[VAC] Re: Vintage tow vehicle (1972); update

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxxxx>]

GAWR ratings only tell us what the axles are rated to carry, not what
the truck is capable of pulling. 

Towing depends on the thrust available to start a combination up a hill.
That depends on tire diameter and axle torque, which depends on axle
ratio, transmission ratio, torque converter ratio, and engine torque.
When you can't easily change engine torque, torque converter, or
transmission ratio, that leaves axle ratio and tires. You can improve
towing thrust by reducing the tire diameter, e.g. low profile tires on
the same rims. The available thrust will be improved by the ratio of the
existing tire diameter divided by the new tire diameter. You can also
kill towing thrust by going to oversize tires. You can correct for tire
size and axle gear changes by changing the speedometer drive gears in
the back of the transmission. After I changed my F-150 from 3.08 to 4.11
gears, I had to do that.

With your 3.54 gears (13 tooth pinion, 42 tooth ring gear) you would
increase the GCWR by a factor of 4.11/3.54 = 16% with 4.11 gears. If the
truck weighs 5000 pounds and the GCWR was 11,000 (6000 pounds trailer)
that would make GCWR 1.16 x 11,000 or 12,771 and so increase the trailer
rating to 7771 pounds. 4.33 or 4.55 gears would supply a more dramatic
increase in towing capacity, probably at a cost in unloaded fuel
efficiency. The 390 that I have LOVEs fuel when turned faster even if
not doing much more work.

A couple 50 series low profile tires in place of the 85 series will do
as much or more to improve thrust and will improve braking the same
amount for about the same cost as a gear change. With one other cost of
loss of ground clearance. It is harder to find low profile tires with
the load rating of truck tires.

The other determination of towing capacity is the truck braking, though
that's less critical with proper trailer brakes.

There ought to be something in the dealer books of that truck's era
giving it a towing rating based on the engine, transmission, axle, and
tires. I don't know what it is, might be in the original owner's manual,
probably isn't in the shop manual. Some imports limit the towing rating
because they don't have a sturdy frame anywhere near the back of the
truck for holding up the hitch. I don't think that's the case with a 3/4
ton Ford of any era. It shouldn't prove to be a wimpy tow vehicle as it
sits.

Gerald J.