Does anyone know the MPG of a 81 Datsun pickup 4x4?

Just be glad it is running at all. 28 year old trucks with dinosaur like carburator's and to top it off a 4x4 system with or with out locking front hubs. While not really designed to run on 15% ethanol fuels today the 1981 Datsun/Nissan z24 or is it a L20b motor should get between 17 to 21 highway MPG

This would depend on the cars current condition, whether you are driving it in 4wd, whether you're carrying a load etc. Is it a 4 or six cylinder? You haven't given enough information to answer this question.