I have 3.1 block and 2.8 head with fierostore performance exhaust manifold. My engine drink 10 L per 100 KM (sorry don't know gallon per mile) I don't push it over 115 km per hour. I heard that a fiero should use like 8 L per 100 km. No check engine code no bad idle. Hyperchip and 180 c thermostat.
Where should I look first to reduce the fuel consumption and get a regular 8 L per 100 km
These would be things to check when the engine otherwise runs good. Properly working fuel pressure regulator and engine coolant temperature sensor. On the fuel pressure regulator, there is a habit of cracked nylon vacuum lines causing lightly higher than normal cruise fuel pressure. On the coolant temp sensor. You need to watch it with a scanner while it is warming up and when it gets to normal temperature. It may not be out of range enough to cause a code but if it is off by 15 degrees and you are already running a cooler thermostat, the ECM may see that as a cold engine, fattening up the mixture.
If it's a manual, it depends on whether you have the 4speed or 5speed. The 5speed delivers better fuel economy. Also, which four speed. The my17 Muncie had basically racing gears and runs high rpms at hwy speeds. The other "economy" 4speed manual had taller final gears and was better for fuel economy.