[VAC] Re: Towing With A Manual 5Speed Transmission

[Dr. Gerald N. Johnson, electrical engineer]

The tow truck that smokes clutches suffers from the factory quest for
economy and narrow range transmissions with excessively low rear axle
gear ratios. I've seen Ford trucks with that problem for nigh on 40
years. Even farm trucks.

ANY engine needs to turn up to produce horsepower. Say 1500 RPM while
starting a load. Then the factory axle and first gear set that ground
speed too fast, the clutch has to be slipped to get the rig moving.
That's inherent in the automatic. The starting torque of the automatic
depends on a 3 to 3.5:1 low gear and a 2 to 2.5: 1 ratio in the torque
converter. Over all ratios 6 to 8.8:1. Attach that to a 4.10 rear axle
and you have an overall ratio of 24 to 35:1. Meaning that 1500 RPM at
the engine corresponds to about 1 to 1.5 revolution per second at the
axle. Ground speed 8 to 12 feet per second, 5 to 7 mph in round numbers
with 30 inch O.D. tires.

Getting a 20:1 over all ratio with a manual transmission having a low
gear of 4.72 (ford 5 speed heavy duty) means the axle ratio has to be
4.24

Last winter I spent a lot of time looking at a smaller tow rig, F-150
with small V-8 and low gear in the manual transmission only 3.92. I
installed 4.10 axle gears. I'm quite happy with the arrangement, there's
enough torque in first gear to move my Caravel anywhere I want to move
it in first or reverse (though another 50% speed reduction would
occasionally be useful and lead to killing the engine less often while
trying for precision positioning), and enough torque in 3rd gear to get
up any hill in Missouri at 50 MPH. I toyed with going to 4.56 gears but
decided I didn't need the additional 1000 pounds starting capacity,
besides I could get there by change the rear tires from 29.1" OD to 25"
OD with really low profile 16" tires... A set of tires costs about what
the gear change did.

Bigger loads require a higher rear axle ratio. I don't think the V10 low
speed torque will overcome the need for stump pulling torque for
starting with the 3.83 gears. With that much engine, I'd lean towards
4.33 or higher.

In my F-150, the gas mileage was 16 with the old 3.08 gears, and without
Caravel is till 16 with the 4.10 gears, providing I don't win too many
times at the stop lights. The Caravel (at 50 mph) dropped the mileage
down to about 15. Unhooking the air conditioning compressor raised the
unloaded mileage to 17.8. Running unloaded I generally start in second
gear because that's the same overall ratio NOW as the prior truck had in
second gear and that any truck normally has had starting with a 3 or
3.5: 1 first gear.

Be very careful about oversized tires, they can kill off the thrust
gained by high axle ratios.

My neighbor has a 98 Dodge 3/4 ton 4X4 with a V-10 gas. It gets about 7
mpg empty or loaded (automatic transmission) and with a 32+ foot SOB
gets that same 7 whether towing at 50 or 70. I have yet to convince him
that his life is much shorter in emergencies at 70 when stopping and
control may be a severe problem.

Buying a manual transmission truck these days is harder than buying a
Lamborghini or new Ford T-bird (sold out in 2 hours I heard). The
dealers don't want to touch them (their sales staff probably doesn't
know how to drive one to demonstrate it), and claims they can't sell a
used one. Several dealers I've visited plain out say, they won't have a
used manual transmission vehicle on their lot. If they take it in on
trade they will get rid of some other way than selling it on their lot.
I bought a '98 F-150 last January that I think had sat of the lot most
of a year because the claim was that the owner bought a new truck each
year. They were willing to sell it at a fairly decent price because of
that time on the lot. I answer the trade in question by pointing out my
current truck for trading is 14 years old, and my big truck is 35. (Hmm.
Maybe I should have traded them both?). As for transmission I point out
that I don't want to pay for the automatic, I don't want to pay for the
fuel to run the automatic and I don't want to pay for fixing the
automatic. Fixing and automatic is several times the cost of a clutch
even in a Ford which may destroy the clutch when the hydraulic clutch
cylinder fails and sprays the clutch plate with oil. I'd prefer a
mechanical push rod but they don't make 'em that way these days. I'd
prefer a NP-435  or Warner T-19 transmission with a 7.3:1 low gear too,
but that's not available to fit the modern truck.

The stock 5 speed and stock (usually 3.08) gears are NOT suited for
towing. The highest ratio axle gears the factory installs are often not
very well suited for towing. I went to a hot rod shop that installs
gears of any ratio (in police cars too).

I'd get bored with an automatic and frustrated when it shifted when I
didn't want it to shift. When the day comes that I can't be bothered
shifting gears, I probably shouldn't be driving but should have a
chauffeur handling the wheel chair anyway.

Gerald J.



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