Bill,
As an alternate route you might investigate staying on 93 past I40(talk about your super slab!!!) going toward PHX. And it appears that about 40-50 miles south of I40, you make a left on AZ-97(NE bound) and then a right on AZ-96(SE bound) to Kirkland. Then take a right on AZ-15(SE bound) at Kirkland, to the AZ89 and take a left(NE bound) and up over the mountains right into Prescott, and through the Prescott National Forest.
This looks to be close to the same mileage from L.V. to Prescott as droning on I40(or Old Route66) and then taking AZ-89 south.
I haven't been on those roads, but they appear to be pretty decent looking according to my map, and it also looks like you'd be riding some nice mountain roads instead of that flat straight stuff.
