I've been dealing with an inaccurate speedometer on my 2013 F-150 SCAB 5.0. I had the computer checked out by Ford techs to make sure tire size (265/65r18) matched and everything was fine according to them. It was still off by about 15% (Reads about 80 with a GPS reading of 70). Ended up getting an SCT tuner and was able to get the speedometer to match GPS when I changed the tire diameter all the way down to 26" (775 rev/mi).

In a series of missteps by me, I had switched back to the stock tune to deal with the recent recall and bricked the SCT tuner before I was able to get the tune back on the truck. I figured while I wait for my fixed tuner to arrive, I might as well figure out the real issue with the speedometer readings. I have tried changing the tire size using Forscan but haven't been able to get much closer than 10% (and that was with changing the rear end ratio to 4.1). Anyone have any clue as to what my issue is?

 

The speedometer needle is intentionally off, many vehicles are like this, its even worst with the European cars. When comparing the OBD readout vs. a GPS reading on my truck, both are pretty close within 2% or less.

 

I wouldn't say it is intentionally off. But it is impossible to get the speedometer or odometer accurate to within less than about 5% error. And unless someone is running tires that are significantly larger or smaller than factory I don't recall ever seeing one more than 5% off. This is due to the fact that tires get smaller in diameter as they wear down and the speedometer is actually just counting wheel revolutions. Then using the tire circumference X revolutions to calculate speed and distance. But the circumference is always changing. As the tire gets smaller it takes more revolutions to travel a mile and the vehicle thinks it is going faster and traveling farther.

Something is wrong. If your tires have over 40,000 miles on them that would account for about 5% of the difference. The other 5% could be an inaccurate GPS or something else.

 

Not 15% off. My ‘03 is a flat 3 MPH off no matter which radar I pass by. Probably a nice way for Ford to dodge the warranty a few thousand miles early (assuming average speed of 30 MPH, which is reasonable) for new buyers. Sucks to the previous owner of my truck, though it’s got a 4.2 in it. No biggie.

 

Like the others have said, less than 5% off is normal. 15% off is not intentional. It puts the speed limiter at 85 instead of 100 which is plain annoying in Texas.