so why did there car still run. Surely it would of wrecked the gearbox or blew the engine.
Because it's a movie and movies generally take dramatic license, exaggerate things, depict things that are unrealistic, etc. for the sake of entertainment.
It amazes me how many people watch horror, thriller, sci fi, and fantasy movies and then sit there and pick apart every little thing that was unrealistic. If you want films to reflect reality why would you be watching these types to films in the first place? Stick to historical films, biopics, drama, documentaries, etc. they tend to be more reality based. But even then you can generally find things that are inaccurate if youre looking for it.

This Saab would have dusted this old fart no performance rwd Cadillac but somehow it didn't happen. Maybe it was a standard 2.0 instead of turbo
A 57 Caddy most likely would have had a 365 engine with over 300 HP and would be no slouch. But it was obvious by the sound of the Caddy in the movie that it was supposed to be "souped up" so it would have been faster than a stock one. Not that it matters cuzit's just a movie but if you're gonna nitpick at least get your facts straight.
What are words for when no one listens anymore

Saab 900s are slow, turbo or not.
State champ in martial arts, trained with firearms, I eFF'n dare you!

This Saab would have dusted this old fart no performance rwd Cadillac but somehow it didn't happen. Maybe it was a standard 2.0 instead of turbo

When he's chasing the Cadillac across the lake, the speedometer is shown at somewhere between 100 and 110 mph. Then the engine blows up.

Actually it had both mph and km/h. Those rims on that saab were nice.

As far as I remeber the speedometer was not showing mph but km/h.

It's been awhile since I'v seen the movie, but it seems to me there was ice on the road any car that hits ice can peg the speedometer in forward or reverse if you hold the pedal down.

It must have been a SAAB 1.1 litre injection turbo - those things fly!