It's a lot about additives. I rate BP Ultimate diesel as best all-round for fuel economy as well as fuel system treatment. Shell V-Power diesel has the best lubricity for the fuel injection pump but does not achieve quite the same economy. Shell V-Power and its predecessor Shell Optimax are the best petrols because of their fuel system cleaners and high octane. The combination allows engines to run cleanly at lower revs and if this is taken advantage of then better fuel economy will result. Ignore any tests that refute this because the testers were not driving the engines to the best advantage.