Try this formula!
April 6, 2012 9:29 AM Subscribe
H1 (%hway) * H2 (hwayMPG) = B
A + B / 100 = EC
This formula uses the manufacturer data for consumption, and your own estimates of the city/highway split, this gives an expected consumption (EC). This lets you know how your car is actually performing against the manufacturer's values. The reason I do it this way is because a manufacturer's average is only relevant at exactly 50% city, 50% highway miles. This formula gives you the relevant everage at your current distance and city/highway percentage.