Those with GPS- How accurate is your speedo

[04SCTLS]

I recently put in a Kenwood 9140 nav/stereo unit and have noticed that the garmin gps reads my car as going 4-5 mph less than the speedometer.

I have 245/40/18's on the car.

The guys on the kenwood forums say their speed readings are dead on mostly.

I can use my SCT flasher to reset the tire height but was wondering if anyone else had any experience with this.

 

[oandtmaster]

when im going 75 on the spedo, my tom tom gps, and cell phone (spedo app) and the police speed radar sign all say 73....

 

[owlman]

Well, I don't have a gps but whenever I drive by those "Your Speed Is..." signs they always read 2-3mph less than the speedometer. :shifty:

 

[Beamer]

My speedo reads 2mph slower than the GPS. I have a new Garmin Nuvi btw

 

[Adam Moore]

My nav/gpd reads the same as speedo

 

[CrackerJack]

+1 for the Your Speed Is sign, I always get around 2 or 3 under from my speedo. Maybe its built in anti-speeding technology from Lincoln, they know we'll speed in this car so they help us out by having us think were 3 MPH faster than we really are

 

[Dacket]

My f90bt is dead on. I don't have the speed wire hooked up, because I can't find it. I have stock wheels too. Also it yells at me for speeding too. lol

 

[mrzeee]

My wifes tom tom shows about 2-3 miles more that the speedo in either car I have. My magellan gps (not a street finder) that you carry into the woods is dead on....but it uses all 8 satelites and WAAS or whatever that is.

Max

 

[ground_zero298]

Last 3 garmin units I have had have always been 2 miles over what the speedo says, I also have the external ant. and am running 3 foot acuracy. Tracking 12 sats, but I also have the cell ant. feature enabled, which could be a factor of that.

 

[1LoudLS]

4 @ 80, and around 2-3 @ 55

my Avic F900bt (w/ VSS hooked up) and my blackberry read the exact same speed

 

[Fla02LS]

I have found my car to be alittle slower with the speed sign as compared to the speedo. But when you think of it, the LS speedo cant really be viewed as being dead accurate, the numbers jump in 10 mph increments with no 1 mph lines in between and the dial itself due to its size/width looks like it could have a few mph variation. I'm glad the LS may show a faster than actual speed, i usually set my cruise at 76 mph on the highway and i have blown by some state troopers sitting in the median and they usually give you about a 10 mph freebie. Never been stopped yet thankfully.

 

[Quik LS]

the GPS units are typically bang on - since it is fixed position with measurement of difference in distance over time. Speedos are usually a 'tad' high - since there are laws governing their accuracy - focusing on 'not to be lower than'.

couple of things come into play though - tire wear (reduction in thread depth) and the 'smoothing' of calculations that occur as error in the GPS when you go around corners. Since the GPS does not read every milisecond, the distance calculated is different than the actual distance traveled in a corner.

so in a perfect straight line - the GPS should either be bang on to the speedo or slightly lower.