A little of what Mkirsh says, IE the speed of which you travel is true!! altho frankly, to me slowing from say 60mph on a typical 3-5% freeway mtn grade is not a big deal to me. BUT, as noted before, when I can not get up a given grade in 1st or reverse, still under gcwr etc. NOW I have an issue. As I can not GET TO the end place I am trying to get to.
Being over a gcwr is generally speaking, nothing unsafe per say. You only slow down, or may not meat the engineer minimum max grade one can go up before literally stalling out. For the new SEA specs, this is a whopping 12%. My sister has an 18% side road in front of her house. I have a client with a 30% grade driveway from a water front home on Lake Washington. So I am under gcwr by a few hundred lbs at the bottom of either dead end place. Only way out is up, BUT, I can not pull the grade with the trailer/load at hand! If you are an out of the way forest service road camper, unlike a lot of folks that are only on asphalt or concrete. you could be REALLY SCREWED in a situation like this!
This is not so much a total HP or torque issue as it is how low an overall low first or reverse you have. Some can be helped with 4lo. BUT, if your grade to get out of, is a rake roughed up concrete 25-30% grade with a few hairpins, you've now blown up a transfer case or equal!
For me it is NOT the freeway grade slowing down, but as it was with the examples I gave earlier, not getting up the hill in first! After that incident, I figured out, one can find formula's for how steep a grade you can go in low gear based on power, turns out torque is used in this formula. If you want to know how fast or fastest you can go on say a 5% grade, that will be based on HP. There are a number of other factors to be included. Wind-age, frontal area, aerodynamics, tires as in number of them, type ie bias vs radial and traction vs a hwy and st 75-85% sidewall to tread width ratio vs lower profile in the 50-75 range. Lower profile gets more hp to the ground, as does radial to bias, and a hwy tire uses less HP than a traction tire. Using AC, or for that matter, JUST having an AC pump uses 5 or so HP at the flywheel. A single wheel rig with 20" of tire per side, uses less HP than an equal dually, as a 20" wide single weighs less than two 10" tires on rims totaling 20" of tread width to the ground.
If you go to some of the truck manufactures that build MD to HDT trucks, you can find the formulas to figure this out. It is not just an engine axel ratio that can make a difference as the LD truck manufactures, and some members of RV net will lead you to believe.
The first thing I do when specing a truck, is get the payload needs I need, then spec the drive train to HOW I am going to use the rig. Not how some engineer in Detroit thinks I should be speced. My 96 6.5td could pull a 20K load up a 30% grade. My 2005 dmax, with 50% more hp and torque, might make it up a low to mid 20% grade before stalling out at 20K lbs. Yet the 05 is rated some 10Klbs MORE than the 96. All because the 05 at 20K on a freeway grade can go 55-60, where as the 96 was doing 40-45 or so.
Marty
Marty
