Seriously?!
Most if not all petrol engines between 1.6 and 2.5 litres are capable of averaging MPG in the 30's. Some of the ones towards the lower end of that scale will do 40's. It's only around town you'll find the MPG will drop into the 20's.
Even if I'm in a hurry on the Motorway the average mpg on a long trip in my 3 litre petrol car will be about 31-32mpg, observing the speed limit its much higher. Same with smaller engined cars. Most 1.6-2.5 litre petrol cars will average mpg both in town and out of town thats higher than the average you get from an automatic diesel X5.
Even the current Porsche 911 has a COMBINED fuel consumption figure of 29.7mpg which means on a run, you are north of 30mpg.
Hint: We do not live in a world where every petrol does 15mpg and every diesel 60mpg.