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I'm Buster Brown, I live in a shoe. This is my dog Tige, he lives there too.

Heavy Duty Truck (HDT) Conversion

Red Rover, Red Rover, Send Tige Over
Red Rover, Red Rover, Send Tige Over

Last Changed 8/12/2012
In Medium Duty Truck the case was made for a Medium Duty Truck (MDT) over a Dually.  Now the case will be built for our decision for a Heavy Duty Truck (HDT) Conversion.

A Definition of Truck Classes:

Class Weight Range
Typical Manufacturer
1 0 - 6000 small 1/2 ton pickup  
2 6,001 - 10,000 1/2 & 3/4 ton pickup  
3 10,001 - 14,000 1 ton dually pickup Chevrolet = Cnnnn
4 14,001 - 16,000 MDT, F450, C4500, Ram 4500 Dodge = Ram nnnn
5 16,001 - 19,500 MDT, F550, C5500, Ram 5500 Ford = Fnnn
6 19,501 - 26,000 MDT, F650, C6500, FL60 Freightliner = FLnn
7 26,001 - 33,000 HDT, Local tractor, F750, C7500, FL70 Kenworth = T2000
8 33,000 + HDT, Over-the road tractor C8500, VNL, T2000 Volvo = VNL
By general definition MDT means those trucks between pickups local/over-the-road tractors.  However, the real MDT characteristics that make them better tow vehicles start in the Class 6 category.

HDT Conversion

A HDT Conversion is taking a used semi-tractor and modifying it for fifth-wheel towing.  The normal conversion for a HDT tractor is to remove one of the rear axles.  This lowers the weight class from Class 8 to Class 7.  Often, the rear axle is moved forward and the frame is shortened.  This makes the converted truck more maneuverable.
HDT Conversion A Volvo VNL 610 (left) and a Converted 610 (right). HDT Conversion

Here is a simple comparison of what a Class 6 generally offers compared to a HDT Conversion:

  Class 6 MDT HDT Conversion
Horsepower 250 - 350 HP 400 - 500 HP
Torque 800 - 860 lb-ft 1,400 - 1,600 lb-ft
Transmission Allison Automatic Eaton Auto-Shift, stick
Automatic starting 2004
Engine Brake Exhaust Jacobs Brake
Sleeper No Yes
Rear Seats Probably Rarely

Make it a Motorhome

Depending on the state, with the addition of a few items, a converted HDT with a sleeper usually can be registered as a motorhome.  The reason for a motorhome registration is to have a private vehicle registration.  Some states allow an HDT to be registered as private without needing the motorhome items.

These are the usual additions:
Usual Requirement Usual Solution
Cooking A microwave oven
Refrigeration or ice box A small refrigerator
Self-contained toilet A Porta-Potty
Heating, air-conditioning, or both Usually standard with the sleeper
A potable water supply system including a faucet and sink Many variations
Separate 110-125 volt electrical power supply Add AC Extension cord outlet and add an inverter
An LP gas supply Not usually added
Usually the addition of four of the seven items qualifies the converted HDT as a motorhome.  These additions can change the driver license requirements and insurance.

Used Truck and FET

We wanted a used truck because the Federal Government collects a 12% tax on new tow vehicles called the Federal Exercise Tax (FET).  This FET is bad in regards to RV towers because it is a disincentive to have the correct vehicle for pulling large trailers.  The tax originally was to collect funds from over-the-road truckers to help finance the highway system.  But as things go, any tax tends to get stretched to include sources that were not planned for.  And once the government starts collecting tax money, it is very hard to stop.  Be careful of this FET because many truck sellers are really not clear of when the tax applies and they are often wrong.  The buyer is responsible for the FET even if the seller does not collect it.

Some sellers are posing that to avoid the FET, you buy the truck stripped and then add the hitch afterwards.  The FET applies to the truck and anything added to it in the first 6 months.  This includes a truck that did not qualify for FET at time of purchase but has the qualifying equipment added in the first 6 months.  That truck becomes subject to the FET.

