I have 2 speed limit signs in my neighborhood that inform you of your speed as you approach. I drove past them many times in the F-150 (traded in for the new F-250) and the signs always indicated the same speed as the speedometer.

I noticed that the F-250 and the sign I pass every day were off. The speedometer shows that I am going 35 MPH and the sign indicates 32 MPH and I just thought that they needed to calibrate the sign radar. Yesterday I went past the other neighborhood sign and found the exact same thing.

While the speed aspect is not a major concern, the effects on the odometer, MPG, etc do concern me. I am going to check for a couple of days to determine if it is 3MPH off at all speeds or if it is actually 9% off regardless of speed.

I'm sure that I can have this corrected, but I was wondering if anyone else has noticed anything similar.

 

Do you have a portable GPS - like a Garmin? I put 35" tires on mine and was sure I'd need the speedo recalibrated. However, I put my portable GPS in and selected the speed screen. Mine was dead on even after the new tires (I didn't check before as I only had the stock tires for 12 miles). I checked it at speeds ranging from 10 to 75. I was surprised it was accurate after the new tires, but maybe it was off with the stock tires?

 

Great idea. My portable GPS is for marine use, but does indicate speed.

 

A 3-5% variance between stated and actual is pretty common.

 

Mine was the same way. Ford sets the speedo's with a huge margin for error and in my case, my larger tires helped correct it some. There is a federal law (Federal Odometer Statute) that among other things, applies to automobile manufacturers and states that if they deliver a new vehicle with a speedometer that reads below actual speed, they can be fined huge amounts per vehicle. Every manufacturer is bound by the law and therefore builds in a fair margin. The margin is like playing Russian roulette if it is too tight. If the margin is too close, the native stacked tolerances may cause the speedo to read low and therefore subject the manufacturer to penalty.

The statute was created to protect consumers against odometer fraud and speeding fines due to no fault of their own. The unintended consequence however, can be that the consumer gets less factory warranty simply due to the accumulated inaccuracy of their speedo's. As PowerStrokeHD just said, 5% less warranty can be a significant amount if a warrant-able failure happens right at the end of the warranty.

 

Mine against GPS - The speed seems about right... But get a consistant 1 mile off every 17 miles - Odometer - 17 GPS-18 Odometer 34/ GPS 36

Looks like I get a few more miles of warranty...

 

I haven't tracked the miles against the GPS. I'll have to do that as well just out of curiousity now.

 

You couldn't wait to use that term for Paul. "stacked tolerances " LOL

 

lol, I was wondering if anyone was going to pick that up. See? You are paying attention...

 

Just want to thank everyone for their replies.

Tested the speed shown on the speedometer VS. the sign display and they were the same as mentioned in the original post.

Then took the Garmin GPSmap76 (marine GPS that I use while saltwater fishing) and the result is:

Speedometer GPS % difference

50MPH 48.4MPH 3.2%
35MPH 33MPH 5.8%
25MPH 23.8MPH 4.8%
20MPH 19.3MPH 3.5%

Considering that we have an analog display for the speedometer and I could have been .5 to 1 MPH off (driving and looking at a GPS), I think it is safe to assume that the comments regarding 3-5% suppression on the trucks is correct.





Randy

 

I will be checking mine today via GPS. FWIW - use cruise control to accurately set your speed, then measure. results in less looking around at the various gauges to determine difference

 

Should have thought of that (didn't even know that the CC could be set at 20MPH) but like with Byram's suggestion, it didn't even cross my mind.
I'll learn.

Randy

 

For those without gps, don't forget you can always use the stopwatch/mile marker method. Find a nice flat stretch of freeway and set your cruise at say 60. Start the watch as you pass a mile marker and stop the watch the moment you pass the 3rd marker. Do the math and you have a very accurate velocity over 3 miles.

 

I have a Garmin 1490T and am observing the same as noted here - about 1-3mph slower on the Garmin than the truck (oops, had it backwards).

 

Good idea on the miles.
My truck is 2 MPH faster than my GPS.
A malibu rental I drove two weeks ago was exact with my GPS.
I need to further test with my topo GPS in my truck and my vehicle GPS in another vehicle and see what I find out.