An Automobile Manufacturer Increased the Average Mileage on a Car $\%$

An automobile manufacturer increased the average mileage on a car from 20 miles per gallon to 20.4 miles per gallon. Find the percent increase in mileage. Round your answer to the nearest tenth of a percent.

A) 2.1 $\%$
B) 0.4 $\%$
C) 0.2 $\%$
D) 2.0 $\%$
E) 3.0 $\%$