Cascadias with dd13 and dd15 mpg

I’m surprised no one has asked about the op engine. Do you have a glider or did you mean c-15?

 

I've had 3 Cascadias. A 2017 lightweight with the dd13/auto I used to get mid 8's mpg. Smaller engine but when heavy it strains more. (was new truck)

2018 Cascadia with dd15/auto my average was 8.5mpg but I drove 60mpg 90% of the time. (New truck)

2016 Cascadia dd15/10speed I'm getting a average of 7.5mpg. Doesn't seem to matter if I drive 60-65 I have a very hard to e hitting 8mpg like I did with the autos. (439k when I got it)


I'll say straight up you will not hit 8's running 70mph but when you can save that much money by slowing down if you have the time, isn't it worth it?

 

How does the rear gear ratios affect fuel mileage? Also, what is the difference between 3.25, 3.58 and 2.75? Thanks.

 

3.25 and higher , probably an over drive transmission. 2.75 and lower, a direct drive transmission.
Generally, the lower the ratio, the better the fuel economy with a modern engine. The big factor being how big of a load you hauling on a consistent basis and at what speeds.

 

Generally speaking, I never go past 65. I'm usually around the 62-63 sweet spot. So the lower the gear ratio, the less pulling power?

 

They all pull the same if you drop enough gears with the transmission.
But there is a limit to how low of a ratio you can go and still be able to launch the truck fully loaded on a hill without burning out the clutch. Startability.
It’s all about compromise.

 

I went from a 9 speed cascadia manual to a dd12 cascadia auto and get the same mpg, maybe even a tad bit better with the manual ..but im in flatbed so fuel mileage always sux i guess

 

The higher the ratio, the less your mileage is going to be at highway speeds.

Truck with a higher ratio has to turn more rpms. More rpms means more fuel used.