Is my car over-revving

Hello everyone,

I have a 2005 1.4 TDCi. I brought the car 8 months ago and have always thought the mileage is rather rubbish. The tyres are all new and aligned so theres no problem there. Generally speaking Im only getting 50mpg driving around the country at around 50mph on the 15 mile trip to work. I dont break heavily, and the tyres are all well inflated.

Im not used to a diesel engine, and I wonder if its over-revving slightly. This may sound strange but does anyone know what the revs should be. For example, when doing 70mph the car is revving at just over 2 and a half. When at 80mph the car is over 3000rpm. Does this sound normal? If I take the car on a 100 mile long motorway trip with no stopping I would expect the mph to go up, but whilst doing a steady 80mph it appears like its gone down.

Im wondering if there is a sensor problem somewhere thats making the revs high. When first started the engine idls at just under 1.5000 rpm, but after that it just about drops under 1000. Any ideas?

Thanks