How can I get the other driver's insurance co. to pay the full cost of rental car?

I was hit from behind and the other driver's insurance company has accepted liability but, they only want to reimburse me about $27/day for a rental car when I'll have to pay around $50/day. They are also dictating to me some terms about the duration of the rental car period based on the number of shop hours estimated for repair. I am wondering if there is some leverage I can use to make them pay 100% of the rental car cost for a reasonable car and duration.