When a car is moving at constant speed, is chemical energy in the engine still being converted to kinetic energy?

It depends how you think of the energy budget of the car. If the engine were not running then the kinetic energy of the car would gradually be converted to thermal energy (via friction) as the car slowed down. So from one point of view you can think of the chemical energy released in the engine as maintaining the kinetic energy of the car at a constant value.

Or you can say the kinetic energy of the car is constant so the chemical energy released must equal the thermal energy lost. From this point of view the chemical energy goes straight into thermal energy, bypassing the kinetic energy “pot”.

If person A pays you $100$ dollars and you use that to pay $100$ dollars that you owe to person B, do you think of the $100$ dollars as going from A to you to B or straight from A to B ? The outcome is the same in either case.

There is the process $$mathrm{A rightarrow B rightarrow C},$$

where

• $mathrm{A}$ is the chemical energy of gas
• $mathrm{B}$ is kinetic energy of the car, which is in a steady state
• $mathrm{C}$ is thermal energy, dissipated during car motion via friction.

Loses of $mathrm{B}$ via $mathrm{B rightarrow C}$ are compensated by the equal rate of conversion $mathrm{A rightarrow B}$.

There is also the more direct parallel process

$$mathrm{A rightarrow C}$$

during the engine actions, taking the most of $mathrm{A}$, as the inevitable consequence of the 2nd law of thermodynamics.