Asking near the same question as I did in another thread but this is more 300 specific.

What would be the peak RPM's for a 300 and manual tranny for top MPG efficiency?
I'm turning 1700 R's to make 55 MPH with the manual 3-gear, a 2.75 9" rear, and oversized tires. (235/75's instead of stock 195/75's) This equals just 47 or 48 MPH on the speedo - but around 55 on the road. I'm afraid I may be lugging it at such a slow RPM - or am I just zonkers?

THEN.. also considering swapping in the 4 speed OD manual from a near same year truck. (mine's an '80) Overkill? Good idea?
And MPG's are top priority since I can drop back to 3rd for trailering or with a load.

 

As answered in your other thread, the 4od trans will work fine with your current engine, rear axle, and tire size.
Properly tuned, you could see around 25 MPG hwy, a figure new trucks have a hard time matching.

 

That's the same tire & gearing I have on my 67. On the highway, I was thinking I should have 2.50s in it. With a totally empty truck it's nowhere near lugging. I just swapped out 3.25s last week. I'm striving for better economy myself. I think you would like an overdrive.

 

There will be no one particular RPM or secret numbers where the whole world of 300s will operate efficiently. Each engine is different and there is an infinite number of variables and conditions that impact mpg.