Percentage increase in MPGs for simple mods?
 

I recently added partial grille block to my minvan, and in a half tanks worth of driving saw the difference on the tripometer. So I did the same thing to the Max and noticed the difference on the scanguage.

While I realize a grille block will have varying results according to average speed and size of grille opening etc, it struck me that I increased the mpg possible from my van about 10% with a bit of old vinyl siding and some zip ties. Are my results just not completely tested, or is a grille block (or other smaller mods) much more effective than I am led to believe?

take a look at the wiki for some real life mod data:
Car MPG Efficiency Modifications Main - EcoModder

your numbers sound a bit high, but not past the range of mod + normal driving variation.

What was your test speed?

nearly everywhere I drive has a speed limit of 40 or higher, and there is a stretch of about 5 miles to town over 50, so I'd say aero mods would make a big difference. I'd chock this discussion to more info needed for a couple weeks til I can get a tankful of miles behind me. Tends to take a while with a 20 gallon tank though.

6% tops!
 

The SAE credits the cooling system at about 12% of overall aerodynamic drag.
Completely eliminating the cooling system would provide a 6% mpg improvement at 55-mph,a little more at higher speed.
Accordingly,10% would be a little high,although on a case-specific basis it is possible.

I wonder what the aero drag percentage of a minivan is.... I suspect a bit lower, due to larger frontal area.

yea 10% is a bit high i think, if you look at your frontal area of your car and look at what percentage of frontal area your grill block is its probably less than 10%.
You might however be getting an advantage from higher engine bay temperatures, quicker engine heat up times.
Id still be surprised with 10% but would be great if you are getting that much improvement.

regarding the fact that its a simple mod, whenever i do another mod to my car i am continually surprised by how easy they all are (so far), makes me think i should have done this a long time ago.