Everyone who's ever found themselves at the side of the road in a car that's died has thought, "What's this gonna cost?" and suspected they're about to get reamed. It's a common feeling because most people have no idea what it really costs to, say, replace a fuel pump.

A California start-up has launched a Web site that will give you an idea of what it ought to cost so you know if your mechanic is on the level. RepairPal.com used reams of auto industry data and insights from more than two dozen mechanics to create a database it says covers more than 20 billion estimates to repair just about anything that could happen to just about anything on the road. Think of it as the Kelley Blue Book of auto repair.

"We're bringing transparency to a huge industry," says CEO and co-founder David Sturtz. "Our goal is to minimize the anxiety and uncertainty that consumers typically feel when they service their cars."

Sturtz says Americans make about 2 million visits to a repair shop every day, and in most cases they have no idea if they're paying too much. He didn't, which is one reason he launched the site.

"I didn't like the fact I couldn't tell if I was getting screwed or not," he told Wired.com.

He started building the site about 18 months ago, when he called several mechanics to get estimates for basic repairs like changing the oil and replacing brake pads. The estimates were all over the map, even within the same city, with no rhyme or reason to it. Then he had women make the same calls to the same shops - and found the quotes they got averaged 17 percent more than the ones he got.

"Women are clearly a big target" user for the site, Sturtz says. "They almost universally feel they're getting ripped off, an our data suggests that they are."

Sturtz enlisted the help of more than two dozen mechanics and an army of number-crunchers to build the database and search engine that drives RepairPal.com. The dataset includes everything from the cost of a spark plug to the time required to install a clutch. They drew on their own experiences, automakers' factory labor time guides - huge books, generally available only to professional mechanics, that outline how long specific repairs should take - and piles of service receipts. "We've ... run statistical analyses to determine pricing patterns across geographies, brands and service channels," Sturtz says.

In other words, you can use the site to figure replacing the clutch in your 1993 Civic will run you $658 to $1,032 if you live in Oakland or putting a muffler on a 2005 Pontiac G6 will set you back somewhere between $283 and $482 if you're in Boston. That's a ballpark as big as Yankee Stadium, but Sturtz says it accounts for price variances between dealerships and independent shops as well between genuine OEM and generic parts.

The site covers 500 makes and models going back to 1990, a figure Sturtz says includes 94 percent of the cars on the road. It covers about 75 percent of the repairs anyone might ever have to make - from replacing a headlamp bulb to installing new CV joints - and offers a rundown of things that might have caused the problem in the first place.

Of course, all that info isn't much help if you have the car hauled to a shady shop, so RepairPal.com has a database of 286,000 repair shops and will recommend some in your area. As people use the site, they'll be asked to provide reviews of the shops they use. The site's free - Sturtz plans to use targeted advertising to pay the bills - but if you're willing to spend a few bucks, you can get advice from a certified mechanic.

Photo by Flickr user Ryan Stone.