So let's talk gears

[kuskoal]

Double OD is a huge parasitic loss, though. Turning all the gears through 2 or 3 transmissions is a nice drag on the engine at speed.

OTR trucks use about a 3.55 if memory serves right. If 1:1 is the most efficient gear, why not underdrive the whole geartrain and run somewhere in the higher gear sets and an old-school crashbox? So 3.55 and 35" with a very nice gear spread and ending at 1:1 nets 2200 rpm at 65 mph. Sound right?

 

[War Wagon]

Why not 3.73? Good medium on ratios. If I were towing I wouldn't do more than 65 mph. Something about a big load going that fast in a light duty truck kinda scares me. It would be 2500 rpm with 1:1 and 1800 with a .76 od.

This would be a better choice for me than the current 4.10's. I drive quite a bit of Interstate and the speed limit is 75, but everyone goes 80+. It's frustrating for me to have the engine roaring when I don't need any extra grunt; I'm just trying to go close to the speed limit, and the 4.10's kill my milage at highway speeds. There's a lot of merit to what Goose says too. I just hate being revved out before I even get up to speed.

 

[Black dawg]

I am really liking my us gear, I can leave it in high range, making 1st gear very usable, and better than starting out it 2. Most of the time unloaded or light, I just leave it in high. My cruising rpms are higher than I like, (4.1 with a t19 and .80 us gear) but would probably work really well with the zf. And as far as double overdrive and frictional losses, these trucks do crappy on fuel at 80 regardless of rpm.

 

[GOOSE]

RLDSL uses a 5000 series Brownie box and Gary uses a GV. To the best of my knowledge both report more gains in useability than losses due to extra driveline components being added. These are not triple countershaft units like some of the crash boxes that big rigs use, the parasitic loss is minimal.

 

[NTOLERANCE]

"37's with 4.10's is like swapping to 3.55 and running oem tires "

No

A taller tire is harder to turn that a shorter one as well as weighing more
While your effective gear ratio may be the same performance and mpg will not
Assuming we are talking highway / road use.

Think about this extreme example

You have an axle shaft with a rim bolted to it. No tire. Now grab the splines with one hand and try to turn the shaft.

Now mount a tire on that rim and try to spin it. A lot harder right?

Same holds true for going up in tire size just to a lesser degree.

I sold tires for years. Every truck owner wanted bigger tires. Even going up one size would lower mileage and acceleration
Changing the ratio can offset most of this but not all.
Will you notice it? Maybe.....maybe not. Just food for though n

 

[Greg5OH]

I like the tall tire and low gear combo. I have 373s in my 05 SD axles and going 37s myself. Puts me at 1700~ in 5th doing 60 i believe, so to conquer hills or if i were to tow heavy, drop it in 4th and your at 2000. Keeps stress off the tranny this way as well!

 

[hotshotidi]

i think either 3.73 or 4.10 depends on were your driving if highway id say 3.73 if all around mixed driving 4.10