I ralize this has been addressed in other threads but I thought my particuliar case was a little wierd. My truck has had 35" Tires on it since I had it, I recently switched back to a stock size and now my speedometer reads high, as much as 10mph off on the innerstate. I understand it's a software issue but my truck has no programmer on it so why would it be off?
At any rate I guess my only fix would be to buy a Smarty to correct it?

 

Whomever put the 35's on probably calibrated the speedo.

 

Isn't that done through a programmer though? The truck had a diablosport programmer on it but I removed it completely, I reset the unit to stock tuning when I removed it and sold it. You would think that would have put the speedometer back as well right?

 

It can be done at a dealer service department with their diagnostic tool. Some dealers charge for this, some don't if you are a regular maintenance customer. They charge anywhere from $35-$75 to reprogram. Cheaper than buying a downloader that can do it.

Some auto/diesel shops have tools that can do it also.

Smarty is one of three or four downloaders than can change the programming for stock tires.

 

The ECM does the speed, the programmer was used to change the ECM but the programmer doesn't calculate the speed itself.