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Message: [QUOTE="me2, post: 384507, member: 3825"] Here is how towing works with the 5.9 CR/ZF6 in my F250. The ZF6 ratios are 0.73, 1.0 and 1.31 IIRC. At 60 MPH the engine speed is 605 x 3.73 x 0.73 = 1650 RPM. At 65 MPH, its 1784 RPM. The 5.9CR is stock 610/325, except that I advanced the timing, so its probably making 630-650 ftlbs, all at 1600 RPM. 650 ftlbs x 1650 RPM/ 5252 = 204 HP. This is nearly 40 HP more than the 6.4 makes at the same truck speed, 60 MPH. 650 ftlbs x 1784/ 5252 = 220 HP. Again, 35 more HP than the 6.4 at the same speed. My F250 pulled the trailer fairly nice in OD and could pull moderate hills without too much loss in speed. Being the ZF6 is a manual transmission, I could control when and if it downshifted. If there was no traffic behind me, I often let the engine pull down and torque its way up the hills. On larger/longer grades, I'd have to downshift into direct. When I say grades, generally it would take a mile long climb at 3-5% before I'd need to downshift into direct. Engine speed at 60 MPH in direct is 3.73 x 605 = 2250 RPM. I don't have an engine curve for the 5.9CR, but it probably makes about 600 ftlbs here, so the engine power is 2250 x 600 / 5252 = 257 HP. This is about 20 HP less than the 6.4. It takes a pretty good grade in both steepness and length to require a downshift from direct, especially if you let the speed fall back a bit, to say, 50-55 MPH. Being that the ZF6 is a manual transmission, you can do that. Most mountain passes can be climbed at 50-55 MPH in direct. No fuss, no muss, its a bit slow, but if you are patient and enjoying the scenary, its a great way to go. The next gear down is 3rd. Engine RPM at 60 MPH is 605 x 3.73 x 1.31, which is 2950 RPM. This is pretty much wound out on a stock 5.9CR, but I usually let the speed drop to 50 MPH or so before downshifting into 3rd, so the RPM is more like 2450 RPM. The truck will pull a house up pretty steep inclines in this gear and I can only think of a time or two that I have ever shifted into second. 3rd gear is also a great downhill gear, as long as you keep the speed below about 65MPH. The 5.9CR provides pretty good engine braking even without an exhaust brake at this RPM and I can usually come down 5% grades without touching the brakes at all. Fuel consumption wise, the 5.9CR uses about 25-40% less fuel than the 6.4, though the 5.9 might be slightly slower due to letting it pull down on hills. However, the 6.4 is no speed deamon either if you are feathering the throttle in direct trying to keep it from screaming its brains out in 3rd gear. So even though the 6.4L is rated at 650/350 I far prefer a 5.9CR with a ZF6 for heavy towing. My apologies for boring people to death with this stuff. Our dog needs more tests. [/QUOTE]