Self Correcting Speedometer

[lowmpg]

245/75/17 have roughly 641 revolutions per mile
285/70/17 have roughly 616 revolutions per mile

difference of less than 4%

Fact is your tire isn't big enough for you to really notice, and "aftermarket" GPS in general isn't calibrated to be this accurate. You just got lucky in that your tire is hardly bigger than what the vehicle had OEM. Had you gone with a 35" tire, you'd be off by nearly 7% and you would absolutely notice that considering at 60 on the speedo you'd be going nearly 67mph.

Here ya go if you want specifics on the mph changes:

Speedo Reading	Actual Speed
20 mph (32.19 km/h)	20.79 mph (33.46 km/h)
25 mph (40.23 km/h)	25.99 mph (41.82 km/h)
30 mph (48.28 km/h)	31.18 mph (50.18 km/h)
35 mph (56.33 km/h)	36.38 mph (58.55 km/h)
40 mph (64.37 km/h)	41.58 mph (66.91 km/h)
45 mph (72.42 km/h)	46.77 mph (75.27 km/h)
50 mph (80.47 km/h)	51.97 mph (83.64 km/h)
55 mph (88.51 km/h)	57.17 mph (92 km/h)
60 mph (96.56 km/h)	62.36 mph (100.37 km/h)
65 mph (104.61 km/h)	67.56 mph (108.73 km/h)
70 mph (112.65 km/h)	72.76 mph (117.09 km/h)
75 mph (120.7 km/h)	77.96 mph (125.46 km/h)
80 mph (128.75 km/h)	83.15 mph (133.82 km/h)
85 mph (136.79 km/h)	88.35 mph (142.19 km/h)
90 mph (144.84 km/h)	93.55 mph (150.55 km/h)
95 mph (152.89 km/h)	98.74 mph (158.91 km/h)

Sponsored

 

[hockeynut258]

Read some of these threads and just bought a 2022 JLUR w/ 8.4” screen and thought it’d be fun to test the theory.

put on 315/70/17 tires and immediately did a speedcheck. Speedometer was off by 5.7%, read 70mph GPS was 74mph.

checked the trip meter, off by 7%, drove 40 miles using mile markers and trip read 37.4.

after putting on a few hundred miles on a Highway road trip, sure enough the Speedometer and GPS match! 70mph and GPS reads 69.8mph. At different speeds it was usually 0.2mph off. Trip meter was still 7% off. Checked multiple times later and remained consistent

UPDATE:
After playing with phone settings, wired connection and Bluetooth, seems the phone gathers speed info from the Jeep while plugged it. With the phone in airplane mode the speed is off. 70/74