Apart form driving style and poor maintenance, incorrect tyre pressures is one of the main causes of poor fuel economy. I was amazed by the difference it made when i drove 150 miles with only 18psi in a rear tyre on my van due to a slow puncture. I must have 10 mpg on that one journey i reckon easily!!!!

By thy way, i work out my vans mpg every run so i know if theres a problem somewhere. Sad i know, but good on the pocket and i use that as a measure so i know when to service it.