Since money (dollars) is fungible, don't look at it as your costs being covered. You get that money, and you can either spend all of it on the actual costs, or you can spend 2/3 of it on the actual costs and pocket the rest. (I'm assuming you are reimbursed based on a per mile formula, not actual costs.) So consider that reimbursement a bit like an unpredictable salary bonus, and look at those dollars as no different than your salary. Do you want to spend all of the monthly bonus on gas and other car expenses, or do you want to be able to keep half of it? Also, when you say that your costs are covered, are you including just gas, or also things like pro-rated oil changes, tires, tune ups, and other more infrequent maintenance?
I suspect you can do some research on smaller cars that are still comfortable for larger people. See if there is something about there that will get you another 6-8MPG, which you can buy for roughly the same amount for which you can sell the town car. Test drive it to make sure it's comfortable, but if you are intent on maximizing your finances in this area, I think there's definitely room to do so.