Used MDTs

Used MDT haulers run in the $60,000 to $65,000 range.  They have the advantage of having automatic transmissions.  The disadvantage to us is the choice of crew or extended cab bodies only.  For some, the issue of the automatic transmission can be the deciding factor.


HDT Conversion Club Cab

For families where more than two will travel, the club cab (two doors, two seat rows).
click on image to enlarge
HDT Conversion Crew Cab

The crew cab (four doors, two seat rows) available with MDTs can be the deciding factor.
We did find one Crew Chief Aero-Condo during our search.  We really liked the layout of the Crew Chief but the one we found was a stick shift and that was not acceptable for us.

Auto-Shift Transmission

The Eaton Auto-Shift is a standard 10-speed transmission with a computer and servos added to disengage the clutch and change gears at the proper time.  Since there is no torque converter as in an automatic transmission to provide slip, the clutch does need to be fully disengaged at stop.  This means the Auto-Shift Transmission has to have the clutch pushed in at stop.

The clutch is released to start.  From that point, the transmission computer with synchronize the clutch as engine speed to shift through the gears like an automatic transmission.

The clutch does not need to be used as much as it does in a car at low speeds because the low gears of the Eaton 10-speed are very low.  You can creep at 1 or 2 mph in second gear.

The Auto-Shift is not as easy as an automatic transmission but for many, it is far easier to drive that a straight manual shift transmission.

Starting in 2004, full automatic transmissions were available.  These are sometimes referred to as two-pedal transmissions (fuel and brake).  These automatic transmissions have brand names of Freedomline and Ultrashift.

Engine Brake

A HDT Conversion engine will almost always have a "Jake Brake".  The proper name is Jacobs Brake System.  The Jake Brake is an extensive modification of the engine valve train that when turned on, alters the valve timing to change the engine into an air compressor which gives a braking action to the engine.  Typically, the Jake Brake is adjustable to use 2, 4, or 6 cylinders giving the driver an adjustable braking effort.  In a downhill situation, the driver turns on the Jake Brake (actually it is usually set up to automatically come on).  The speed of descent can then be controller by switching from 3 to 6 cylinders to develop the amount of braking action needed.

This is a superior braking system compared to a simple on-off exhaust brake.

HDT Conversion Costs

Many are getting Volvo 610/630 Conversions (more on this later) in the $30,000 to $40,000 range.  Add a deluxe bed and you would be around $50,000.  So you can have more truck for less dollars than a used MDT.  If you are handy, many owners have made their own beds.

For around the same money as a used MDT, we could get a HDT conversion with a sleeper that would give us a small motorhome capability.  It would have more miles on it but it still would be well within the useful life of the truck.  We would have more torque, horsepower.  We would have to deal with an Auto-Shift transmission instead of an automatic.

Drivers Licensing

Drivers licensing for MDT and HDTs varies highly between states.  But let's start with a primary rule;  what is legal for your state in regards to vehicle and drivers licensing is valid for all the other 49 states under the "full faith" clause of the Constitution of the United States of America, Article IV., Section 1.  No state law enforcement officer can override the Constitution though some may try to overstate their authority.  Not that we recommend arguing with law enforcement officials but they can be wrong and they can't ruin you day for being wrong.

If you MDT or HDT is licensed as a motorhome, then there is a good chance that a Class C (auto) license is all you need.  Some states have started to add a license requirement for RVs and RV combos over 26,001 lb.  This includes motorhomes and motorhomes with trailers.

For more discussion on licensing, see Drivers Licenses

SD Info SD Info In South Dakota, a non-commercial truck pulling an RV only needs a Class C license.  This is one of the reasons we choose South Dakota as our residence state.

The current SD Code covering this subject is 32-12a-9.  see Drivers Licenses
and State Listing of RV Drivers Licenses.

HDT Financing

See HDT Financing


Therefore we decided that we wanted a HDT Conversion over a MDT.  Now the question was, which HDT to convert.  Our research found the Volvo VNL series to be the truck of choice.

Disclaimer: The information in this site is a collection of data we derived from the vendors and from our personal experiences.  This information is meant as a learning guide for you to  make your own decisions  Best practices and code should always be followed.  The recommendations we make are from our personal experiences and we do not receive any compensation for those recommendations.