Carbureted Mehran or fuel injected? For the carbureted, you cana install a vacuum gauge to get a ballpark of the engine's load based on vacuum.
For EFI, look up the MPGuino on the ecomodder forums. It's a simple Arduino based unit that uses VSS and injector pulses to calculate the instantaneous and average fuel economy. You will have to build it yourself; the design is open source.