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Upgrading -looking for hp, tq, wt, gear info

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Been reading tons of posts on this forum as well as all the other online info i can find about RV conversion/toter/haulers for 5th wheels.  Amazing amount of experience and knowledge contributed here.  New to RV's, have a F350 Dually gasser pulling a 37' toyhauler around the northwest - i.e. over Cascade and Rocky Mountain passes.  First trip over the continental divide, 3500# under my expected usual weight of 15500# (trailer grosses 17k) convinced me that truck doesnt have the power or more importantly, the brakes for towing a trailer that heavy in my part of the country.  I'm a MDT delivery truck driver during the week so big doesnt bother me, sooooo, thinking I am looking for an HDT hauler.

Already have one truck not big enough for the job, dont want to make that mistake on an HDT.  Gonna be looking for an older unit, preferably with a deck to fit ATV or Razer,  to use primarily for 600-1000 mile weekends 10-12 times per year plus a couple of longer trips.  I have a good friend who has a coach type conversion pulling an enclosed trailer and another who's a HDT mechanic, primarily on log trucks so I have some good resources locally.  However, I'm hoping some folks here can point me in the right direction in determining what my requirements are.

How can I figure out what combination of horsepower, torque, tire size, rear end ratio, transmission choice/speeds, HDT weight will pull 20k at the 70-75mph.  I did find the advice of a minimum 425hp/1450 torque, but that looked like for a 30k trailer plus a smart car.   I'm ok with some drop on long grades, but based on my current truck if its close to maxxed out pulling flat ground at 65mph you have dang little left for hills.  6 plus percenters have me down to 30mph.  Not an experience I want to repeat.

Any info from WA state residents about licensing and registration would be greatly appreciated.  I have read the RCW's and see that if its a motorhome with appropriate equipment its not problem at all.  But I dont want to duplicate the equipment i have in the 5th wheel if I dont have to.  I see an exemption for a truck above 26k gross pulling a personal RV over 10k, but I'm unsure of the restrictions or enforcement  hassles that may come with that setup.

Thanks for any advice, I'll continue to research and  read the forums to learn more.

Happy Trails

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Welcome to the Forum

I'm still learning, but there are a bunch of HDT gurus on here that can answer your questions. Including a couple from WA - but one just "migrated".

If you can make it to Hutchinson, Kansas Oct 8-14th for the National HDT Rally you can see lots of samples of HDT and get all your questions on HDTs answered.  Most of the HDT gurus will be there, Sign up before Friday (Sept 1st) for a better rate and ...


Edited by TheLongWayHome

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As suggested, try to make it to a Rally and check out the resource guide. Jack Mayer's website has a lot of great info.

You will find that most here do not travel at 70-75 mph. It is usually a more sedate 62-65 mph. They are not in a hurry, and if your trailer has tires like mine, they have a maximum speed rating of 65 mph and we keep it below that speed most times. An occasional moment above does not hurt but prolonged running above that speed us a blowout waiting to happen.

You will find that it is hard enough to find an HDT that has the other items you  may want, like no smoke smell, clean interior, lower miles, 2 pedal transmission, that items like gear ratio do not mean much. 

Ours has too big of a motor at 14l (would have wanted a 12.7l) but has enough power and torque ( I would have to look up the numbers to see what it was specified at) to get us up the East coast mountains at a steady 60-65 mph pulling our 18000 trailer. It will go down with mostly the using just the Jake brake if I have it in the correct gear (usually 9th) with only limited amount of braking. We have 3:73 gears. Would have like lower gears for mpg savings but it is not enough to worry about.

If you are ordering new, then there are some people on here that can help you spec out the truck that will meet your wants. Otherwise just look for a truck with the creature comforts you want. Remember most of these are set up to haul 80000#.


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I am basically in the same boat. I just did a search on truckpaper.com using the advanced search feature and you can put in class you are looking for, minimum hp, even which transmission you want. I put in a 400hp minimum, and searched for class 8 trucks with sleepers and varied the transmission I was looking for. Most adds have gear ratio listed as well, but it is not a search option. I also looked up different engine families for each one I was interested in. Basically most truckers said to stay away from mercedes engines (don't last), volvo engines(proprietary systems/durability), newer cat engines(egr problems expensive parts), basically anything newer than 2007(emissions). Cummins and Detroits seem to be favorites. 

I completely understand the gear ratio question and the way I see it, the weight we are pulling, get the highest gears you can. With the amount of torque these trucks make even out west you aren't going to make them struggle. The difference in 8mpg and 10 mpg is huge, if higher gearing can help me achieve that, even a regear is cheaper in the long run.

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DOn't paint yourself into a corner with limitations.

There are a bunch of Volvos (like mine) on this forum that are working very well.  They are no more expensive or problematic than any of the other brand engines/chassis.  Mine is a 425 hp Ecco-Torque 13 liter engine with a non-overdrive iShift and 2.47 ratio.

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We ordered our truck with similar needs to yours. We have a 21K loaded fifth wheel and have a smart car on the deck. I wasn't worried too much about fuel economy but we still get decent numbers when hauling as long as I keep my foot off of the skinny pedal. We started with a Cummins 15 liter ISX as they can be worked on anywhere and is the only engine available in all makes of trucks and ordered the performance model to get the HP and torque range that I wanted (500 HP and 1650 torque). We were after a 1850 torque engine but KW limits you to 1650 when using a single rear axle (in our case a 23K rear). It was set up with a 2 speed Horton fan clutch, 37.3 CFM compressor and will be turned up to 1850 torque when the warranty runs out.

The transmission choice was a 13 speed Eaton Ultra Shift Plus with a double overdrive pushing a 3.07 rear gear. This is not the advantage transmission as I wanted a cast iron case and filters while the lighter transmissions do not have a filter and have an aluminum case. The "Plus" is a lot different than the earlier Ultra Shift in that it uses a regular clutch that is servo engaged. The Plus will also skip shift, has the option to start in different gears, has hill start aid and a lot more options than the earlier version. Having 13 speeds will help in the hills compared to a 10 speed and the equivalent 12 speed transmissions of the DT12 in a Freightliner or I shift in a Volvo will also help in the hills.

The Cummins has a sweet spot of 1,400 rpm so the gearing I chose lets me run roughly 55 mph in 11th, 65 mph in 12th and 75 mph in 13th (within 2 or 3 mph of those speeds in those gears). I went with standard 295/75R22.5 tires as they are the most common size available and did not go with super singles.

The wife and I did have a lot of help from Scrap on the specs as he really did get into the project (as did a few others including my salesman) but in the end, you need to make you own choices based upon your needs and wants. I happen to be a gear head and lean more towards the performance end and went against Scrap's and my salesmen advise for the engine and rear gear ratio knowing what I wanted to do with the truck. Scrap was in the 2.90 range and my salesman was in the 3.20 range but in the end, I set it up for what I needed and wanted for our use.

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