'Which?' had a go at this, but I don't trust the findings because the tanks would need to be completely drained and the same cars would need to be driven identical (considerable) distances in identical conditions. Michelin manages this for its tyre tests by constantly rotating tyres, cars and drivers over 10,000 miles, but the exercise costs at least £100,000 and no magazine, website or 'consumer' organisation would ever spend this on a test because it could not finance it. I simply stick to one brand: Shell. And in switching from Shell Fuelsave to Shell V-Power Nitro+ I have seen a 9 per cent improvement in economy from a 5.7 per cent increase in outlay.