I haven't had any experience with the Saleen, but are you getting any traction? cause sometimes your tires might be just spinning making your rev to just keep bouncing to w/e the shift point is to fast
What I did was buy the fully customisable transmission, find the actual gear ratios of the car, and copy them into the fully customisable transmission menu. Problem solved
It's not the length of the gear or traction. It's like the rpm start to fall back then it catches. All other gears the rpm fall back then shifts. Just get in it and try it. It's really different . Like different clutches in that gear