This one runs off of the acelarometer and the gps I believe. I tried one of the free performance apps on my wife's deodorant incredible when I still had a blackberry and it too had a had Time picking up on satellites. Compared to what I've seen this is fairly accurate for the most part. Bad thing is my Yukon is faster than my 92 z28 lol but not by much I had traction issues with the Camaro and it's 2.73 gears kill it bad but that car is stock with just a 3 in fatback and 254k on the clock.
03 yukon xl:intake, longtubes ory,performabuilt lvl2,circle D 278,tune, tint, lt1 fans
92 z28: lb9 air foil vette servo magnaflow catback adjustable pan hard rod Lcas lcarbs
86 trans am: lm7 hot cam th350 3500 stall