1. idle circuit: not particularly 'wasteful', and VERY accurate fuel metering. Not crude at all. MAINS are crude, since it's just a fixed-orifice jet. Idle is fine tuned with a screw very accurately. It keeps flowing throughout the rpm range, but the % of all fuel metered through the idle circuit diminishes as the main jet circuit takes over. (venturi velocity at the idle hole is reduced too but overall velocity still exists, drawing small amounts of fuel through the idle circuit, it does not close or stop flowing at WOT)
Idle circuit has an important role at cruising speeds, light throttle.
Lower geared rear ends mean the engine has to work harder to accelerate the mass of the whole vehicle. At cruising speeds once the speed has been reached, the lower rpms required from the engine "often" means better fuel economy... so you save a little fuel after spending more during acceleration. Unless you are way overgeared for the speed and lugging the engine too low. An automatic transmission would then kick into a smaller gear to stay in the powerband.
MBHD
