I'm no gear-head mechanic guy, but does it ever shift to 4th in
Drive? I drive around town in 3, and when i get on the highway
and pass 55-60 MPH, i push the Shift Lever forward into Drive.
But as far as what it'll do, it'd probably create some excess heat
in the Automatic Transmission. As long as you dont go too fast
and take a few rest stops for your car, make sure fluid levels are
good, you shouldnt have an exploded Transmission...

But then I also could be seriously WRONG and you mess
your car up real bad. So in conclusion, just ignore what I just
said above.. Would hate for you to mess your SVX up.

Good luck, hope you get some solid posts.