D1 and D2 are unnecessary but harmless. All they do is prevent the transmission from shifting into any higher a gear than the one the shifter designates. Put it in D2 and it’ll not shift above second gear. Put it on D1 and it will not shift above 1st gear. The ECU will prevent the engine from overreving. However, there may be additional wear on the drivetrain.
i’ve learned that with my car, which uses a Camry drivetrain, if I leav it in “D” I cannot use the engine compression to slow the car, but by shifting into “3” I can. In “D” the engine drops immediately to an idle position, in “3” the engine revs drops slowly and I can feel the compression helping. I can also feel the vacuum on the fromt of the booster diaphragm helping, as when the gas is lifted in a car in gear the vacuum spikes in the cylinders. Is this what hubby is actually doing?