What really limits the towing capacity of the V-6ers is the drivetrain, no? What if I was to change my 3.55 to a 3.73, would that increase my towing capacity? I think it will also increase acceleration as well as kill a bit more mileage. But by my logic, the 3.73 gear would improve mileage on uphill and city driving and hauling, but cruising on the freeway at a steady pace will decrease the it, right? Via the Ford manual, a 3.08 to a 3.55 has a 1000 lb towing difference, both same configuration elsewhere. How much mileage would be lost if I move up to 3.73, and would I have to recalibrate the speedometer? I haul overloads alot, haven't towed yet, but I would like more oomph when needed especially on city driving, and would feel comfortable towing at maximum. I have to deduct 1000 lbs at the rated maximum which supposably gives me a "comfy" zone.

Leo dC

 

Biggest thing in an axle ratio change is getting the engine up into the torque and HP range. While this sounds like a no-brainer, take a look at the HP available at your cruising speed. By going to 3.73 vs 3.55, I raised the 70 mph rpm to around 3K vs some 150 rpm less with the 3.55. 150 rpm is nothing -- however it can make 10 or more HP difference at that speed, since the engine HP curve is at a pretty steep ramp at that RPM. If you have a 3.08, you would notice a significant driveability difference by going to a 3.73 -- with little real penalty in gas mileage.

As a reference, assuming my 3.73 cruises (OD) at 2K rpm at 70 MPH now, it would cruise at the same speed at 1651 rpm (OD) with the 3.08. Do you think you'd have heartburn over a cruise of 2K RPM? 2K converts to around 3K in direct (the gear in which I tow) at 70 MPH, which is around 2786 RPM at 65 -- a more natural place to tow.

That 2786 would be 2300 RPM in direct at 65 -- which is midway into the torque range, but way down in the HP range. Remember that when Ford was running the 292 in-line six as a towing engine, they spun them in front of a 4.56 ratio direct (no-OD) tranny -- which would have a 65 MPH RPM of around 3406 for the 4.56 -- well into the HP range, and well above the peak torque, meaning that when it pulled down for up-hill jaunts, it would pull down INTO the torque range, rather than pulling down UNDER it.

When I was a kid, and quick was the goal, the rule was "The answer for a lack of Cubic Inches is Cubic Dollars", while now for towing, the rule is "the answer for a lack of cubic inches is RPM".

You can tow a boxcar with a Briggs & Stratten with the right gearbox -- but you can't do it quickly without HP.

It's all simple ratios -- hope this helps.

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Y2K XLT EC 5.4L 7700# SB 4x4 w/LS 3.73 (repeat buyer)

 

But how much mileage would I lose with the gear swap? How much additional lbs of tow do I gain? How much m/s/s of acceleration is added? Would I need to recalibrate the speedometer?

Leo dC

 

The answer was close...
The Torque your engine develops will have more affect when TOWing than HP. I don't know the Torque curve for your engine, so I cannot suggest a particular RPM - which would translate into MPH with some simple calculations/observations.

Leo,
I've seen your various posts about your limited hauling capacity but I never paid much attention to what engine you had. However, now that you seem interedted in TOWing rather than LOADing your truck, I must recommend a V-8 (or "bigger").
See your friendly Ford dealer and get a copy of their "Towing Guide" - IGNORE ANY SALESMAN that says, "Oh, sure! That truck will handle the load." or anything similar. Remember he's a "salesman" and gets a commission on what he sells. A SuperDuty hauls more and tows more than a light duty and COSTS LESS.
Good Luck.

 

Oh yea, JC is so right! I have never seen a salesman that would not say "your truck will handle that no problem!"

And JGorka is also right. I have pulled stumps out of the ground with a 25 horse Power tractor. But I could barely hit 35 MPH with it. In fact a 35 or 40 HP tractor is a big one! All in the gearing.

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97 F-150 SC Lariat, Styleside, White over Gold, 4.6L (Windsor), Automatic, 3.55 rear end, electronic 4X4, w/ every option of Lariat and ORP packages except the 17" wheels (I even got the OR decals now!) MOD's so far: K&N air box mod, Gibson Super Truck, clear Bugflector II, Leer TSC 600 hard cover, Bridgestone Dueler OWL ATP 265/75 R-16's, TrailMaster Invader N7 shocks, Smittybilt step bars, Ford moulded mud guards, "White-white" head lamps, Cloud Rider Design's SS Grill and bumper opening, Bosch Platinum + 4's, Hayden 30K+ Tranny Cooler,Class III Reece load distributing hitch system, full wiring including trailer battery charging system, Escort Sensor II Brake controller, Superchip