Can someone explain to me what a doubler is. I searched but didn't come up with any real explanation. It seems to me the purpose of a doubler is to improve highway driving without taking away from the low gears needed for wheelin'. In building my truck, I have a 465 & a T400. I'm trying to decide which to use. On one hand I like the 465 for wheelin', but I don't want the engine to be screaming at 55mph. What do you guys do in this situation?