Hyundai i20 1.4 glide 2012 model

Evening guys, my car has a very serious problem, it consumes too much fuel and i have noticed that my rev counter is very high when driving, if am driving 100kmh the rev counter would be at 4100rpm, if am driving 120kmh it would be around 4500rpm, 140 its around 5000rpm and that consumes too much fuel,i visited some mechanics and they dont seem to know what the problem is..Help please I am in polokwane