Well, if you're talking MPH, you must have the Enhanced Edition ROMs, and the dipswitches set to MPH... because the original was only in km/h.
The Enhanced Edition used a slightly simplified conversion from km/h to mph. It divides by 8 then multiplies by 5. This is within about 0.5% of the "proper" conversion, so I considered it good enough.
The original max, IIRC, is 293 km/h. Converting (and truncating) gives 183 MPH. So, yes, I think that's the maximum speed in the game.
Wow that is great information! You are correct about enhanced edition. My board came in the mail on Saturday and I burned the new roms immediately. Forgot it was all in km/hr.