Does anybody know what happens if we return a rental car earlier than the specified time period originally set up? We can actually get a car cheaper if we rent it for an extra day than needed, so I'm considering that option.

returned a weekly rental one day early last month (think it was Triangle). NO problem. Just one experience, though.

I've also returned early - no problem. I think it's a common practice, a win-win situation for both customer and the rental company.

It may cost you more.

If you read the terms and conditions of the rate, the company may "rerate" if you bring it back early.

For instance, if you rent it weekly and return in less than 5 days, they can rerate you and charge for 4 days. The 4 day individual rate could be more expensive than the weekly promotional rate.

cheap,
I should have noted that when I rented, I returned the car only one day early, in six days, instead of seven.
I would follow Dick's advise and check the terms carefully. And, the more you get in writing, the better!

One day early for us - no problem, mon!

Dcik has it exactly--if you look at the details on most contracts, you must have a car 4 days and some hours to qualify for the often lower weekly rates. If that's the situation you're in, no need to fudge the return date, just tell them you need it for 5 daysyou;ll still offically qualify for the weekly rate.
They can actually charge you extra any time you change the terms even if the rates would have been the same. For example, even if the rate for an airport dropoff is the same as a city dropoff, if you contract for one and them do the other, addtional charges may be incurred.
Other factors that affect rates--maybe in order to get a specific rate you have to return it on a specific day because that's when they need the cars in that location. If you return it early, the companies inventory could be skewed.

Do it at your own risk if you must, but don't be shocked by how much extra you can be charged!

Weekly rentals are for 5 6 or 7 day keeps. If you get a rate of say, $159 a week, and return in sooner than 4 days and a half, the contract will be changed to the daily rate, and it will probably be a higher daily rate than would have been available with an advance reservation.

How about only for 5 hours off? I found out we can get a cheaper rate for just that much difference in the time we'll be dropping the car off.

Cheap, read what everyone has written. The company can charge you for ANY change because you have not complied with the terms of the contract you sign.
5 hours might just make the difference between a 4 day rental and closer to a 5 day rental , which usually counts as a week

Seems to me we talked about this on this forum last winter, so you may want to do a search for that thread. I remember because I was in a similar situation--I was renting at DEN Sat-Tues am. as I remember and thought we might return the car Mon pm and take the hotel shuttle to the airport on Tues. The daily (not weekly or weekend special) rate was so much cheaper for Sat-Tues than Sat-Mon that we paid less total if we kept the car and returned it a day later. So, we kept it and didn't try to return early.