I'm new to this site so I'm not sure if this post in the correct location.

I just purchased a used 2004 F150 FX4 w/ 18" tires. Well, I hooked my GPS up and wanted to see if the speedometer was accurate. According to the GPS when the speedometer is at 60 the GPS says 56? I have used the GPS to check speeds before and it has always been correct. I am thinking it's an issue with the speedometer? Has anyone else ever seen this?

Thank you – Ray

 

4-5 mph difference isn't unusual, I wouldn't lose any sleep over it. I'd be more interested in what the VSS actually said as opposed to the speedometer.

 

I have a 2004 F150 Lariat w/ 18" Wheels.
With my truck at 60 on Cruise Control my Garmin shows 56 MPH.
The GPS is right-on in my other trucks (or my speedo is off..hmmmm)
I am taking mine to the dealer for other issues and was going to have them check it out.

Qiuntin,
I am an idiot, but what is a VSS??

 

Vehicle Speed Sensor. I'm more interested in what the VSS says since it'll show a true representation of speed, as opposed to the relatively rough guestimate that the speedometer offers.

 

funny, i was using my gps while driving a couple months ago and i was going 75 gps said 71.. i think its safe to say stock are over by about 4 mph

 

When you wear the tread down on a pair of tires, the speed is going to change a bit.

I know that sounds stupid, but tire tread can change the speedo by 2-3 mph.


Besides, most speedometers are off a mile or 2 any ways.

If you want a 100% accurate indicator of speed, then you probably shouldnt use your speedometer.

 

Speddo is off, Odometer is probably close to dead on. There is a 1-5% standard error allowed in the US auto industry on speedos. My Japanese motorcycle is off by nearly 10%...that's why when I go to larger tires I may not readjust the computer for those larger tires. The 265/70/17's I want will just about offset the current error.

 

I thought they used the same sensor???

 

They most likely do, but how that signal is used and displayed is easily manipulated electronically. For instance, the Speedo and Odo on my bike are mechanical, driven off a cable from the front tire. When I am running an indicated 60 MPH my GPS tells me I am only doing about 54-55 MPH. But my Odo and the GPS are clicking off the miles nearly exactly together. In this case, the faceplate of the speedo is off by about 5-10 percent. The gearing is correct to indicate MPH and distance, but the faceplate is deliberately off. The same can be done with an electronic speedo and odometer.

 

Ray,

I just drove my new F150 FX4 with 18" tires off the used car lot last evening. On the way home, I set the Cruise at 60 and my wife (following) said why and I driving 56?

I will use my GPS to confirm the inaccuracy of the Speedometer.

Matt