Thought I'd post some fun news!

It's been some time since I've taken my '81 for a drive. Got out of town this weekend and drove down to my parents. Beautiful day, no wind, about 85° out. Thought it'd be ideal for a gas mileage test. Filled it up as I left town.

I kept it at 65mph the whole way and the vacuum was much higher than it's been before. I don't remember if I posted it anywhere, but my (new!) brake booster had a massive vacuum leak in it. Even worse than the original one I replaced it with. The original one would hiss when I'd push ON the brake, so it was only leaking while braking. The new one I got hissed when I took my foot OFF the brake, meaning it was leaking all the time. However, I could never hear it while driving so I didn't know. It's been nice having that replaced.

Either way, around 11 - 14hg vacuum at cruise, about 7 - 9 going up grades. Only occasionally did it dip into the 6.5hg power valve. Much better than before over all. From my calculations, my timing should have been sitting right around 48° advance. (16° initial + 14° mechanical + 18° vacuum).
I also kept it in 3rd the whole way and never used overdrive, so I was in my 1:1 and my RPMs were around 2150 the entire trip.

Pulled into port and the mileage was 131.2 on the odometer. I filled up and it kicked off at 6.1 gallons. Generally, where it kicks off is where I leave it, but I figured that couldn't be right, so I tried to put more in. It kicked off at 6.4 gallons. Then again at 6.6. 6.7. 6.75 (started sloshing out the top). It was definitely topped off.

If my mileage was correct where it first kicked off:

131.2 / 6.1 = 21.5mpg

According to where it finally wouldn't fill anymore:

131.2 / 6.7 = 19.6mpg


I'm floored and shocked! Again, this is with the 600cfm Holley. That's on par, if not better, than the 1bbl when it had smaller tires! It's also way, way better than that 390cfm I used (I averaged 11 - 13 with that).



I wanted to re-test the results on the trip back but it was a 20 - 30mph direct headwind for the first 100 miles, which really sucked. The 3.00 gears really don't like that and I think I averaged around 14mpg on the way back. I'm thinking if I increase the mechanical timing and decrease the vacuum advance timing, I may have a little more strength. Too much of my timing seems based on the vacuum advance. But, that's just a theory.

Either way, shows it's possible, and that bigger carbs don't necessarily mean worse mpg.

 

The number of bbl's or CFM rating of a carb has less impact on MPG than *proper* tuning does. Driving style also has a major impact.

19-21 MPG in a Bronco with 31" tires, is rather good.

 

Excellent! And X2 on what RW said.