The logical school of thought being that most people have a bit more excessive external rotation in the right ankle (rather pronation/eversion/etc.) is that they drive with their right foot and often don't move the whole leg to hit the brake and gas pedal and instead just move their ankle.
Since we're on the gas most of the time, the right foot gets in the habit of being pronated/everted.
It's true if you have really tight adductors or predisposition to valgus knees (caving knees weakness) the foot does tend to evert/pronate though.
You more see chronic external rotation in the hip in old people as the hip flexors get really tight (as psoas major, iliacus, etc.) are external rotators of the hip.
Depending on how you sit (legs crossed or not) also becomes an issue sometimes.
So usually it's multiple issues with people.
If it's only the right foot I would tend to think it's just the gas pedal thing and things we do unilaterally since most of the time if it's due to a weakness or tightness issue you're gonna get it bilaterally or on the weak leg (left leg for most people).
I have a bit of the same thing, and now hae to consciously rotate my leg or move the whole leg when I'm driving a bunch. So I know the feeling of how annoying this is to correct.
If that's not the case we'll try to figure it out from there, but gas pedal thing makes the most sense to me.