Differential swap

Good to hear. I gained about the same amount when I went from 3.73 to 4.30. and the truck works a lot less with the 4.30`s!

Here is the gas mileage report. I took Tuesday off and towed the 5er up to St. Cloud so Pleasureland could finish the job they didn't complete the last time.

Our truck with 3.73 gears averaged between 7 and 9.5 mpg towing. On this last trip the mileage was 11.1. I'm happy with that. So all in all I'm very satisfied with the differential swap.

Thanks for all your help.

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SLE wrote:
Us Nodak boys know one thing and thats wind! I helped a few buddies tune up their oil burners a few years a ago and found a comment pretty funny. I was on the phone with a shop in Denver ordering a few go fast parts and getting a custom tune set up. The shop owner asked what we were after and I said that we were looking for better towing performance and to increase mileage. I then said we don't need anything crazy power wise as we stay pretty local to our flat lands of ND, nothing like the mountains you guys gotta deal with. The shop owner laughed a little and said, the tuffest pull I ever had was in ND bucking a 30 mph head wind. At least in the mountains you eventually get to the top and get to go back down.

Its one of those comments that didn't strike me until I got off the phone. thought it was worth sharing.

X2 on the ND wind. And it's real. We are right behind Texas as the windiest states in the union.

The Denver shop owner had it right. At least the mountain (or hill) is still aimed the same way when a guy makes a return trip, and what goes up must come down. I might be a little paranoid, but I think me inevitably buckin wind both ways in ND, is part of the plot. ;)

Wes
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SLE wrote:
Us Nodak boys know one thing and thats wind! I helped a few buddies tune up their oil burners a few years a ago and found a comment pretty funny. I was on the phone with a shop in Denver ordering a few go fast parts and getting a custom tune set up. The shop owner asked what we were after and I said that we were looking for better towing performance and to increase mileage. I then said we don't need anything crazy power wise as we stay pretty local to our flat lands of ND, nothing like the mountains you guys gotta deal with. The shop owner laughed a little and said, the tuffest pull I ever had was in ND bucking a 30 mph head wind. At least in the mountains you eventually get to the top and get to go back down.

Its one of those comments that didn't strike me until I got off the phone. thought it was worth sharing.

I went down yesterday to get my TT from the rv shop. On the way down I was bucking a strong head wind with gusts to 40 mph. I was using more boost solo going down then coming back with the trailer and a tailwind.

Us Nodak boys know one thing and thats wind! I helped a few buddies tune up their oil burners a few years a ago and found a comment pretty funny. I was on the phone with a shop in Denver ordering a few go fast parts and getting a custom tune set up. The shop owner asked what we were after and I said that we were looking for better towing performance and to increase mileage. I then said we don't need anything crazy power wise as we stay pretty local to our flat lands of ND, nothing like the mountains you guys gotta deal with. The shop owner laughed a little and said, the tuffest pull I ever had was in ND bucking a 30 mph head wind. At least in the mountains you eventually get to the top and get to go back down.

Its one of those comments that didn't strike me until I got off the phone. thought it was worth sharing.

If I remember right, mine cost around $3,700 for full axle rebuilds for front and rear and also a freshening of the ol 4R100 including a new HD torque converter. $1,700 sounds right in line, 5k is way to much.

As far as mileage goes, mine was shitty before the new gears and is still shitty after, lol.

Well, all I can say is that you No Dak boys sure know a lot more about all this than I do. And I appreciate your knowledge.

Just an FYI: the differential gears were changed by a shop in St. Cloud, MN and the fee for changing front and rear was 1750.00. Some folks were saying the cost is normally 2-3000, and Boyer Ford in Mpls wants 5000 so that St. Cloud shop is very reasonable in their pricing.

I haven't checked MPG yet, but I will in the very near future, and will report back here.

The other thing to consider is your effectively increasing your torque to the ground in any given gear and at any given RPM at the cost of speed. By going from 3.73s to 4.56s you effectively increase your gearing advantage by 22%. That means 22% more torque to the ground, all else being equal.

Think of gears as a lever or cheater bar. The longer the lever (deeper gears) the more torque you can put on a given bolt or object. If the object turns freely however, you will not be able to spin the large lever as fast as a small lever.

Remember torque is an actually measurement of force, hp adds the factor of time and the constant of 5252 into the equation. If you compare results on a rear wheel dyno, you won't make any more HP on the Dyno but you'll make a whole lot more torque. In all reality, even though you won't make any more peak HP, you'll build hp more quickly. This makes a difference in every gear. the downside is, with deeper gears, you'll decrease the top speed of each gear.

For those of us with V10/4R100 combos, think of it like this. if we tow at 65mph, we effectively increase our torque to the ground by 22% in both 3rd and 4th gear by changing from 3.73s to 4.56s. On top of this, we raise our rpms closer to the torque and hp peak of the engine. With 3.73s I turned 1800rpm in
4th and if I remember correctly around 2600 in 3rd. This engine makes peak torque at around 3,200 rpm and peak HP at around 4,700 rpm. So moving the rpm band closer to those peaks at the same speed ultimately also adds to the overall equation of increasing HP and TQ at a usable RPM. If I remember correctly the net overall effect was around 30% torque advantage at the same speed. As is expected, it's a noticeable difference!

RCMAN46 wrote:

Ivylog wrote:
Why does a tranny have gears... It's to change the gear ratios. Manually gear down and keep the RPMs up... Same going down the pass.

X2

Changing gears will not increase your horsepower period.

What is so important in pulling hills in 5 th gear with a 4+ rear gear when you can do the same with a 3.xx gear in 4 th gear?

No the overall power is not increased... but...

Most importantly, I think that the available transmission gears are effectively closer together with higher numbered axle ratios. With 4.56's as an example, the numerical jump from 4th(OD) to 3rd causes less engine rpm change than a jump from 4th to 3rd, or 3rd to 2nd, with 3.73's. In other words, the difference in transmission ratios really becomes mathematically a smaller percentage of the overall gear reduction. So with lower gears, when the transmission finally does kick out of 4th on a hill, it feels like not such a drastic change into 3rd... and it isn't.

For myself (and yourself) and many others, the swap is probably unnecessary. But for dedicated towing, a gear swap can be pretty valuable. This is especially true when one only has 4 gears to start with (4R100). They are spread pretty wide as opposed to a 5 or 6 speed. From experience, I found this to be true when I casually swapped sprockets back and forth on a vintage 4 speed motorcycle (highway vs off-road). The advantage is great enough to actually be felt seat-of-the-pants. All newer bikes usually have 6 speeds.

For a 3/4 or 1 ton gas truck, 3.73's are probably as tall as anyone might want to go, and that point represents a reasonable compromise at the OEM's best word-of-mouth maximum fuel economy, avoiding a distinct sales disadvantage. Some folks drive these "tractors" back and forth daily to work. When I picked up my "pre-owned" Excursion, the lady had to take her child-seat out of it first. The digital mpg readout said the truck was getting 6 mpg (winter, city) and it has never done better than a dismal 14 highway for me.

The trade-off to installing new, lower truck gears is a probable reduction in unloaded, or light-load, fuel economy and a significant upfront investment expense. For a 90+ percent tow duty and a planned extended ownership, lower gears may make the most sense. I'm satisfied the guys that have done this swap here, have spent a lot of time carefully considering the modification and it seems well spent to me.

Wes
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