Need some info on mph in vcm scanner not being equal to actual speed

I'm just getting started with tuning, I haven't installed wide band o2s yet but I have done some data logging just for practice. I noticed once I returned to my garage to check out the data that the mph only showed like a max of 75 mph for the fastest pull. Which should have been well over 100 probably like 125. I'm not sure why this is.
I have a 2013 C6 grand sport with LTH and high flow cats with corsa cold air intake. I bought it with these mods aside from the cold air. It's def not stock as far as the tune goes. Found the front o2 wires just hanging. High flow cats were installed before I picked it up so there was no tune done for those for sure.
I'm learning to tune because like many others I don't trust anyone to work on my car. Also I believe it's running lean at times and I want to make sure if it is I can correct it before any damage.
Any help or suggestions would be awesome!:pray: