Regear for MPG?

Would not mind going to 24s if I did not Just replace 3 of 4 Super singles in the past 2 months. ( 2 roadside from debris one was just too worn out for winter)

 

Gotta argue this one a bit, Cowpie. On these RTLO 13/18's it's not a double overdrive in high gear. Two overdrive ratios doesn't necessarily equal double overdrive. The main box is in OD but the aux is in direct. Dropping to 12/17 means running the aux in underdrive, and an increase in parasitic losses.

 

I have a 2009 Pete with an ISX and 3:36s. I'd be willing to trade you for your 3:55s. At 65 I'm at 1350rpm which is optimal for your CAT. For my ISX optimal is closer to 1400. If you're interested PM me.

 

Taller rubber is the easiest fix here. The investment is about the same as regearing and mostly in parts and very little labor. You'll never recoup labor costs if it doesn't go your way.
And you'll eventually need tires either way.

So if it doesn't work the way you want just sell the tires/rims. If it does work, sell the old tires/rims.

 

one of things no one talks about when running long hard and heavy a gear down on a 13 or 18 is trans temp runs high if you have high hp. slowing down to 63 mph would solve your problems. if you are only going to keep the truck for a year that would be the best way to go. you would save 10 to 12 grand in fuel. if you have a dpf no need to change mufflers doesn't do a thing. better air clearer,slowing down , closing the gap between truck and trailer,max air preasure should get you to 7 mpg or close to it. keeping the left door closed will save time time as well as money.

 

This is correct. The rear section in a common 18/13 speed is a direct/underdrive unit - NOT a direct/overdrive. The final drive of the main section is .74, so when you're in 18th/13th,the main section is in overdrive, and the rear section is in direct. In 17th/12th, the rear section is in underdrive. In 16th/11th, (the most efficient gear) the main is in direct, ad well as the rear section. When talking parasitic losses, I would tend to believe 18th/13th is more efficient than 17th/12th, and 16th/11th being the most efficient.

 

He know's the 16th part, he's got a glider set up to run an 18 in direct. It's just one those things, people need to understand that they aren't double overs if they are playing with gearing calculations.

 

Seems like it could be a matter of getting bogged down in semantics. Most folks would visualize that those two overdrive gears on the 18 as being "double over". Whether it is in fact or not is really just picking at straws. As the last poster mentioned, and as I thought I was clearly referring to, was that running in direct (16th 1.00) is the best, but going to the first overdrive (17th .83) above that with a higher numbered rear ratio is not as parasitic as moving on up to 18th (.74). But ideally, staying out of the overdrives gives the most efficiency. That was the primary reason I went with 2.64 and an 18. It could have been done with a 10 over even a 13, but the 10 would not have been available with the torque rating I needed, and the 13 would not have given me the flexibility of splitting the lower end in a real tough pull.

 

I'm pretty sure it's not semantics but rather mathematics. If the gears used to make 8L are an overdrive and an underdrive, it stands to reason that 8L is not as efficient as 8H which uses an overdrive and a direct.

At any rate, I look at your combo and just can't see it being practical anywhere but the Midwest. I regularly pull hills in 6L and some in 5L with your set up I'd be in low range on those. Oh joy. You've got a gear for 60 &70 and a complete useless top gear.

 

I'm not trying to start an argument, but rather am always open to new ideas and theories. Please explain how you come to the conclusion that 17th would be less parasitic than 18th when 17th requires 2 gear meshes and 18th requires only 1?