I don't think we need a full-blown "Definition of a sports car" argument here.
Since I distrust anything on Wikipedia that doesn't include a proper reference, here's the Merriam Webster definition:
Sports car: noun -
a low small usually 2-passenger automobile designed for quick response, easy maneuverability, and high-speed driving
http://www.m-w.com
Whatever your personal definition of "sports car" is, the classic definition of sports cars does
not require the car to be rear-wheel drive, powerful or
expensive.
Rear-wheel drive?... back in the days before increasing engine power made it impractical to use front-wheel drive, Miller was racing front-wheel drive cars on the banked ovals in the US. (Have one issue of Auto Afficionado dedicated solely to these strange cars... great book sale find!) Their compact and light design made them very successful racing machines, and they're prized by collectors today.
Powerful? Expensive?... almost every motoring journalist or afficionado agrees that the Miata is a terrific sports car, yet it can't even outdrag a Corolla... and it's cheap.
The Probe GT is, by today's standards, relatively light, decently quick and certainly easily maneuverable. Doesn't matter if it's based on a pedestrian car.
And yes, it can do high-speed driving... for quibbles about this, note that many old sports cars had trouble exceeding 100 mph... while the Probe can do 130-140 mph. High-speed in this case would mean mountain carving and not straight-line speed.
The Mustang? I guess that would depend on which one. Certainly some Mustangs weren't quite up to the handling of other cars of their day, but I suppose most would still count as sports cars.
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RE: Probe versus Si: While Mazda/Ford make somewhat inefficient transmissions, the older Civics didn't have much in terms of torque. I've only got about 150 hp, and I don't have trouble walking them from stoplights. It's actually pretty easy to walk them in a drag race, as optimal launch rpms on B16s are so high that you will almost always bog or boil off the line.
By 'trashy' all I meant was slow - I give no opinions on its reliability, except to say that it was used in a lot of products so it's got to hold up pretty well.
Hahah... was just ribbing you man... not really offended. I mean, my opinion of the motor itself is the same as you, because I've owned one version or another of it for the past five or six years, and yeah, it is trashy and horribly underpowered compared to the competition and takes to modification about as eagerly as a cat to water. I'm throwing everything in but the kitchen sink, and those K20 boys are still laughing at me...
Yes, I've got an intake and an exhaust... pure rice...

...but boy does it feel wonderful on the racetrack.