UPS is up front about it these days. They publish a zone map for Ground showing where it's next day, 2 days, etc. And they generally keep to those times. In round numbers, I can get a UPS ground shipment from N Calif to almost (maybe not Eureka or Alturas or east of the Sierra or south of Monterey on the coast) anyplace in N Calif next day, SoCal in 2 days. Next Day Air does the same in the North, 2nd Day Air in the South. So why pay extra? Unless you're crossing state lines; for some reason, it's next day Ground from Bay Area to Fresno or Redding, but not to Reno.

FedEx, however, has no service guarantee for Ground. I've had things go missing in SoCal and not get to NorCal for weeks (eventually, all arrived though - they weren't pilfered, just lost). I guess they learned from doing the backhaul for USPS...