Australia

In any car built in the last ~20 years, you can monitor your real time consumption either on the dashboard or else by hooking up your phone into the mechanic's diagnostics port (there are cheap bluetooth dongles).

In general, fuel consumption is infinite when the engine is running while you're not moving. At very low speeds economy is terrible and as you increase speed fuel efficiency improves until the sweet spot which is usually at about 60km/h. That sweet spot is fairly wide - up to around 80km/h in most cars and then it starts getting bad again.

It's different for every car - but as a rule of thumb if your car uses X amount of fuel at 60-80km/h, then it probably uses about twice that much fuel at 20km/h and 130km/h.

HOWEVER that 130km/h number assumes the car hasn't been modified. If you've installed a roof rack for example then it could be more like triple the consumption you had at 80km/h! Low speed would be less affected by modifications.

Ultimately the only number that matters is the number for your car, so why not measure it? Modern cars use a computer to calculate the fuel injection speed and it's possible to monitor that number.