so for the longest time ive been using the proper speedomenter gear (according to the charts) for a 31" tire with a 4.10 diff ratio. but my speedometer is still off compared to several GPS apps. According to General Tire my Grabber AT2 tires are 30.8" tall, but when i measure them on the jeep the front is 29" and the rear 29.5" I also don't know if using full time 4wd affects the speedometer.
So my question is after checking my tire pressure again when i get home. should i use a gear that matches the manufacturer tire size or actual installed tire height?