We had a mid range Cruize from Avis, that did not have GPS.
Don't pay to rent one, I brought a GPS with USA Maps off eBay for $70 before I left and resold it for the same on my return.
GPS is good to have to know you are going in the right direction, I would not have done without it but you cannot really follow Route 66 on it.
Plan out your route using EZ66 Guide and Google Maps.
The maps on this site are a great help too
June 15th to July 13th 2016
Did the whole thing, Chicago to Santa Monica