If $\frac34$ inch is $120$ miles, then $\frac43\cdot\frac34$ inch is $\frac43\cdot120$ miles. Doing the multiplications, we find that $1$ inch is $160$ miles. Now just multiply $6.5\cdot160$ to get $1040$ miles altogether; there’s no need to deal with the extra half inch separately.
In general, if $a$ inches represents $b$ miles, we can divide both numbers by $a$ to find that $1$ inch represents $\frac{b}a$ miles. Then, given a measurement of $c$ inches, we know right away that it represents $c\cdot\frac{b}a$ miles. (That’s really what I did in the first paragraph: multiplying by $\frac43$ is the same as dividing by $\frac34$, which was the $a$ in that particular case. The $b$ was $120$, and the $\frac{b}a$ was therefore $160$.)