Speedometer not reading correctly.

[Marky Dissod]

Using gps it says I’m going 70MpH
but the speedometer says 80MpH
with the scanner I have ecu says I’m going 80MpH
and with a car pacing me actual speed is 70MpH

Speedo in the driver instrument cluster reads fast,
ECU says same as DIC-speedo,
and GPS & some other car says actual?

Your ecm needs to be reprogrammed.

 

[PanchohasAyukonno]

Speedo in the driver instrument cluster reads fast,
ECU says same as DIC-speedo,
and GPS & some other car says actual?

Your ecm needs to be reprogrammed.

How would I go about reprogramming? Where do I start?

 

[Marky Dissod]

Unfortunately, best bang for your buck would be to have both your ecm & tcm tuned.
BlackBear Performance comes highly recommended here, but there are other tuners with good reputations.
Do your due diligence before getting your vehicle tuned.
'Best bang for your buck' does not mean 'cheapest'.

Hang on a sec ...

Are you running a different size tire then stock?

Running equivalent size to stock.

Pray tell, what is the actual size of the TIRES on your Yukon?
Is this the same size as the tire size mentioned in the door?

If it's not, and your ecm & tcm have already been tuned, then you should ask the previous owner who the previous tuner was.
Updating a tune is always cheaper than a fresh baked tune.

 

[PanchohasAyukonno]

Unfortunately, best bang for your buck would be to have both your ecm & tcm tuned.
BlackBear Performance comes highly recommended here, but there are other tuners with good reputations.
Do your due diligence before getting your vehicle tuned.
'Best bang for your buck' does not mean 'cheapest'.

Hang on a sec ...Pray tell, what is the actual size of the TIRES on your Yukon?
Is this the same size as the tire size mentioned in the door?

If it's not, and your ecm & tcm have already been tuned, then you should ask the previous owner who the previous tuner was.
Updating a tune is always cheaper than a fresh baked tune.

That’s where I’m hurting I’ll give you the size I have on now later today and what the door says as well, I can’t contact previous owner because I got it at an auction