The guys at the automotive blog Jalopnik have obviously been playing Grand Theft Auto IV. In The Real Life Cars Behind Grand Theft Auto IV, they examine many of Liberty City's fictional vehicles for elements of their real-world counterparts. You can read commentary such as:

Though the headlights scream Charger , a little closer examination of the rear taillights would suggest the Presidente is all about the Cadillac CTS...and the 175 mph top speed would hint CTS-V.

For amusement value, it's worth continuing down through the comments, where you'll read tidbits such as:

Good lord you guys are dense. The Coquette is almost exclusively based upon the C5 Corvette. In fact, excluding the taillights, you could actually modify a C5 to look exactly like the Coquette with existing aftermarket body parts and wheels. Hell, the wheels on the car and non-popup headlights are in fact very common mods seen on the c5, and the slits coming out of the side-gills are an accessory kit.

Gearheads are a picky bunch. Nearly as bad as gamers.

Jalopnik's car critiques led me to a great resource at the GTA Network's fan site for GTA4: a list of all the cars along with statistics taken straight from the game's files. Ever wonder who would win in a head-on collision between a Patriot and a Cavalcade? Every wonder how fast an Infernus can go? Every wonder why the Dukes and Stallion look so cool but suck to drive? Look it up here.

(The answers are "they have equal collision durability, but the Patriot has 500kg advantage", "208mph, but with an acceleration as good as the fastest bike in the game", and "they have the exact same suspension as a crappy old rusty Vigero".)