AWESOME GUITAR STUFF FOR DRIVING:
Crosstown Traffic by Hendrix is a good one to drive to, but is more of a cruising song. For awesome full on Hendrix see "Here He Comes (lover man)", "Ezy Rider", "Voodoo Child (slight return)" all great for driving. "Killing Floor" (BBC sessions) also rips it pretty awesomely. "All Along The Watchtower" is another fabulous Hendrix tune (Dylan cover though!) that has a wonderful rhythm for driving. There's loads more besides these!
Good driving ones by Joe Satriani: Anything that isn't a slow lovey-dovey ballad. Good examples: "Flying in a Blue Dream", "Summer Song", "Satch Boogie", "Mind Storm", "Crystal Planet" etc etc etc etc.
Steve Vai can also help with the guitar solo servings. See the baddass "Giant Balls Of Gold" & "Bad Horsie". All his more full-on stuff is good for driving fast. "Frank" is beautiful relaxed rocker by Vai. Also his earlier stuff with Whitesnake for instance is more ballsy and good solid rock!
If you're into just inspiring uplifting melodies see "Going Home" by Mark Knopfler (beautiful irish sounding tune), and "Whispering a Prayer" & "For the Love of God" by Steve Vai. Yes it can be cheesey, but oh the emotion in the playing and the sheer skill of it...
For balls to the wall hard rock you can't beat AC/DC classics like "Back in Black", "Thunderstruck", "Highway to Hell" etc etc etc kicks ten kinds of arse.
Perhaps the 2 most cathartic songs I've ever heard are Rage Against the Machine's "Killing in the Name Of" and "Wake Up" - their 2 best songs by far, followed closely by "Bullet In The Head" and "Bombtrack". Awesome driving material again, but not if your in the Prius!
Another awesome Van Halen tune other than "Panama" (the best one) is "Hot For Teacher" another fun driving song, that has a good fast tempo.
Perhaps one of the most fitting songs that should've definately made the cut into GT4 is Jeff Beck's super cool hot-rod inspired song "Roy's Toy" from his album "You Had It Comin'". You'll KNOW why if get to hear the start of the song - a huge V8 hot rod revving up! That whole album has some of the coolest guitar ever (the guy is a genius), and is appropriately ballsy with awesome driving rhythms! Awesome guitar riffs, with cool Prodigy style drums underpinning... I urge people to check it out ASAP...
After all the full-on stuff, I usually like some nice rockin' but less aggressive stuff like cool blues like Albert King, Cream era Clapton, Stevie Ray Vaughan (hang on that can be pretty full on!), JJ Cale creates wonderful tunes for cruising to, see his original version of "cocaine"- totally creams the clapton cover version! For this more relaxed vibe, some Blues Brothers music always goes down well, like "She Caught the Katy", "Shake Your Tail-Feather", "Sweet Home Chicago", "Jailhouse Rock" etc.
"Layla (Derek & the Dominos era)" and "Crossroads (Cream era)" are Eric Clapton's 2 best electric guitar songs that are cool to drive to. When either of these tunes come on in PGR2 when a London track loads up it creates the London mood SOO well...
Now for something completely different!
Also all the old WipEout2097 (and some earlier) techno is pure gold for racing to. "Firestarter" by the Prodigy is so perfect its a crime. Also "Loops of Fury" by the Chemical Brothers and everything off WipEout games in general (except WipEout Fusion, that sucked hard). The key for techno to be good for racing is simple. There should be minimal vocals, and the sounds have to be gritty and ballsy, not all wishy-washy and nancy-boyish like the Chemical Brother's newer work...
Hope I gave some people some good ideas!