Unlimited mileage is standard for most domestic rentals in the US, but it's far from guaranteed everywhere — some international rentals, one-way rentals, luxury and exotic car categories, and RV or campervan rentals commonly cap mileage at a set number of miles or kilometers per day or per rental period.
When a cap applies, overage fees are charged per mile or kilometer beyond the limit, and they add up quickly — often $0.25 to $1+ per unit depending on the car category, which on a long road trip can turn an attractive daily rate into an expensive final bill.
The mileage cap (when there is one) is usually listed in the fare rules at booking, but it's easy to miss since it's rarely shown prominently next to the daily price. Always check this specifically if you're planning a genuine road trip, renting a luxury or specialty vehicle, or renting in a country where unlimited mileage isn't the default.
If you know your trip will involve significant driving, it's almost always cheaper to pay slightly more upfront for a confirmed unlimited-mileage rate than to risk overage fees calculated after the fact, when you have no leverage to negotiate.
