brian9

This table should help. You can change some assumptions at the top. Basically, look up the baseline MPG for any vehicle in column B and the improvement in MPG in columns E-H. The intersection is the expected fuel savings over 5 years. As you can see, its basically a wash for a 2 MPG improvement from 19 to 21 for a $1,000 upgrade. You can also see, the improvement diminished with higher MPG baseline vehicles.