2000 era, Beaver or Foretravel

I can speak to the retarder vs. Jake issue as I have had both (99 Foretravel with retarder and 95 with Jake). I really like my 2 speed Jake but it really can't compare to the Allison transmission retarder. The retarder gives you total control of braking and you just can't believe the stopping distance with the disk brakes and full retarder. Regarding the transmission heating issue, yes, the retarder heats the fluid but ours always stayed well below the Allison limit going down those long mountain grades. So the retarder, in my opinion wins hands down but the Jake is a good and safe second choice.
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