Ignore the owner's manual. That was for tire technology of the 60's and 70's. Tires today are constructed differently, and the correct pressure for your tires will depend on many factors including tire construction, driving style and environmental conditions. The typical MGB runs approximately 28F/30R give or take a couple PSI. I run 29/32 in one MGB and 30/30 in another. The difference is tire size, stiffness, rubber compound and more weight over the rear wheel on one of them (GT versus roadster).