It doesn't matter. Yes I could send it a fake speed signal to go along with it (which would show up on the speedo and put miles on your car, who would want that?) but I did even more testing (I REALLY wanted this to work) and found it's just not going to work. The dash messes with the value too much.
The dash dampens the temp more going up than it does going down. So the temp will drop almost instantly, but takes a very long time to increase. There seems to be a time lockout where it will not change for the first 5 minutes after start, then it seems to update a bit faster. It also seems to be speed related. It seems to update much faster above 65.
I ended up replacing the temperature sensor with a fixed resistor for some tests so I could take the actual temperature out of the equation. It took a full 10 minutes before it even got close to the temperature it should have read. and even then, sending fake messages seemed to result in an averaged display, not the number I asked for.
It seems an external display is the way to go. So now the question is, do I come out with something simple and quick or more elaborate that might take longer...
Ford F450 Crew Cab Hybrid Conversion