Subject: Medium Duty Trucks

Instructor: Bill Farlow

This class explored the need for and advantages of medium duty trucks for towing larger (over 30 feet) fifth wheels.


Why diesel is important in a tow truck: Better torque characteristics. Horsepower is really not that meaningful -- it is not a "measured" number. It's derived by engineers. Torque is measureable. And it's what really counts. It's a "twisting force". Nothing has to move to have "torque". HP includes speed and distance over which something moves. Thus until you have enough torque to move the vehicle, there is no HP. HP determines how fast you'll go over the hill, but torque determines whether you will go over the hill at all.


A fundamental characteristic of diesel engines is longer engine life. Caterpillar estimates average time to major overhaul, around 1 million miles. Most diesel owners are unaware of a potentially damaging condition common to all diesel engines, "diesel cavitation erosion". This refers to tiny bubbles forming on the water jacket side of the cylinder walls. They're jarred loose by engine firing, and with each implosion the bubble is jarred loose carrying a tiny molecule of the cylinder wall. "DCA" is a water additive that precludes that from happening. Cummins claims 5.9 is not subject to cavitation erosion, but speaker cautions that all diesels, including Cummins owners, should use the additive.


Diesels enjoy better fuel economy -- on average 1.5 times the mileage of a comparable gas engine. But of course the biggest variable in fuel economy is the driver!

Finally, diesel power results in lower cost per miles of operation. That means initial cost is higher, but resale is higher too, with engine life being several times that of a comparable gas engine.

The Case For Medium Duty Trucks: (Also called Class 4, 5, 6, 7 trucks)

The trailer towing capacity numbers for most light duty trucks are not too helpful in calculating the "real" towing capacity of the vehicle. They assume no options, cargo, hitch, etc, which means the maximum trailer weight available goes down by the weight of these items. In the opinion of the instructor, the maximum length fifth wheel towable by the biggest "regular" truck is around 28' - 30'. In a nutshell, that's a major portion of the case for medium duty trucks.

Another reason is braking, as medium duty trucks have adequate braking even if trailer brakes fail.
And they have a much stronger transmission, often rated at 70,000 pounds or more. With the proper engine option, they have more than adequate power for any size fifth wheel. They have less tendency to overheat, and offer far better overall safety. And piece of mind as well: you don't have to worry about getting down the mountain, or having things break.



The initial cost is typically 1.5 to 2.0 times what a regular pickup. $60K should be and adequate budget for purchasing. And it will outlast 5 pickup trucks. Unless you wreck it, it will be the last truck you ever will have.

It's large appearance is deceiving, in that its :"footprint" is within inches of a crew cab dually. It "looks" big, but aside from height it's virtually the same size. The longer wheelbase makes it look awkward, and longer. But the turning radius is much tighter than a standard wheelbase pickup truck. This makes backing into a campsite much easier.

How to Choose a Medium Duty Truck

Comfort: Buy them w/the lightest weight springs you can get. Most dealers sell trucks to carry big loads. But compared to what the usual service of these trucks, you're carrying a very light load, and you simply won't need the heavier springs and attendant rougher ride.

Capacity of front axle -- 8,000; with 12,000 - 13,000 on the rear.

Air suspension seats are a must

Wheelbase: Longer wheelbase means more comfort, and less bouncing.

Conventional versus cabover. Cabovers are less expensive, and are more maneuverable. But there typically are made in Japan, and the engine torque is relatively low. Also, some object to the ride, as many don't have enough space under the seat for air suspension. Note: to get to engine, you need to tilt cab forward -- and everything in the cab spills!

Driveability: 19 1/2 " wheel is usually adequate, but a few require the 22 1/2" wheel (International). The larger wheels are better for ground clearance and comfort.

Transmission: Automatics are nice. 600 series Allison transmission. The 500 series comes with lower HP and torque engines. More slippage. Most RVers will like the Allison 3060 "World" transmission, which is a $12,000 option. And a year's waiting time!

Limited slip differential will give as good traction as 4WD. And unless you have it, you'll find yourself slipping on wet grass.

Stick shift: Don't get 5 speed. The 6 and 7 speed transmissions are much better. They'll be a bit stiff for the first 15-20,000 miles. They're different than a pickup's stick shift, but once you're used to it they're a lot of fun.

Gear ratios: Consider engine speed for maximum torque, as well as tire size.

Tire pressure: Many dealers and tire salesmen will inflate those big tires to 115 pounds or more. But to safely carry your weight, you'll need far less than truckers hauling heavier loads. Some will be around 80 pounds in front, and 55 on the rear dual wheels. However, just how much air pressure needs to be determined by careful reference to the tire company's weight/pressure chart.

How much power is needed? 600 ft lbs minimum. Torque at the crankshaft, not at the wheels. The gears increase the torque. HP minimum of 215. And it goes up from there.

Horsepower and fuel economy: Diesel engines only develop as much HP as you tell it to. And you can get more HP w/out appreciable increase in fuel cost. So increasing HP and torque will not materially increase fuel economy.

Air brakes? Not really needed, as 4 wheel disk brakes will typically be rated to 40,000 pounds or more just for the truck alone. Standard usually is a backup hydraulic system in the event of engine failure. But be sure to check that it is included when you're shopping for your medium duty truck..