Geoff Weeks Posted 3 hours ago Share Posted 3 hours ago If your going to run a lot of highway miles a steep O/D gearing is going to put a lot of engine power into heat in the transmission and rear axle. In an ideal world, you would have top gear direct and be able to get all the road speed you want. The world is not ideal, and for many reasons that is not easy to achieve and be usable. Most efficient is straight through drive, where all the gears are just "along for the ride" in top gear. Gearing up in front and down in back has lots of losses in the form of heat. It does allow smaller driveline to a point. That is why it was used of old. @Mark T is spot on about driveshaft speeds and critical rpm. It is easy to drive a driveshaft at speed that it will come apart. Shorter shafts and carrier bearing are required to keep the shaft from "whipping" at higher speeds. Spicer has a good online critical speed calculator. Quote Link to comment https://www.bigmacktrucks.com/topic/76520-9-to-8-speed/page/2/#findComment-593800 Share on other sites More sharing options...
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