Fuel Economy

What is the transmission in front of both of them and what size tires are being run. Lots more to think about than just the rear ratio and HP. Do both have the same torque even with different HP? The question is not that simple as you state it. I would not be running in the same gear with 3.25 as I would be with 2.64, even with the same gear box. For instance, with the 18 speed in my truck, tied to 2.64 rears which it is, at 70 I have two options, run at 1600 in 16th direct drive or shift up to .86 overdrive and do 1350. Depends on the load and terrain. For the hills, I would stay in 16th. With 3.25, this all changes. Get the idea?

 

Now you don't specifically state that the trucks themselves are identical models or makes. A Cascadia or Volvo is more aerodynamic than say a Peterbilt or KW. Obviously a more aerodynamic truck will have less wind drag.

 

Again, all else being equal except the rear ratios, would depend on what RPM and gear selection to keep the engine in most efficient range.

The I-12 has a 11th direct and a .78 overdrive 12th. At 70 mph, you will be turning 1250 RPM in 12th with a 2.64 rear. With a 3.25 rear, you would be turning about 1525 RPM. Given that the D13 is designed as a low RPM motor when it comes to efficiency, on flat ground at 70 mph, the 2.64 would work best for economy. With, say, a 3.08, you would be turning about 1450 at 70 mph. With 2.93, you would be turning roughly 1375. So it is a matter of where you want the motor to run for maximum efficiency. I am not all that familiar with the D13 (and don't plan to be), so you have to decide where the motor does it's best work and does it the most efficiently.

HP is not really relevant except on those really hard pulls, and then you are not really doing that on flat ground. How you drive determines more about fuel economy than the power. I, personally, prefer the higher power. Remember, whether you have a 425, 500, or even a 350 hp motor, at cruising speed, on flat ground, you are only using a portion of the total power. Chances are good that most times you are not even using 300 HP while cruising. So It really doesn't matter the engine rating, you only use the fuel to get the power you need to get the job done. I have averaged 7.93 over 494,000 miles with a 500 hp motor. I drive it properly and have it geared right to keep it in the most efficient band of the motor. I know this might sound bizarre, but aerodynamics is going to play into all of this. It is a very rare truck that will get jaw dropping good mpg at 70 mph.