My new job allows me to rent a car for trips over 120 round trip. But I'm thinking it's better to drive and take the reimbursement and pocket the money.
Mileage reimbursement was my only option at old jobs. I'm getting the full federal rate. My car gets 35-40mpg and is only a year old with 25k miles on it. I estimate 4 trips per month of 200+ miles each. There are other shorter trips but those can only be expenses through mileage, no rental option.
I had an extremely similar situation about 10 years ago - I drove ~1,500mi/month for work on long trips and could either use my car and claim mileage or rent a car. Briefly, my experiences.
I went with the mileage - my car got 30mpg (towards the higher end in 2007), i owned it outright (no interest) and it was about 3 years old at the time. At first it felt like I was really coming out ahead because I got a check each month for ~$600, and about half of that went towards fuel. After two years of this I had saved up about $8k. By year two there were some expensive (but predictable) repairs; new tires, new brakes/rotors, CV joints, etc. that sucked back a couple grand from what I had "saved." However, combined with my daily commute I was driving my car close to 30,000mi/year and in just two years the wear/tear was very evident. I ate so many meals in that car and spent so many hours in the drivers seat that the interior suffered (badly). While 95% of the miles were highway the end of each trip often involved a few miles of dirt roads in unknown locales and I put a couple scratches into the paint where bushes/trees were crowding the road.
In a way it was good for me because it made me stop viewing my car as some precious thing that reflected who I was, and instead I started seeing my car as just a tool to get my from point A to B. That was what made it worth it to me. Ultimately I feel I made some profit on the deal, but it wasn't a huge amount. Driving a car I was familiar with was certainly a plus, and I never had to deal with going to the car rental and denying (yet again) the stupid "rental car insurance" upsell.
YMMV (pun intended).