5.5 is more like a 9T or less. I use to have a GTX/orion 10x1 in my t4...the 10t was way slower them the 5.5. That was for outdoors.. I had a 5.5 in the truck, 6.5 in the buggy. was great for this large outdoor track. Indoors w/ only 100' straight I had to put the 6.5 in the truck.. 5.5 w/ the radio dailed way down still too much motor for the traction. Funny thing is I run much quicker laps w/ the 10.5 in the buggy comparted to the truck...granted I got alot more laps in and fine tuning on the buggy. But sometimes bigger isn't always better. If I had another 10.5 I'd prolly replace the 6.5 with it...6.5 still too much for the traction I have available.