I have a 2005 prius purchased on june 06 and have already 13325 miles. I use for work. It usually does 49 mpg average, but when my average goes down to 47, i check the tires and all the time when happens, it's underinflated. If i run very slow, like 40-50 miles per hour the average goes up to 52mpg. any sujestions to improve mpg?