Wow, you guys are way more advanced at this than I. I'm trying to find something that will calculate mpg from an OBD1 port, I'm guessing there isn't enough data from an OBD1, but am not familiar enough with the process to know.
There is no standard for OBD1 interfaces/protocols so there's a large variety, too large (and with too few customers) to bring a product to market. Also, as you suggest, plenty (most? all?) of OBD1 interfaces do not carry the necessary information.