Speedometer calibration

[Spock1996]

I bought a 2001 expedition a few weeks ago, it has stock size tires on but for some reason the speedometer is off. The speedometer reads faster than the gps. The scantool says the ecm reads the same as the gps. I've read a bunch of speedometer posts talk about reprograming the ecm but my ecm seems to be correct. I have a ford scantool with the right cards for reprogramming but it shows nothing for the 2002 & older f150/expedition tire change.
can someone explain this to me?

 

[0xowner]

Hey Spock... can you give real numbers and specs / tire sizes?
maybe the door replaced, does the sticker match the vin?

 

[Spock1996]

Hey Spock... can you give real numbers and specs / tire sizes?
maybe the door replaced, does the sticker match the vin?

It has the 265/70r17 on it just like it says on the sticker & what tire rack says it should have.

 

[ExplorerTom]

How worn are your tires?

 

[Spock1996]

How worn are your tires?

My tires are about half.

My stock tires are about 31" tall, it would take 36" tall tires for the speedometer to be correct.

 

[ExplorerTom]

Wonder if someone replaced the gears.

Look up the axle code on the door tag. Then jack up the rear axle (block it, put it in neutral), mark the tire and driveshaft so you can count rotations, rotate, count, compare.

 

[Spock1996]

Wonder if someone replaced the gears.

Look up the axle code on the door tag. Then jack up the rear axle (block it, put it in neutral), mark the tire and driveshaft so you can count rotations, rotate, count, compare.

Door code is h9, 3.55?

It's gonna sit at work for a week & I don't have a jack there so I drove it at 60 on the gps, in 3rd (1:1)gear. The rpms were about 2500(1800 in od(1:0.71)), the math says it should have 4.10 gears. But that's double what the speedometer is off then unless the tach isn't correct.

 

[Spock1996]

I drove my wife's expedition today has axle code 19 3.55 open diff her rpms were exactly the same as mine so I doubt the gears have been changed in mine and I must have done my calculations wrong