May sound like a dumb inquiry, but if my overdrive is not functioning what safe mph can I drive without doing mechanical harm to the engine? Or, will I be able to attain high speeds without the fuel economy?

It really depends on how many RPM's you're turning. The engine doesn't know what gear you're in, just how fast it's spinning. Maintaining 4500-5000 RPM shouldn't be much of a problem, as long as the engine is solid. I'm sure that drivers on the Autobahn turn that in top gear regularly cruising at 110mph or so. I just receintly had an Audi 100 with transmission problems, and for a year, basically drove in one gear. 60mph= 4500rpm. FOR A YEAR! No engine problems. The Audi 5cyl was a tough motor, but so's the Volvo...