Why is our Renault Scenic averaging only 26mpg?

Our 2002 Renault Scenic 1.6 automatic has clocked exactly 45,000 miles. On a recent motorway journey of 150 miles I decided to check the economy and was horrified to find it was only 26mpg. We only use supermarket petrol, which, I know, you do not recommend but could there be other reasons why the mpg is so low?
Maybe it isn't changing into fourth gear. Maybe it needs a new air filter. Maybe the fuel system is gummed up. If you're sure you're getting fourth (2000rpm should correspond to about 50mph), then change the air filter and try using Shell V-Power Nitro plus for a couple of tankfuls. You could also change the spark plugs.
Answered by Honest John on

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