To be a true sleeper, it has to look like anythig but fast. It's supposed to be a surprise. The first tip off is the tail lights disappearing into the distance.
I had a lot of fun with a Dodge shortbed truck in the early eighties here in Germany. Back in those days, the typical German pickup (kind of) was a four banger diesel with like 60-70 HP. If they ever mad eit to the fast lane the Mercedes drivers would harass them with blinking headlights and horns blaring.
My truck was painted the same color as the interstate service vehicles are here. Looked like a rest stop trash truck. The only real tipoff was the whistle from the blower and the wide tires. I could burn the Mercedes of that period by about 7-8 MPH top end and get there a whole lot faster.
I think the sleeper that impressed me the most was full size four door Chev. it was quiet, kind of a dark grey metallic, had slightly oversized Michelens on it and left me sitting at a stop light, without squeeling a tire or making any huffing and puffing noises and just disappeared into the distance. The part that impressed me the most was, I was sitting on a 120 horsepower motorcycle and was throttle down trying hard to keep the front tire on the ground at the time.
My favorite was an early sixties, Valiant station wagon, flat and peeling red paint, four door, with a built Hemi, load flite manual shift auto tranny and a 4.88 Dana rear (full floating axles
). A conservative estimate of 400 HP, which wasn't the whole story, the torque that sucker produced was awesome. The body would flex and the tail gate would pop open. 10.50/16 Town and Country snow treads, aired down to about 15 PSI (they actually lauched really well). The tires would regularly disintigrate, but they were a large part of the fun. Tire chunks pelting the poor sap behind me. Had a sign on the door (hand painted) that said "Need your pipes cleaned" call Chuck.
It has too look anything but quick, with a large surprise factor, to be any fun.