Matt Nager for The New York Times

Last week, the U.S. Department of Transportation announced that it would begin requiring airlines to “prominently disclose all potential fees on their Web sites.” Great. Now, can we just get rental car companies to do the same?

Finding the most frugal car rental that meets your needs (and protects your safety and your financial liability) is a process fraught with complications. I’m never sure what I’m going to pay before all is said and done, I’m insured and the car is filled up with unleaded.

The confusion begins on the rental car Web sites, where most companies start out giving you a base rate that is rendered utterly useless by a random assortment of “concession fees” and “facilities charges” and “bus cost recovery fees” (not to mention taxes). It’s as if the price tag on a carton of milk covered the liquid alone, with refrigeration cost, cow maintenance charges and expiration-date ink-recuperation fees added at check-out.

But that’s just the beginning of potential additional charges. For example, when I stepped up to the Alamo counter in Jacksonville during a recent road trip in Florida, I realized that I had no idea what they charged for insurance. (Alamo’s Jacksonville fee was on the low end, $11.95 a day for liability coverage, but even the rental agent looked shocked. “It used to be $17.99,” she said.)



Then there was the time I took the commuter train from New York City to Jersey City to save a few bucks on a rental for a ski trip, only to find that Enterprise had upgraded me from a compact to an S.U.V. It sure was comfy, but it nearly doubled my gas costs, obliterating my savings and expanding my carbon footprint to at least a size 15 EEE.

I realized that it was time, once and for all, to figure out how to get the best rental deal, fees included. My method would be to book an imaginary trip: a weekend in Houston. I’d be arriving at George Bush International Airport on Friday and returning the car the Sunday after that. The result: a guide to help remove some of the shock from that final rental bill.

Shopping Around

Comparing prices on every individual company’s Web site is time consuming. There’s a shortcut, of course, because the same places where you find plane tickets (kayak.com, expedia.com, orbitz.com) will all help you compare car rentals as well. They also cut right through the tax-and-fees absurdity by listing both the base rate and added fees on the first screen. But beware: Expedia buries certain companies that aren’t “preferred vendors” and Orbitz leaves some out entirely. Kayak works fine, but my nod goes to VroomVroomVroom, which is utterly intuitive and presents your options in a simple list with no bells or whistles.

You do miss one money-saver on those sites. Some companies now offer lower rates if you pay in advance, essentially guaranteeing that you’ll show up. (Most reservations are made with no deposit and no penalty if you don’t show up, a system that creates a vexing no-show problem for rental companies in the United States.) For my Houston trip, Avis and Alamo offered pay-in-advance discounts, which didn’t appear on the aggregators.

At the end (which was really just the beginning), I found a wide range of prices for what was variably called a “subcompact” or “economy” or “extra small” car: from $50.78 with Advantage to $93.52 on Hertz. Which is not to say that Hertz is always the most expensive, nor Advantage the cheapest. (That would be ironic, since Hertz owns Advantage.) Rental rates change by the day, hour, minute and second, depending on available stock. So reserve as far in advance as you can, and don’t rule out any company just because it’s usually more expensive.

Fees and Taxes

Companies come clean at different speeds. Some display the full price (in smaller print) right on that first page (that’s Hertz and Enterprise being half-reasonable); others reveal it on the next screen. Or on the third screen, after you scroll down – thanks, Thrifty. For my imaginary trip in Houston, in some cases, fees and taxes almost doubled the price.

Those taxes and fees vary widely across locations. When I did random searches for similar dates in major airports across the country, I found the fees for picking up at Seattle-Tacoma International Airport could be more than three times as much as the cost at Chicago Midway. (But the taxes are higher at Midway.)

One almost universal difference: Airport locations have higher fees, as they try to recover concession fees that they must pay to airports, the shuttle buses that take you to their lots and the like. You’ll occasionally find deals at neighborhood locations that are so much better than those at airports that it’s worth looking into how easy it would be to get there by public transportation or even taxi. (You can find nearby neighborhood locations by finding the airport on Google Maps and then searching “car rental agencies”).

