18" and 19" slicks are what we run on the 1/8. Longer than that, American Racer is better.
You could also play with your gearing. We run 18-19 front sprocket and small rear (36-37-38). If your motor can pull a 19 front, play with the rear one because it's faster to change it.
Bigger front sprocket does put less stress on the output shaft, so it's a win/win.