Analysis

This question mainly examines the positional relationship between a line and a circle, and the application of completing the square, which is a basic problem. According to the problem, the line $4ax-3by+48=0$ passes through the center of the circle $(-3,4)$, hence we have $a+b=4$. Then, using $ab=(4-b)b=-b^2+4b=-(b-2)^2+4$, we can find the range of values for $ab$.

Solution

Given that the line $4ax-3by+48=0$ bisects the circumference of the circle $x^2+y^2+6x−8y+1=0$,

It follows that the line $4ax-3by+48=0$ passes through the center of the circle $(-3,4)$,

Therefore, we have $a+b=4$,

Thus, $ab=(4-b)b=-b^2+4b=-(b-2)^2+4\leqslant 4$,

Therefore, the range of values for $ab$ is $(-\infty,4]$.

Hence, the answer is $\boxed{(-\infty,4]}$.