Allison Transmission Questions Allison Man, I just bought a 1986 Diplomat by Executive (now Monarch). It has a 454 / 400. I noticed that an Allison 4 speed was available as on option and am curious as to whether it would be like an "overdrive" unit or one that gave me more grunt (don't need more..). What model would it have been? The p-30 chassis is an 85, the motorhome an 86. Are they still available? New/used? Expensive? Just curious about what I can do to lower rpm when driving over 55. Vehicle weighs 16000 and as near as I can research has a gross vehicle tow rate of 19000. Thanks.