on a 70X48 track in 12th scale... I ran a lrp speedo with the novak 17.5.
I ended up running around a 3.16-3.19" rollout it temps out around 150-155 any higher and its over 160...
the 17.5 definately feels spongier (sp) than the 13.5 did on the bottom end... probably all that gear we have to run on it.