[Titanium48]

If you want to be comfortable and not worry too much about your car's limits, avoid mountains, if and when moving from East Coast to West Coast. Try to avoid those Rockies... Although it may take more time and it's a longer drive, take I-10 West (from JAX) and hook up with I-5 in SoCal North....
Also, drive most of the time at night...
This may be water under the bridge since the original inquiry was placed here 2 years ago....

That's a long way south. If the origin is in the US northeast and the final destination is the BC coast, the best mountain-avoiding route would be to cross the border in Manitoba and follow the yellowhead highway (#16 and #5), then take highway 1 at Kamloops. Still a bit longer than taking I-90 to Seattle, but the highest elevation on the whole trip is under 1200 m (4000 ft).