If you can afford more CPU, then the FMOD Convolution Reverb is the next thing to try after Multiband EQ. You can create a more complex and smoother sounding reverb effect in your DAW, then “bake” it into a reverb impulse by driving a single byte click through it, and use that as the impulse response in the Convolution.
Convolution Reverb can be added to an event (it doesn’t have to be in the mixer) so each car engine event can have its own internal version.
However, if you really prefer to save CPU, you might consider baking interior versions of the engine sound wavs themselves, and switching to those in the interior view. As in, just bake the effect into the wavs. However reverb and filtering, if baked into wavs, doesn’t handle pitch changes very well. So if you are using loops that get pitched up and down a lot, baked reverb may end up sounding really odd.