Are gps units accurate to check speedo?
Champion
Joined: Aug 2007
Posts: 4,702
Likes: 2
From: N. Florida
I went from 265s to 285s and read here and other places that this would make my speedo 2 miles or so slower then my actual speed. I had to see for myself as every vehicle/situation is different. I checked first with the GPS on my phone and at 60 it said I was going 62. I checked next with a Tom Tom and it read the same, so I adjusted accordingly. After passing a mobile radar unit that the police put up it said that at 60 I was going 60, so I repeated both previous checks with GPS as well as borrowing a co workers Garmin and they all read the same, 2 mph over. I'm not going to go out and spend money on either a programmer or the dealer to calibrate it for 2 mph so I just adjust my speed. If I needed/ wanted to see actual sppeed all the time I would buy another GPS and leave it up all the time.
Administrator
Joined: Jul 2007
Posts: 24,686
Likes: 21
From: South Georgia/East Florida
NO TIRE is the exact height listed on the sidewall (and very few are true to the "listed" specs either). MY "35x12.5x17" Cepeks measure just a hair over 34 1/4" when filled to the max. air pressure as on the side of the tire. More like just barely over 34" when I run them at 40 lbs. (max is 50).
Measure them by hand and plug in that diameter and the GPS will be just about dead on...
Measure them by hand and plug in that diameter and the GPS will be just about dead on...
Site Moderator
Joined: May 2008
Posts: 9,657
Likes: 24
From: Louisville, Ky
You driving more miles than your odometer is showing. I'm calling the NTSB and reporting you right now 
I have found that the mobile radar machines on the side of the road are quite inaccurate. My speedo is dead on and it shows me a few mph slower than I am really going. My Mustang reads about 3mph higher than actual speed, yet the roadside radars show me going the speed on my speedo.
I have found that the mobile radar machines on the side of the road are quite inaccurate. My speedo is dead on and it shows me a few mph slower than I am really going. My Mustang reads about 3mph higher than actual speed, yet the roadside radars show me going the speed on my speedo.