lower grille and letter slot partially blocked (depending on outside air temps), hood gap sealed with bicycle inner tube and pipe insulation, fog light gaps covered in aluminum tape from the inside
LED bulbs at parking/side marker/license plate lights
Weathertech side window deflectors
antenna removed
removed back seat for sleeping/hauling capacity
added sound deadening material to back seat/trunk area and hatch door (Protecto-Wrap)
Weathertech front floor mats
ScangaugeII, flash drive loaded with music attached to the USB port in the glove box
warm air intake made of 3" dryer duct running from intake snout to exhaust manifold heat shield, wrapped in duct insulation
also insulated the intake tube and airbox
engine blanket
Ultra Racing rear anti-sway bar purchased from Evan at NextGenTuning.com
stock steel wheels have holes covered with aluminum tape, smooth wheel covers made by Hella
You could use a fish scale How many pounds pressure before the car starts to roll on each type of tire?
(At the same pressure, of course.)
Or, what I did: find a gentle hill with a long run-out (I used an empty parking lot). At the same tire pressure, roll the car in N from the same start position multiple times, and mark the pavement with chalk wherever it stops. Change tires, rinse, repeat. Compare the 2 sets of chalk marks.
Since you have 2 Mirages, you could do the comparison without having to change tires! It'll take you 15 minutes, tops.
There was a big difference in rolling resistance when I did my own tire comparisons, here: