A few years ago I flew into Atlanta and had a rental car already booked for two weeks. The car turned out to be what looked like an old Ford Mondeo so I asked if they had a Ford Mustang, they didn't but did have a Dodge Charger. I swapped for that and took a long route to Nashville. I am a tight bug** and certainly not rich so it must have been very reasonable or I wouldn't have afforded it. Petrol was very cheap but one American told me I had rented a gas guzzler that would cost a lot to run. I told him I didn't care about that as petrol was cheap compared with the UK. When I told him how much petrol costs here, he nearly fainted. I didn't see many diesel cars, in fact I can't remember seeing one.
My advise would be, just do it. You won't live forever and when you look back you will be glad you did.