Answer
The reading gives a speed that is (a) less than the true linear speed.
Work Step by Step
The wheel of the truck has a rolling motion, so $v=v_T=r\omega$
When the speedometer reads the angular speed $\omega$ of the wheel, it applies the radius of the old tires $r_{old}$ to give $v_{old}$.
Therefore, when new tires are replaced, with $r_{new}\gt r_{old}$, it follows that $v_{new}\gt v_{old}$. Since the speedometer still reads the old numbers, the reading gives a speed that is (a) less than the true linear speed.
