Shell did not come out of it too well – you would earn a lot more Avios by buying your petrol at a Tesco garage, or an Esso garage with a Tesco convenience store attached.
Shell is fighting back, however, with this new promotion:
All Shell Drivers’ Club members will receive triple points when they purchase Shell V-Power Nitro+ fuel during September but we’ve got an exclusive offer for our Avios collectors.
In addition to collecting triple Avios on Shell V-Power Nitro+ you’ll also be able to receive triple Avios on our regular fuel. In short this means 60 Avios per 20 litres on Shell V-Power Nitro+ and 30 Avios per 20 litres on regular fuel. We’ve also extended the offer period…fill up between 5th September and 13th October and see the difference in your Avios balance!
To qualify for this offer, your Shell Drivers Club accounts must be switched over to send your points to Avios.
There is some detail here although the text above was taken from an email.
Ironically, even with triple Avios, you are still better off filling up at Tesco (look at the calculations in the original post I link to above) – but the gap is narrower.