in my opinion, a sleeper is a car that's faster than it looks to the average shmoe..

a 5g sedan w/ DX badges and no exterior work done w/ a built H22+turbo is my idea of a pretty good sleeper

a riced out civic w/ a huge kit and wing may be a sleeper to us cuz we don't expect it to be fast, but it's not a sleeper to the average bum on the streets cuz he duzn't kno better and thinks it's fast

the opposite of a sleeper would be a show car, i would think