The rpm sensor counts the bolt heads. On the 6.5, there are 4 lobes on the timing gear, so I figured the 4 bolts on the 4BT would give the correct rpm input. The X frame was done because there wasn't much room at the crossmember for the big hydraulic mounts, and I wanted the engine down in as far as possible. The vibration is considerable, but I don't really have anything else to compare it to with a 4BT. It is enough that my wife doesn't like driving it anymore. My last tank of #2 averaged 26.6 mpg, that is with the help of lp with a mix of highway and stop and go driving. I don't know how much lp it is using yet, I need a long trip to check that. I also need a long trip to see how it does on just #2. I filled it before and after a 150 mile trip today, and it got 33.6 on #2 with the lp on.