GPS for steep grades

Good Grief! as Charley Brown says. You shouldn't be pulling a Toad if you can't do the grades. There are only about 3 over 7% going west if you go go out I40 then up I5 or 99 in Ca. As I recall after driving fro OKC to the NW about 200 times in a big truck. The highest elevation on I-40 is 7,330 ft, at the Arizona Divide just west of Flagstaff. The steepest grade is around 4% for about 3 miles near Harriman. I ran trucks that had a maximum of 230 Horsepower, hauling cattle all over the West. It took a half hour to climb La Veta in Co at 13 MPH. I you can't go faster than 45 mph you will get a ticket in NM or AZ for going to slow. Ask my Aunt.
A driver can receive a traffic citation for violating the Basic Speed Law even if their speed is below the "maximum speed limit" if road, weather, or traffic conditions make that speed unsafe.