the car was designed around a large, tall grill.
I only closed the lower grill because it was easier on my car, and also a little larger than the upper openings. My car does have a small underbelly, forcing all the grill air through the radiator.
I drove for about 10 minutes before starting the test so my car was not fuly warmed up. Since the runs with and without the grill block were alternating I didn't think it would matter much but maybe it did. One thing to note is the first runs on each course were the coldest and showed the best mpg (grill block was ran first).
That's a nice looking grill block you made. I want mine to look like that. Your upper grill is much larger than on the sedan version.