215/75/15 is 27.5" vs 31" tire for a diff of 3-1/2" diff.. That's pretty a big diff... correcting the speedo is good. It will drive with the tire change, but it be sluggish. A gear change would bring it back to a good performance level.
2004 Wrangler Rubicon 4.0 w/Auto, 3.5" Rubicon Express Super Flex lift w/Currie Control Arms, RE Mono Tube Shocks, RE CV Drive Shaft, Engine/belly pan skid plate, Co2 Powertank. Hi Lift Jack.