I've ran both in my TC and 27t is faster slightly. At the start of the run 27t has alot more power, but they come together later in the run. My 17.5 had to be geared to the point of meltdown to get the times I was running with my brushed. At the last carpet nats I went thru 2 17.5's in qualifying and melted one down in the B main. If I was paying for these I would have spent over $200 in motors that weekend. Oh and by the way all the cars that finished above me were running brushed. Thats why I now run the 13.5 class until they get rid of the 17.5/27T class. It just not equal in TC's.