I have a Edge CTS. There is a difference between the speedo, GPS and the CTS. The speedo and GPS match. The CTS is 1-2 mph slower. The CTS is set for my tires. I've run the speedo against mile marker and stopwatch, finding that plus the GPS is on.
Also the CTS odometer is fast by about 2%.

Which one is really accurate? I know the mpg on the CTS is higher than reality by about 2-3 mpg.

 

The fact that the MPG is optimistic and reads slower speeds, I would say the GPS and speedo are correct.

 

These things are always going to read slightly different from your gauges because your gauges are mechanical. As for the mpg the programmers give you an average for how you are driving at the time you are driving so it constantly fluctuates. Stop and go will make it read lower and highway travel it will obviously go up. I have an edge evo and I think it's great it's the only mod I have and it's helped tremendously. Everyone results may differ.

 

From edge products after my inquiry that the edge MPH was lower than the gauges were reading:

"There is a small amount of speed added to the true reading on the factory cluster so that may be part of it but if you put the true tire size in along with making sure the gear ratio is correct the Evolution should be accurate."

 

I've plugged in the correct tire size and gearing.

I question which is correct since the speedo matches the GPS. I probably should contact Edge to ask about this and oil temp. My CTS reads either 53 or 250+ degrees.