7.3L Gearing Question

[121x]

Theoretically speaking, what if any mpg change do you think there would be between the current 4.30's with 37" and 3.55 gearing on these trucks? Nevermind the rock crawl mode that may somehow be programmed for the 4.30 or less pickup off the line while towing heavy. Just wondering if there would be any perceivable mpg improvement unloaded or towing less than 7k.

 

[RedZilla]

The 37's are just going to make your mileage worse regardless. Turn more weight and a larger diameter. Changing out to 3.55 gears would probably make it even worse, not too sure. There's a lot going on there. You would probably want to reset the TCM as well so it could learn with the new tires and gear setup. In the end you're still moving the same mass around powered by a gas engine. If you are strictly driving flat highway you may see little change, but I'm betting the mileage will go down, who knows how much. Also you mentioned in a previous thread about lifting the front. This will also move your mileage to the negative due to wind resistance. Your money, when you figure it out let us know.

 

[Modman]

Also consider a 4.30 with 37" tires is equivalent to ~4.07 with 35" tires. You already are numerically lowering the effective gear ratio with the larger diameter tire. I think with the 10spd you can reduce the final drive some if you are willing to take the tradeoffs, and our engine has plenty of torque, but at some point you will be on the losing side of physics. Probably not relevant if you are staying under 7k towing.

Do you have a good baseline for fuel economy with both the OEM tires and 37's? Alot will impact this, tire width, compound, mass (as @RedZilla stated). Are the 37's on an 18" rim, or larger/smaller?

That would be a good starting point of where you would end up with a gearing change. I would guess with just the tire size change you would decrease fuel economy and maybe claw back a little with a gearing change. Gut feel is not to go lower than 3.73 (this would be like 35s with a 3.55).