I wanted to get some ideas from the experts on here on what I should check or try for to adjust/tune for better fuel mileage on my ‘75 D100. Current powertrain set up is 225 with Holley 2300 2bbl, 727 auto, and 3.55 rear. Current fuel mileage on rebuilt engine with 10k miles. I have been consistently getting 7.5-8.5mpg with latest carb. When I had the 1BBl set up, I could only squeeze out 10mpg average, then went to a BBd set up and still stayed almost the same mpg. Then decided I would go for broke with the Holley 2300 which I felt would at least give me the Hwy speed & power I need and be a hair more efficient with fuel but so far not the case. I do have great take off power now compared to the BBD, and while cruising at 70mph, are only at 3/4 pedal. Truck top speed tops out around 77mph on the flat. I have tried every trick/adjustment trick in the book I know and read up on and nothing seems to help. The Holley 2300 is a brand new carb and adjusted for a relatively clean burn producing 20in vacuum at 750ish idle. Ignition timing is set at 14 deg advance at 750rpm. The only part I have not tried to adjust is the vacuum advance pod on distributer. The truck cruises down the Hwy at a comfortable 70mph. I feel this truck should be getting 12-13mpg minimum all day long and as high as 17mpg or so on a good day. Thanks ahead of time.

That's likely too much; a setting of 5° to 7° BTDC (depending on the particulars of your distributor), with attendant carburetor adjustments to keep your appropriate idle speed, might work out better for you overall. What spark plugs are you running, and are you burning high-test, mid-test, or regular gasoline?

That's a medium-reasonable expectation.

This might be harder to achieve. Remember, you are pushing a brick through the wind with a stone-age motor.

Check the other usual stuff: exhaust restriction, tire pressure, transmission or clutch slippage, brake drag.

…and cam timing. What kind of mileage were you seeing before the engine was rebuilt?

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

The 83 D150 has a slant, compression raised to 8.5 Holley 2280, hei ignition, cold air induction, dual Dutra’s, a833 OD and 3:73 rear gears.
City stop and go driving 14 to 15 mpg, and that is driving with fuel economy in mind.
Highway driving, staying in the right lane with speeds at no more than 65 mph it will get 18 to right at 20 mpg. Run in the left lane at 70 to 75 mph and the mileage drops to 16 to 17 mpg.

Like Dan mentioned, tire pressures, dragging brakes can have an impact.
If you are watching mpg, be sure your odometer is accurate. Small errors in distance traveled will have a big impact on the calculated mpg.
The 727 transmission is not as efficient as a 904. The a833 is even more efficient at highway speeds. That alone could easily be one mpg.
I run synthetic oil in the engine, transmission and differential.
Your driving style and technique will make a big mpg difference.
Mount a manifold vacuum gauge in the cab and work the throttle to maximize manifold vacuum and watch the mpg go up.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

The Holley 2300 series of carburetors are performance orientated. It may need to be rejetted to improve mileage. The best way to do that is to install a wide band O2 sensor and adjust the carburetor as needed.

Wow. Though many /6s weren't what I'd call "great" on fuel, with the mileage figures the OP lists I would think you'd have liquid fuel dripping out the exhaust at that rate. If it even ran at all ..... I drove my son's ram charger home to Chicago area from Vegas, and not realizing it had a pinched return line (had to drop tank and swap pump in a truck stop Parking lot and missed the position of the line as we were trying to hurry so we could get back in the road) it's TBI 360 got 7 mpg the whole way because of the excess fuel pressure the pinched line caused and I had a headache much if the ride. My son followed me in our wrangler the whole way and he said he couldn't ride very close to me because the exhaust was so strong......
I can imagine the huge cylinder wash that's going on with that kind of mileage

I was going to say the same thing. Get a wideband AFR sensor on there to see what your fuel is doing.

_________________
Matt Cramer
1966 Dodge Dart turbo / EFI project

Make sure you see if the vacuum advance pod is actually working. If you can pull the hose off the carb and suck on it and it does not leak, then likely the pod is working. If you can keep sucking air, the pod is blown and you have no vacuum advance. This will kill 10-20% MPG.

Best of luck, and keep feeding us new tests/observations!
Lou

_________________
Home of Slant6-powered fun machines since 1988