Ed, I just spent some time searching the web for info on your question, but wasn't able to turn anything up about a 175 mph speedometer for the Corvette. Maybe the Sharks will know about that option (if it was an option).
I don't know about a 175MPH, but I do know that a 145MPH speedo from a 78 or 79 will fit right in with no changes except for the lens for the Choke light...........It does seem to me that Mid America did, at one time offer another speedo for the 80 to 81.........I had one of these in my 1980..............Steve
From everything I've read, when GM decided to go with the 85 mph speedo, there were no other options, even in Corvettes. 85 was standard in hopes to keep people from going so fast, and for insurance reasons or something. It's already been mentioned that Eckler's sells 165 mph speedos, and so does Mid-America. But I've never seen or heard of 175 in an 80 - 82. Maybe it was sold by a Vette parts place that is no longer in business?
That photo from Drive-Shaft, in my opinion, is an aftermarket replacement speedo and not factory. It does not look anything like mine. Mine does not have that square box around "55" and it does not have orange "55" numbers. If you look at the crossflags and "Corvette" script on the speedo face, the script is not centered over the crossflags logo. It's a poor example of a reproduction product. Also, factory stock speedo's in 1979 only went to 140 mph. My car is original and I am the original owner.
Mine is different, faded orange to yellow the X flag is similar to the late 70's. The speed sensor is in the rear of the speedo and is used by the ECM. Definitely an aftermarket of some sort and is accurate with the GPS.
I also have an aftermarket speedo 240 km/h (150mph) but only shows the km/h range (europe). If I remember correctly the company makes custom speedo's so you can have them the way you want it. Friend of mine ordered them, sorry don't know at which company.