I have the scan gauge II in my 451 smart fortwo, have done a little install on it so it sits in the far corner of the dash, works great on the 451 without any issues. Love the real time horse power and air temp, plus since I didn't have the big pods I now have rpm readout via the scan gauge.
We are doing a further custom install of the scan gauge but are waiting on a part for the install, but we will be mounting it into the car so that it appears to be factory as we are trying to keep a very clean look.
I highly recommend it to anyone, I'll do a full review on the unit once I have it built into the car, but might be another week as the parts for a bunch of installs are delayed