In my previous role with my company I drove 20-35k per year in my personal car, but only flew around 20k. In my new role, I am on track to fly 200k this year, but only drive my car about 42 miles per week - 21 to the airport on Monday, 21 back home on Friday. Rental car miles vary quite a bit - some weeks are less than 100 miles, some are approaching 800. Depends a lot on where the work is in relation to the nearest airport.