San Diego to Denver

LewBob is right on.
take the Hatch shortcut from Demming to Hatch

I think I-8/10 east to I-25 north. Raton Pass at NM/CO stateline is one of the lowest passes across the Rockies to the Colorado Front Range. If it's winter, I'd check the weather and road conditions before committing to a particular route.

Colorado Road Conditions

Just checked my phone - pictures - the sign at Raton Pass (CO/NM border) on I-25 says 7,834 feet. It's a 1,200 ft climb in less than 9 miles going up the hill.

I don't know the grade percentage - but it is definitely on the steep side.

US Hwy 287 is 'flatter' but that's probably 100 miles to the east of I-25.

Hi,

Going past Tucson can get above 3,600' elevation towards New Mexico where it can reach 5,000' elevation, before going back down into the Rio Grand Valley. Once you turn north on I25 you will start out around 1,500' elevation if memory serves me well. By Albuquerque it is close to 5,000 again.

You can look at Google Maps and zoom out far enough to click on "Terrain" (it is a option under traffic and other options). It will give you elevation lines for any part of the country. You have to zoom in enough to read some of the lines, it will give you an idea of the elevations.

By the way, going to Flagstaff is around 7,500' elevation and has many hills to climb over, so I would not recommend that way to anyone in the winter.

Yes you will have to climb some mountain passes, even I8 is over 4,000' at mile marker 60 near Boulevard CA. You might check going through Campo, it might be a lower elevation, that is the way the trains went, to avoid going over the mountains.

That's pretty flat. I recall a short section of I25 near the NM/CO border that claims to be 7% grade. Watch the weather as they can get a lot of snow from Colorado Springs to Denver.