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.
Is your car auto or standard? With the auto, getting 10L/100 KM is pretty normal. Despite what people say, these are heavy cars, so you won't get great fuel economy, especially with an automatic.
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.
9l/100km is closer to what you'll get with mostly highway driving. You might be able to squeeze 8l/100km out of it if you drive 100% highway miles at around 100km/h.
Also those numbers are for a stock fiero. With a 3.1, hyperchip, and 180 degree thermostat, you can expect to use more fuel, not less. So 10l/100km seems just about right.