Back in the day, I had a 56 ford customline 4 door, 272 with overdrive. My cousin and I drove to the west coast and back. In the 6 days of actual driving we put on 5000 miles. We averaged 19.9 miles to the gallon. I recall results of the pure oil trials in the high 20's. A guy I knew had a 57 convertable, D code Fordo, could only get about 12. 

I found a Speed/RPM calculator and plugged in the numbers:  215/60/15 tires, 3.00 rear, 2800 rpm, and the speed was 67.  So pretty close to the mark of 65-66.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

I reported above the mileage at 65 mph and 2800.  After Charlie made a comment, I started thinking 65 @ 2800 doesnt make sense. 

On the trip the speedo said 60.  So I figured with it being 10 percent low, we were running 66.  65-66 with a 3.00 rear and 26 inch. tires wouldnt give 2800.  Now I have to go back and recalculate the speedo error.  Once it was 13.2 percent low, this spring did it again and found 10 percent low, so hell I dont know how fast I am going .

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

Well, keep in mind that it's also about 30 years newer technology (1988). But I think our Ys, when set up right can produce very respectable economy.


Lawrenceville, GA

Thats impressive charlie, gonna need an AOD to match that

       

Gary, 55 Vicky, St. Germain, Wisconsin

 

I noted my car's rpm at 70 in overdrive. It's nearly 1800rpm


Lawrenceville, GA

I just finished a 790 round trip weekend, my mpg were very close to yours, 67 mph, 2700 rpm, 3:30 gears, it was a lot hotter on the return trip and I noticed I gassed up a little sooner. At 67 it would run 210 degrees at 90 and 205 at 80 degrees.

       

Gary, 55 Vicky, St. Germain, Wisconsin

 

That is pretty darn good if you are driving the 46 truck, trucks are not noted for low drag.  If you are running a 28" tall tire your rear axle ratio calculates out to about 3.89.  If you wanted to go to about 2500RM @ 65mph you would need something like a 3.50:1 axle ratio.  An overdrive trany with no axle change would give about 1960RPM which I guess would hurt your setup.

But before you do that you might want to get a vacuum meter to play with.  The instructions with my old SW motorminder vacuum gauge said that the higher the vacuum the better the milage. 

Doug T

The Highlands, Louisville, Ky.

I have to say I am actually impressed with "Vicky's" gas mileage! Just did a 460 mile roung trip, with the family of 4, plus the car loaded down with clothes and tools. I found if I keep the old girl at 65mph or less (rpm = 2800 with 3:55 to 1 gears) I managed to squeak 18.7 mpg out of her!

I did notice however that as soon as I hit 70mph, the vacuume secondaries opened up on the 600 Holley and the gas gauge started to move REAL fast!   At over $4.00 a gallon, I tried to keep my passing to a minimum!!!  LOL

I love the smell of burning rubber in the morning!

It's very low. I try not to shift into overdrive until I hit 70mph. At 70, it's around 1900rpm. I need a rear gear change.

An RPM like that for yours will probably be fine, being so light and with a fairly stock engine.

My '88 Firebird with 305 which is stock turns well below 2000rpm at 70 - 80 mph. The best mileage I've ever had with it was I think 31mpg (it was definitely low 30s) but usually it falls between 25 and 30. I've never weighed the car but they're usually around 3500 lbs. It will probably have better aerodynamics than your truck but you have 700lbs less to carry around. I think you can easily hit 23 - 27 with more economy oriented gearing.


Lawrenceville, GA