This is contradictory advice and, of course, depends on the gearing of the car and the torque characteristics of its engine. My FIAT 500 1.2 will take 5th at 30 no bother. The new FIAT 500 TwinAir, on the other hand, actually requires 3rd for 30mph. The IAM advice is partly to remind you of the speed you are doing by engine revs, and partly so you have instant power to accelerate out of danger should any danger occur. It has nothing to do with fuel economy.