Idle circuits do work very well, not overly rich and crude as you point out. Try adjusting one someday using a air/fuel ratio gauge and you will see how sensitive they really are. I do believe they shut down for the most part once the throttle is open. Even if they don't shut off totally, they supply very little fuel once the engine gets up in rpm.
Gearing has a more to do with the motors torque output at the given rpm then just the rpm itself. If the motor make tons of torque at low rpm. then gear to run at the point, but if it make all its torque at a high rpm, low gearing will kill any fuel mileage and driveability.
Lugging a engine down is gearing it to low for the available torque. This requires your foot to far into the throttle just to keep it going. If its just a simple gear swap for mileage, then every car out there would have 2.41 gear ratio's. Some of the early 80's Chevrolet Malibu's had very low gears with lock up converters and small V6 engines, on flat ground at 55 mph they got decent mileage, but put in the hills or at higher speeds, the converters wore out shifting in and out so much, the little engines just didn't make to torque to pull them.
Just as example of gearing and rpm, my 2001 Beetle has a 1.8L turbo engine that runs 3100 rpm @ 70 mph and still gets 31 mpg. It was design for this, if I was to gear it different, the engine wouldn't have enough power to pull it at any lower rpm. Thats why you just can't change gears and expect the carb and engine to respond with better fuel mileage.
Go for a ride in a car with a big cammed engine with a low gear, you will see what lack of torque means!
Joe
