MGLs maxi singles are designed to drop into a standard 3.125-inch hole, while the Smart Singles fit into a 2.25-inch hole. Author Rick Lindstrom found that the Maxi Single E1 engine monitor was just the ticket to get him flying while his other unit was in the shop for repairs.
If you need a horn to sound when the airspeed drops below a certain level or when the power is cut and the landing gear is still up and locked, an electronic circuitry solution is the lightest, most reliable and most cost-effective way to go.