Is it illegal to operate a commercial vehicle without a speedometer?

Federal Motor Carrier Safety Regulation 393.82 states that "each bus, truck and truck-tractor must be equipped with a speedometer indicating speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km.hr (5 mph) at a speed of 80 km/hr (50 mph)."

I can't answer for all states, but as far as I know, all vehicles have to have speedometers. If by commercial vehicle you are referring to something requiring a Commercial drivers license, then yes, it has to have a speedometer.

I am from Canada, and law here for commercial vehicles states that if it had it when it left the factory, it must be present and in working order. This would include the speedometer. If your vehicle had a speedometer when it was new, I am assuming U.S. law requires that it be there and operational the same as in Canada.

there is actually no law stating the speedo has to be working

it also was never covered in a vehicle inspection