best route san jose, ca. to la junta, co.

On GM I40 is 90 miles less, and a hour or two faster, than I80. But with 20+/- hours of driving time, 1 hr isn't much. Distances are accurate, times are fuzzier, quality of the drive even more so.

I can force it to give me something short - using CA120 across the sierras, US6 across Nevada, US50 across Colorado, but that, of course, is impossible while 120 is closed by snow.

Speaking from experience....You won't go through any Colorado mountains via I80 they are on I70 and hwy 50 . The OP is in San Jose, almost on I 80 . Google maps can get you in trouble sometimes. I have been a cross country semi driver for over 35 years and camp coast to coast. You have mountains going I 40 to I 25 too... Tehachapi, Ash Fork, Raton. But chances of snow going south is pretty slim that time of year. Best of luck.

Google Maps gives the south route, I40, I25 as fastest. La Junta is out in the plains east of Pueblo, so there is no need to pass through the Colorado mountains.

I would pick up I 80 to Cheyenne, WY I 25 south to Pueblo, CO then pick up Hwy 50 east to La Junta.
It should be ok weather but I have seen roads closed on I 80 in Wyoming in April. No big passes except for Donner Summit in Truckee, CA. That route is the fastest. Now if you're not in a hurry and want to go slow and sight see take Hwy 50 from CA to CO all back roads....been there done that but.... if you just want to get there.....I 80 I25 Hwy 50.