I've had a fuelly account for a little while now and thought I'd have a closer look at the relative diffences in fuel economy with different brands. I've covered in excess of 30K miles across both cars.

I tend to run my mapped Octavia 1.8T 4x4 on Tesco Momentum (99 octane), as this is readily available and only a few pence / litre more than standard unleaded. To my surprise the car actually averages marginally more to the gallon on this, than with Shell V-Power (or the newer Nitro+ stuff).

The Yeti (2.0 TDi 110) is generally run on either Esso standard diesel or Tesco standard diesel. I've also run a few tanks of both Shell V-Power and the newer Nitro+ through it, Again the standard stuff seems to achieve marginally better MPG than the more expensive stuff. I have noticed however that the car runs significantly smoother (and quieter) on V-power.

Yeti - diesel (overall Fuelly average over >16K miles = 45.8 mpg)

• Esso average = 45.9 mpg
  
• Tesco average = 45.3 mpg
  
• V-Power average = 44.4 mpg
  

Octavia - petrol (overall Fuelly average over >15K miles = 27.8 mpg)

• Esso97 average = 27.8 mpg
  
• Tesco99 average = 28.2 mpg
  
• V-Power average = 27.2 mpg
  
• Sainsbury97 average = 25.8 mpg
  

I realise this is somewhat less than scientific, as I've run different quantities of each type of fuel, however given the cost differential I think I may be as well sticking to the cheaper stuff.

I found my Polo to run better and give 1-3 better MPG too on the good fuels. The cost difference now though offsets the fuel savings, so it's not worth it.

I don't have anything as detailed as your findings though! I did record/compare it in Fuelly, but not to the same extent as you.