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Join Date: Jul 2002
Location: MCI
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Posts: 52,575
Do you plan on returning to Chicago in the middle of your trip? Car rentals become more complex when they are longer than 2 weeks: you usually can't take advantage of the promo rates or use discount/upgrade coupons on longer rentals. Therefore, if you have the opportunity to switch out cars, you'll save some money. (Example: quick search on ORD yields monthly rates for a full-size at $900+, two-week rate is around $300. And that's before applying any discounts/upgrades you might have.)
If you can do the switching to keep these rentals in the 2-week range, then it becomes a normal exercise of finding your best rate across all companies (including airport and off-airport locations), and then letting Priceline try to beat it.
When comparing off-airport vs. airport rental costs, make sure to consider total costs including taxes and fees.