The mileage between Amarillo and Yuma is right around 900 miles, plus or minus depending which route you choose. It is 930 on the interstate route Mark and I both suggest, and driving time is estimated at 12.5 hours.

If you take the US60 route I outlined, with a small change to cut southwest, instead, from Santa Rosa (NM), to Vaughn, Socorro, and Show Low (AZ), etc, then the miles are about 890.5 but the time is 14:00 or thereabouts. You save roughly 40 miles. According to my software, this is the shortest distance between the points.

If you think about WHY the interstate is faster, even though a greater distance, that lends strength to our arguments to use it instead, from the "grades" point of view. It is quite simply an easier drive than most other state or federal roads, all things considered, because it was DESIGNED to be that way -- with an absence of sharp curves and steep grades, as much as possible. However, if the 40 miles makes the difference, and you care not about the time, then the US60 route might work better for you.

I like the US60 route immensely. Although it is two lane blacktop, it is good, wide road and the grades are no worse than those found on the divided roads in most places. Towns are far apart, so you CAN make good time on it, even if it is slower than the "I" route.