speedo question

Hi gang - new here from Michigan but not new to cars and trucks.

My question is in regard to speedometer and calibration of that speedo. I have a 2004.5 Dodge Ram 3500, auto, 4X4, with the 5.9L Cummins. Now I have changed tire size on a number of vehicles in the past so I have an idea of how it should work. For example, if you go with a larger size tire than what the speedo is set for you will now be reading slower than you are actually going - speedo says 65 you might be doing 70.

Now I bought this truck used and the tires currently on it are 285X70X17. The originals were 265s so I have larger tires than the original. I have noticed that when driving on freeways at 70 I get passed rather regularly - I know, I know, everyone drives faster than 70 but this is much more pronounced than when I do 70 in my Grand Cherokee. So I did a GPS check on the truck yesterday and when my speedo says 70, I'm doing 65. That is the opposite of what it should be so I am a bit confused. How did this happen?

Something that may be a contributor. I have a Smarty S06 tuner with this truck and I had recently programmed in the tire size. It might be my imagination but this slower speedo seems to have become more noticeable around that time. Could that have an effect on the actual speedo since the Smarty impacts the ECM or is my speedo more a mechanical one?

Kinda stumped here.

I know nothing about the Smarty, but with a larger tire. You actually speed will be more than indicated.

Your speedo is electric with a signal coming from the rear end or transmission.
The Smarty is supposed to have the capability to correct for tire sizes.
You can confirm it by the mile markers on the interstates.
Reset your trip odometer at a mile marker then see if your trip odometer keeps up with the mile markers.