Different gear ratio MPG around 78 miles per hour

Some guys used to run 2.64 gears with a 18 speed .. they ran 60/65 in direct 16th gear but had two overdrive gears if they needed to boogie..PS if you do a lot of backing this might be better than a 13 speed setup due the gear reduction of the 18 speed .

 

Dropping into 17th on a 18spd or 12th in a 13spd would have cruising rpm the same at 65 with the specs I showed.
But the transmission is not as efficient in that gear. Top Gear better, next gear down is best as that is the direct gear as far as efficiency goes.
I haul 140,000lbs super-B trains. Not at your speed. I recently tried smaller diameter tires 11R22.5 from 11R24.5 in a quest for better mpg.
I saw a 0.3 mpg loss. The smaller tires did give the truck better performance. But it put the engine at 1460 rpm at 62 mph. 20 rpm over the engines optimal rpm of 1440.
I went back to 11R24.5, rpm dropped to 1380. Mpg came back up.

 

If the OP knew what engine he had and how much weight he was hauling and where he was hauling it, myself and others could probably pick the the optimum gear ratio for his needs using two simple tools, a road speed calculator and a power curve.

Using a estimate of 40" tire height and the 600HP X15 in the graph below for an example, and you're always pulling 80,000 lbs, in all conditions, the gear ratios you should be looking at are 3:08, 3:25, 3:36 to keep the engine at the top of the power band and in the economy range between 1500 and 1700 at 78 mph.

I know it's useless information to you because it's not "real world" and you don't understand it, but it's the best I can do given no information to begin with.

Vehicle Speed Calculator

 

If the transmission is driven by the crank shaft for the engine, takes it's power through a big tough long gear to it's connection with the drive shaft in the back, you would want the power, whatever it is to "Flow" straight down that shaft.

Some transmissions want to route power away from that central shaft and around some gears and back onto the shaft or drive a PTO.

 

2.79 rear ratio. 13sp single overdrive. .85

1500rpm at 75mph

I usually run around 72 in direct at about 1700 RPM. I get around 6 miles per gallon with an n14

 

That's actually pretty close to my avg with my c model and 13. No matter what the load or conditions. Lol. How'd you do that?

 

Now that chart I understood perfectly.

It should not be flat at the top eh?> Most all engine charts Ive ever seen usually has a curve somewhere near the sweet spot.