If one is driving a smelly car, it is most likely that their hygiene is horrible.
I would suggest two changes for this sentence.
If you use one, it means that you are talking about people in general, including yourself. I don't think that I would want to include myself in a sentence like this, so someone or somebody are better words to use.
Note that, if you use someone, it is OK to use the gender-neutral their (the genitive form of singular-they). If you want to stick with one, you should be consistent and use one's instead of their.
is driving is present continuous: used with if, the second clause specifies only what happens only during the time that the person is driving,
If you are driving, you should wear your safety belt.
So present continuous in your sentence is likely to be interpreted as
at any time when somebody is driving a smelly car, their personal hygiene is poor (but might be OK when they are not driving).
It would be better to use present simple drives to refer to a habitual action. Used with if the second clause refers to the person's permanent state. This would be interpreted as
If somebody is in the habit of driving a smelly car, their personal hygiene is likely to be poor at all times.