What's your RPM at 120 km/h?

around 2000

Around 2000.

2012 registered, manual, low trim, for household use, less than 70,000 km. GPS shows speed at 120 km/h with around 3500 RPM.

When driving a manual transmission at 120, it's around 3200 RPM.

The car reached speeds of either 150 or 160, I can't quite remember, with the RPM going up to 5,000 or 6,000. The fuel level dropped quickly.

I drive manual, 120 speed at 3000-3200 RPM.

Haven't driven it. Cruising at 80 km/h. Around 2000 RPM.

What could be the reason if it's between 3500 and 3800?

If the dashboard shows 120 km/h and the RPM is around 3200, but in reality at 120 km/h, the RPM is 3500.

Usually between 3100-3200 RPM.