I'm new to the forum and have read several post on mpg improvements.
I have a Rochester HD 4bbl carb with feedback - no cat.
I have a A727 3 spd tranny with a 3.54 heavy duty axle
16.5 x 8.75 tires on 8 bolt rim

I estimate my mpg at 8-12

I am looking for suggestions on ROI & options on the following:
Replacing tranny with newer OD tranny
Improvements or replacement options on the carb
Improvements or replacement on the rear axle
Effectiveness of going to larger tires verses replacing the gearing axle

Any suggestions on these or other mpg options would be appreciated.
Thanks

 

While you can do a lot of the options you list, and each should have a slight improvement in fuel economy, the ROI is the killer.

If you need a new transmission, by all means obtain and bolt up an A518.

If your tires are nearly toast, replace those 16.5 inch rims with some 16's, and put some taller, narrower load range E tires on them. You might notice/have noticed how difficult to obtain 16.5 inch tires are and how pricey they are anyway.

Switch rear gear ratios, and you might as well rebuild the rear axle at the same time.

I'd say just make sure the engine is running as good as it can. Use synthetic fluids, slow down, take out the extra weight, and if/when certain things need to be replaced, upgrade them to something that might benefit mpg.