Some travelers like to plan ahead by comparing prices on flights, hotels and car rentals for weeks before locking in reservations. But sometimes you need to book a car in a hurry or at the last minute. That's when knowing the best apps for renting a car can really come in handy.

The good news for last-minute car renters: NerdWallet analysis found that booking car rentals at the last minute can save you 15%. The bad news is that you can't set up a price tracker through AutoSlash to take advantage of price drops.

To help you figure out which app is best, we put some of the best car rental apps to the test to see which one comes out on top. Here's what we found.

Our car rental app analysis

To help determine the best car rental app, we downloaded several top-rated apps for renting a car. On each app, we searched for a car for the same weeklong period from Denver International Airport.

We recorded the cheapest price and the rental car agency associated with that price. For comparability, we also logged the cost for a Hertz compact car rental. And since time is of the essence when you need to book a rental car at the last minute, we timed how long it took from opening the app to getting search results.

Here are our results.

The best rental car apps

If you’re not brand-sensitive and want to find affordable car rentals, these top three apps will get you there the fastest.

1. Kayak

In our testing, Kayak clearly takes first prize. Kayak search results checked the most car rental companies and offered the lowest overall price and the lowest price on the control car search.

Even better, Kayak's 37-second search was the fastest among the top apps. That speed is especially impressive when we dug in to find that Kayak had searched for prices on Expedia, Priceline, RentalCars.com and several other websites.

2. Priceline

The top three apps — Kayak, Priceline and Expedia — all found the same cheapest price: $344 for a weeklong rental from Fox. However, Priceline and Expedia searched fewer rental car companies and took longer to search.

3. Expedia

Expedia's process was particularly slow due to needing to sign into an account before you could search. However, if you already have an account and are logged in through the app, that would probably cut down some of the time. Also, Expedia didn't quite find as cheap of a price for our control search of a Hertz compact car. Still, Expedia did better than Skyscanner, RentalCars.com and Carngo in almost all metrics.

Tips for booking rental cars with an app

Determine exactly what you're looking to rent

To figure out the best car rental app for you, you need to know what type of car rental you want. That can vary depending on a few things — from the rental length to which credit card you have.

Are you just looking for a quick car rental to run a few errands? You might want to use an hourly car rental service like Zipcar. Or, perhaps you're looking to rent a unique car. In that case, carsharing service Turo may be the best fit.

Check your credit card for existing rental car benefits

Travelers with a travel rewards credit card may want to stick to a traditional car rental service. That's because some of the best travel cards offer rental car damage protection, but the coverage is typically only limited to traditional car rentals — not hourly or peer-to-peer rentals.

Another benefit you might already have is elite status with a rental car company. Even if you don't rent much, you might be able to get rental car elite status through your credit card. Or, you might be able to claim car rental elite status through your current airline or hotel elite status.

Consider using search aggregator apps for renting a car

For travelers looking to rent from a traditional car rental agency — like Hertz or Avis — the best car rental app is likely going to be a rental car search consolidator.

Rather than searching for rental car prices through individual rental car companies, these apps save you time by searching car rentals across their partner car rental companies and compiling all of the results in one spot.

Even travelers who have elite status with a particular car rental company should still start with these rental car search consolidators. After all, another company may offer such a great deal that it's worth "cheating" on your preferred rental car company.

However, if you're set on booking through a particular company, the right car rental app for your situation may just be your preferred rental car company's app.

If you want to use an app to rent a car

The best car rental app for your situation is going to depend on what you're looking to book. For travelers looking for a traditional car rental company, our testing found that the Kayak app searched the most rental car providers and returned the cheapest prices.

However, if you're looking for a shorter-term rental or renting a particular type of car, you may want to check carsharing services like Turo, Zipcar or similar.