Insurance

The most complicated additional fee to figure out is insurance. For my faux rental, collision damage waivers ranged from $19.99 a day at Enterprise to $27.99 a day at Budget, which helpfully listed the cost of insurance on their Web sites. (I had to call Thrifty and Enterprise, whose Web sites omitted that information.) For longer rentals, the difference in rates could easily decide which company provides the best deal.

I would decline collision coverage; like most U.S.-issued credit cards, mine covers it if I pay for the rental with my card, I confirmed via telephone. But those policies vary greatly and change often, said Loretta Worters, a spokeswoman for the Insurance Information Institute, a trade group in New York. Ms. Worters urges consumers not just to call (oops) but to have the company e-mail or fax the details of coverage, since it can change and the customer service representatives may not be aware of the fine print or recent adjustments.

If you do need collision coverage, one money-saver is to book through Priceline. If I did so for my imaginary ride through Houston, I’d be offered theft/collision insurance for $11 a day ($9 for New York State residents). That’s an incredible deal, but unlike the coverage provided by car companies, which simply waive their right to charge you for damage, Priceline reimburses you once the car company charges you. (Credit card coverage works the same way.)

Collision, of course, is only half of the insurance question. There’s liability, which most credit cards do not cover. Rental companies’ liability coverage — including Alamo – provide a $1 million cap. (This compares favorably with the liability coverage that comes with ZipCar, which I’m not including in this article because it is best for shorter-term rentals.)

But if I owned a car, my auto insurance would likely transfer to the rental car. Again, Ms. Worters suggests getting all the details. Your liability coverage may be minimal, and especially if your personal vehicle is old, your policy may not cover damage to a new or expensive rental.

Extras

Sally Ryan for The New York Times

Among common add-ons are child-safety seats and GPS devices. I found those prices on most rental car company sites but not on the aggregators. Those costs can vary widely — Enterprise would charge me $6 per day and Hertz $11 for a GPS in Houston, for example. You will need to consider whether you need a GPS badly enough to pay that sort of money. If you’re nostalgic for paper maps or spousal squabbles over whether or not to stop for directions — so 20th century — that’s a no.

Gas Mileage

Matt Rourke/Associated Press

Then there’s balancing car size versus mileage. The companies will rarely tell you the exact model that you’ll get, but you can get a pretty good idea what you’ll spend on gas by comparing their examples on the Department of Energy’s fueleconomy.gov site. When I noticed that some companies offered a slightly larger compact for the same price as the economy size, I compared the sample models (Chevy Aveo vs. Ford Focus) for gas mileage, finding very little difference.

I had less luck finding any way to guarantee that I would not be upgraded to a gas guzzler. But I was relieved upon calling clerks at several of the companies’ Houston airport counters who told me that weekend renters were more likely to get precisely what they ordered (not to mention cheaper prices overall) because rental traffic is generally slower then.

Other Details

Had I forgotten anything? I talked to two experts: Brian Ek, a spokesman for Priceline who also writes the Priceline blog called the Travel Ekspert, and Peter Thornton, C.E.O. of Vroom Vroom Vroom.

Mr. Ek noted I should have looked into each company’s grace periods and policies on crediting customers for early returns or charging them on late returns, if I thought either might be a possibility. (And it always is: anything could happen.)

Mr. Thornton noted that companies are now trying to differentiate themselves with certain perks that could alter the decision of some consumers. Enterprise Holdings, which owns its namesake brand as well as Alamo and National, charges customers just the market rate for refilling a tank returned at less than full, he said. That sounded good, reducing the “Is this gas station close enough to the airport or do I need to wait for the next one?” stress at the end of a trip. Probably not enough to sway me, though.

He also said that Thrifty and Dollar no longer charge for minor nicks and scratches.

Now that was appealing. Minor damage occasionally appears on my rental cars through no fault of my own (unless you fault my tendency to take compact cars through terrain and into tiny parking spaces they were never meant to see). Eliminating the stress I go through when the inspector looks over my car and I wonder whether he’ll spot the new scrape – that would definitely be worth a few extra bucks.

The Decision

The advantage went to Advantage, where I got an “extra small car” — no model specified but something like a Chevy Aveo with 25 miles per gallon in the city and 34 on the highway — with liability coverage and a GPS, for just under $99. Still a great deal, but quite a difference from the initial $24 base rate.