The Cd of a mirage is 0.27, the micra is 0.32, so that's going to hurt, the mirage is also 500Lbs lighter, so once up to speed it'll cruise will less input.
The radiator on our cars were designed to keep the engine cool in the thin hot air of Mexican mountains, so blocking half of it (the left 70% of the lower grill, and 100% of the upper grill) should have no appreciable effect for us.
All cars (to my knowledge) have a mechanical thermostat, before the engine, and the coolant sitting in it, are up to temp, it is closed, coolant isn't flowing through the rad or the heater core (hence having the "heat" on with a cold engine results in cold air being blown, as soon as you hear the engine warm up and drop rpm, the thermostat has opened and you suddenly start getting warmer air). If you submerged the rad in liquid nitrogen (and the coolant didn't go solid), the engine would heat up in the same amount of time as if it wasn't... the thermostat would likely close again as its temperature dropped though lol
One of the biggest issues to me is tire compound (rolling resistance of it), same size, pressure etc, same car - swapping tire brands can dramatically change fuel consumption, including how that compound deals with different temperatures.
One thing to look at is also how you accelerate - look up the torque curves for the two engines you're used to, see if they overlap? The HR16DE starts to level off at 2400, peaking around 3200, so trying to accelerate below that window may see you spending so long ramping up that you consume more than going slightly faster.
Aside from that, the how and why of getting your micra lower could be any one of a dozen things, from air dams below the bumper, to a flat floor, or swapping out a sticky pulley, ride height inconsistancy from factory... whether your drive is mainly north/south, since the aerodynamic lateral profile of a micra is basically a brick...
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View my fuel log 2015 Micra SV automatic: 7.1 L/100 km ... 39.9 mpg (Imp) ... 14.1 km/L ... 33.3 mpg (US) ...