Last night I had a buddy of mine adjust the tire size on my Gt with Forscan. After the change the speedo was dead on but it now has a stumble/hesitation when driving aggressively.

I did some research and found this post on forscan.org

“…Forscan makes vehicle option changes.
Tuners change the engine/transmission calibration.

There is one area they overlap, final gear ratio and tire sizes, AKA the VID block info.Here is why:

The tune has the option to use the VID block gear ratio/tire size, OR those can be specified in the tune itself. But here is the catch: it is one or the other. So if your tuner is setup to use gear ratio from the tune, you MUST adjust it from the hand held device or tuning software.

If your tune setup to use the VID block, you MUST use Forscan or IDS to calibrate the speedometer for gear and/or tire size changes. Its is one or the other.

I have stopped using my SCT x4 to calibrate my speedo, as it requires a reflash (takes long lol) I now have my tune setup to use the VID block data. It takes about 30 seconds to make a tire size change w/Forscan. And since it is not a reflash of the PCM, there is no chance of bricking my PCM for a tire size change.”

So it looks like this may be the route of the issue. I am going to restore my tire code to stock to see if this alleviates the condition. If so, here is my question. Can a Dyno tuner like Diego adjust for my tire size using my SCTX4 instead? Also, should I flash the tune back to stock or just leave as it was and just restore the tire code with forscan?