Interpretation of the coefficients in the simple linear regression model: for cars with 1 unit higher weight (corresponding to 1,000 pounds more), this is associated with a reduction of 5.5 miles per gallon.
Interpretation of the coefficients in the multiple linear regression model: for cars with 1 unit higher weight (corresponding to 1,000 pounds more), this is associated with a reduction of 7.1miles per gallon, holding all else constant.
Other coefficients are not significantly different from zero.