Real gas mileage on stock slant6/auto and 318/auto

Just would like to know what kid of gas mileage you guys are getting with your slant six auto trans VS 318 auto trans, in basic stock form. What gear ratio if you know also. So basically how much if any advantage does one have over the other. Thanks

I'm running a 318 4 barrel auto with 3:55rears and I can run about 3 hours on the hiway in 18 gal. tank so thats about 250 miles so thats about 14 miles a gal. but with lower gears (3:23) it will get better...

There is probably a 2-3 MPG difference in the 2, with other factors.
There is also a difference in acceleration.
I understand that you want all the facts to make an informed decision, but if MPG is a big factor, you are better off with one of the real light econo cars of the 80s and 90s. Increased crash safety and accessories have caused increased weights over the past decade plus.
What your expected drive is going to be is a big factor too. Anything over 45 MPH is going to decrease MPG more than uphill/downhill due to drag.

according to the log that came with mine ('64 A100 with a 170ci LG and 3 on the tree) This particular van averaged at about 22MPG for 27 years. And that was all at 5600 feet altitude.

I am the 3rd owner, and the second owner was Caltech, and they kept track of every fill up and oil change and maintenance and repair since 1968. I ended up with a file box full of this data when I got the van.

RodStRace wrote:There is probably a 2-3 MPG difference in the 2, with other factors.
There is also a difference in acceleration.
I understand that you want all the facts to make an informed decision, but if MPG is a big factor, you are better off with one of the real light econo cars of the 80s and 90s. Increased crash safety and accessories have caused increased weights over the past decade plus.
What your expected drive is going to be is a big factor too. Anything over 45 MPH is going to decrease MPG more than uphill/downhill due to drag.

I understand its a 60's van and not a 30mpg unit. But I just want to know if the added power is a massive decrease in MPG...or the other way around. I would plan to install AC at some point/ making a V8 possibly the better choice.