speedo accuracy? self adjusting?

[Corb@CorbinShipping]

ok, truck had 3.55s from factory. I put in 4.10s. I figured that I would be off a pretty descent amount, but no, it was still dead on. Registered within 1 MPH at 60, on my buddy's speedo calibration machine at his shop. So, I figured that it must have the speedo gears in the rear diff.

Until, that is, I put new tires on.... not a huge difference, but i went from stock 275/65 r20, to a 325/60 r20. So, from 34.1 inch, to 35.4 inch tires. So, again, I figured that I would be off a bit, maybe a few MPH..... NOPE! still within 1 MPH at 60.

I have never adjusted the speedo, on my tuner. either the spartan, or the H&S...

So, I do have the NAV system from factory, on my 2010, is the speedo self adjusting through there? Or is my truck an alien life form?

 

[eric.davis]

No you haven't drastically changed your tire size. Mine came with 33s and I have 32s on there now. Some speeds gps and speedo read the same and at the most I'm off by 4-5. Your gears aren't going to effect your speed just your rpms will be different. tire size is the only way your speedo will be effected because the amount of revolutions per mile will change between sizes.

 

[WoodBoy]

edit.