I've had the chance to check several vehicles, US, European, Japanese, and Korean. Each and every one has read high by and average of 2 MPH. I know there are standards in Europe (UNECE Regulation No. 39/00) that require speedometers to never, ever, read low but can read high by as much as much as 10%. My '13 F-350 reads 2 MPH high at 60 while the speed reported by my Scan Gauge II is right on the money with the GPS. So the truck knows the true speed but the speedometer shows 2 high to keep me out of trouble and the cover Ford's, well, you know!!

 

My 2011 with Toyo open country AT's 285's are right on the money. I have the cruise control on at 70mph and the GPS read 70mph and the speed trap said I was doing 70mph. I never checked the original tires for speed accuracy.

 

Did manage to get some slightly decent fuel economy running 70 mph at one point during my run out to NM and back...



But couldn't manage quite as much at 76 mph...



Pulling this tiny trailer...



Actually wasn't pulling the trailer for those speedo pics, was on the way back, bobtail.

Kind of fun seeing the mpg readout go that high, wish it would actually stay that high!

 

If you're running a GPS, why in the world would you be using mile markers on the road to check the odometer? And over what distance?

Fire up the GPS, put your truck's display in engineer/debug mode (does it work with the basic readout instead of the info screen?) and see what it says for speed compared to your GPS on the highway. Ignore the speedo needle. The computer uses data from sensors to figure out how far it believes you've rolled. From that it's able to calculate speed and distance traveled (odometer). If it's off due to your tires, you'll need to get it corrected either at a dealer or with a tuner.

 

If you ever get a chance to examine a Ford Police version of any vehicle type (Expedition, Explorer, Crown Vic, etc) look close at the speedometer and there will be a graphic on the plastic screen below the Speedo dial that says something about accurate or true speed. This will only be shown on the Police versions so that they have access to accurate speed. The vehicles are perfectly capable of showing the correct speed on the analog gauge, Ford is deliberately showing the inaccurate value we all experience.