3:55 to 3:08, opinions?

2.64 will not "suffer" in hills with an 18. All that is going on is that the gear to cruise in will be lower than the top hole. There are plenty of examples of individuals that are using 2.64's and 2.79's and pulling heavy. And by running in 16th (direct drive), the transmissions are running cooler and more of the power is not being lost thru the overdrive gearing, and is making it to the axles. There is roughly a 10-12% loss, on average, of HP making it to the rear wheels anyway, and even more of a loss when in overdrives. Upwards of 30% loss according to some who study these things. Even the senior engineer at Eaton is on record as saying that there is a 3% loss of efficiency and economy by running in overdrive, and if one ran primarily in direct drive, there would be essentially very little need for the transmission cooler. That energy loss in overdrive is translated into heat. That is why, when on the dyno, a truck is worked up to direct drive to get an accurate measurement of power making it to the wheels. There is not much parasitic loss of power to the wheels when running in direct or lower gears in the transmission. But overdrive takes parasitic loss to a higher level.

Granted, something tall like 2.79 or 2.64 would not be the ideal for every situation, like grossing 100K coming out of a soft corn field during harvest time. But virtually 90% of what goes on in trucking could use the tall ratios and it work just fine. You seem to be chasing the fuel economy thing by micro blueing components. Why not chase it more by running at your cruise speed in direct drive? That is one of the benefits of using a taller ratio. And micro blueing would just sweeten the deal.

 

Good to hear MicroBlue is working for you!

 

I am really not trying to but I know this will sound "smart**s" but if you are so worried about fuel mileage, why do you have a W-9?
Aren't there options that provide the same driver comfort and drive train options but with a more aerodynamic body?

 

That is the beauty of a 13 or 18. Just drop a half a gear and then those slower speed limits are no problem. There is nothing in the Eaton manual (or any other brand) about having to run in the top gear. Mine has NEVER seen the top gear since I got it, and only 17th 3 times for a short period of time. I primarily run in 16th (direct drive), and when the speed limit is in the 55 mph limit range, I just drop to 15th as I need to. Shorter ratios (i.e. 3.70 on up) limit your flexibility for various road speeds and keeping the engine in the right rpm band. Taller ones like the 3.08 mentioned give a lot more flexibility, and pulling is not an issue with anything 80K or less.

 

Cowpie i haven't been following your other threads where do you run what kinda weights and fuel economy?

I'm in the process of spec'ing a truck if everything works out, i live on the east coast of canada so there is enough hills to annoy a guy and after reading this thread i'm toying with the idea of of using outta the norm gear ratio's to save on some fuel. you guys all say hill climbing isn't affected that i can believe a gear drop is a gear drop whether from 18th to 17th or 9th to 10th how is start ability with the higher ratio's? you know going from stop light to stop light with 80k?? would running in 17th(0.86) be better then 18th(0.73) or is it as soon as you start using the rear box you start loosing power and fuel economy?? Playing with cummins powerspec it looks like in 17th at 62mph would be 1300rpm which i'm htinking is where i want to be although its saying 1400rpm is better for power and less shifting.

My main concern since i see hills on every trip is i don't want my motor to start leaking antifreeze(crying) every time it sees a hill.

 

oh and another question i just thought of do you guys think alot of power 500+ used reasonably vs 450 in the hills would be better for fuel economy with this higher gear ratio's??

 

Running 3.08 rears 22.5 lows was running talls wanted a little better pulling power u will lose about 6% pulling power (torque)for every gear ratio u go up. I.e. 3.55-3.36-3.25-3.08. 65mph sucks for me it's 61-62 17th or 70mph 18th run 80 tho midwest to Cali me I would do 3.25s good pulling in hills yet still run if want to. If want 3.08 do ECM upgrade pdi,pitts, or bullydog otherwise truck will be a Draggonfly. And your mpgs will suffer as we all know the wind never blows in wyo. just my 2cents.

 

By the way the 6% torque loss comes from cat spec sheets they have it broken down to many different variables forgot the addy for that sheet but sure someone on here has it and I have seen it when I went from 3.55 to 3.08s

 

I just ran 45,000 loads both ways in and around Kentucky, including a lot of two lanes with no problem. I will have about 40% heavy loads, but usually the average payload I have in the box is around 35K lb. With 2.64's in my diffs, I have pulled all the hills right along with everyone else. And mpg has averaged around 7.5 for the life of the truck.... hand calculated pump to pump. With the 2.64's and my 18 spd in 16th (direct drive), at 1300 I am at 60 mph, at 1400 I am 62-63 mph, and 1500 I am at about 66-67. On low pro 22.5 rubber.

 

still looking at 3.08. I a ripping them piece of ####s out and putting 3.55 or 3.73 in mine. I had a direct 10 speed in it heard a bearing wine so swapped in a single overdrive 13 now no power and way to much speed. Its so bad i skip 11 go straight to 10th. and thats with a 500 detroit with a modded ecm and holset turbo kit....alittle advise low rpms does not always equal better fuel. our other truck has the 3.55 in it with low 24.5 1400rpms it runs 68 and in high gear gets about 5.5 drop a gear and slow down or keep a gear and speed up to 75 and it gets 6. 75 is about 1600rpms older motors where not designed like newer cummins are....