Diesel cars or petrol cars?

I am looking to turn down to an ecomony car because I don't think it is worth wasting money on the petrol. I hear stories that the new ford fiesta and vauxhall corsa are not getting the mpg that the car should be getting in the car brochure. I want to know what is the best mpg your car is doing ( new cars only ) on a Diesel. Also what is the best petrol car that does more than 40mpg around town and again tell me what your best mpg you've got out of your Petrol car.