Depends what you actually mean by coasting in gear.
Coasting is when there is no connection between engine and wheels, either gearbox is in neutral, or clutch is down. If clutch is down, gear selection is irrelevant.
Do you mean on the overrun, so accelerator off, and momentum of car pushing the engine?
Coasting is bad practice, it destabilises the car, reduces the effect of the steering, and surprisingly increases braking distances, but not due to lack of engine braking. (There's a lot of physics going on there)
Find a safe, quiet, straight road with good visibility. With no other traffic, position yourself in the centre of the road, at around 25-30mph. Gently sway the steering left-right a few times. Important, gently sway, not abrupt swerves. If anyone does not understand the difference, don't do this. Your risk. Feel how the car behaves.
Now repeat with the clutch down. It'll feel loose.
When coasting, the engine has to remain ready for use, so the computer will fuel it at tickover, so will show a fuel consumption, may be mpg (l/100km) or g/hr (l/hr).
On the overrun, no fuel is fed into the engine until it drops near to tickover, when fuel is gently fed in to prevent a stall. If well above tickover, on the overrun, clutch and gear engaged so engine being pushed by momentum, fuel use should be zero. This may show as 99mpg, or just no display.
If display shows fuel being used, it is possible that the accelerator pedal is not telling the ECU that it is fully off. Look for restricted movement, or connect to
MES or a suitable diagnostic program that will show accelerator pedal position.
Or change the display to a different reading and stop worrying.