Southern route avoids elevation. I flat out stopped the I-40 east of Albuquerque in 1992. Turns out the parts store “rebuilt” distributor had the wrong contact points installed ;( . But even after that was fixed, same car going east up to Eisenhower Tunnel on I-70 would slow to a jogging pace. Stupid 67 Chevy II with CA smog pump and Carter YF carb.
Another factor since you are driving an "unknown entity"... Are the towns more spread apart on the southern I-10 route? Just thinking about if you break down.
Does car have A/C? if not go cooler route. Also easier on old car. But, both routes are desert heat.
I think these photographs from that 1992 trip on 1-40. Remember when Dick Simon ran Indy Cars?
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