Crossing the Rockies in a 172

Straight line from ENW to MYF is 1505nm. Throw in LVS (Las Vegas, NM) in
there for terrain avoidance and it only adds 17nm and keeps you south of all
the highest terrain. 10,500 is the ideal altitude west of LVS, at 8,500 you
may need to zig-zag a little between Las Vegas and Grants, NM.
Places you'd fly near or over along the way...
Moline, St. Joseph, Manhattan, Great Bend, Dodge City, Albuquerque, St.
Johns, and a whole lot of nothing until San Diego.

Northern route sure is alot longer. Through Yellowstone adds about 200nm,
figure all with a headwind, and much dicier weather probable. But you'd fly
right over Salt Lake City and the other Las Vegas (NV). Just noticed you
said during the summer. This would a pretty nice scenic route, will have to
zig-zag around the mountains more. The southern route above is surely to be
bumpier especially in NM and AZ. But you can hook up with I-40 and have one
heck of a long emergency runway and it showing the way through the lower
passes.

Another scenic option is LVS towards Shiprock, NM and Monument Valley, then
over the Grand Canyon. Total miles= 1585nm, all comfortably done around
10,500. Watch out for the SFAR over the Grand Canyon though.

Have fun, Chris