Insurance companies oftentimes do their best to not treat you fairly or equitably. One example of this is when the at-fault driver's insurance carrier provides you with a rental car with a "take it or leave it" attitude.
In California, you are entitled to be provided with a vehicle of equal value to the vehicle that was damaged as a result of another driver's negligence. For example, let's say that you get rear-ended in your new Toyota Prius. The other person's insurance company says "yep, our driver was at fault" and then allows you $20 a day for rental car. Well, an equivalent Prius may rent for $50 a day.
You are entitled to rent a $50-a-day car and have the other party's insurance company pay for it (subject to, among other things, his or her property damage policy limits). Period.
Now, let's change that scenario a bit: For whatever reason, you don't want to rent a car during the time that your car is in the shop. You're entitled to make that choice. Let's take it one step further: Your car is in the shop for six weeks (42 days). You will be entitled to $2,100 in real dollars to compensate you for the time that your car is in the shop. In other words, you do not have to actually rent the car in order to be compensated for your loss of use.
OR what if you rent a car for $25 a day? You'll be entitled to the difference in cash (well, in check). So, you rent a car for 42 days at $25 a day for $1,050 but you were entitled to a car renting for $50 a day. The insurance company should pay you the difference of $1,050 in the form of real dollars.
Here's the law (CACI 3903M): To recover damages for loss of use, [name of plaintiff] must prove the reasonable cost to rent a similar [item of personal property] for the amount of time reasonably necessary to repair or replace the [item of personal property].