Nitro Filled Tyres

When the car went in for its 1000km service I was asked whether I wanted to have the tyres filled with Nitrogen.  I was told that Nitrogen filled tyres can provide fuel consumption efficiencies of up to 30%  A grand claim for a car like a Defender that has all the aerodynamics of a brick!  I am keeping an interested eye on the fuel consumption and I have found so far that for urban driving I am getting about 500km per tank.  I filled up two weeks ago, after our trip to the Coromandel  and driven exclusively in and around Auckland.  Today I took the following photo...

On this current tank I have done 542km and still have a quarter of a tank to go.  On the face of it it the fuel consumption savings look like the investment was worth it.