I have a 81 D150 in decent shape with 225 slant6, 86k original miles. Rebuilt carb, new plugs wires cap etc,,,,

I am getting about 13 mpgs average. I was hoping for better and kinda disappointed.

What might the mpgs be on a properly tuned vehicle like mine?

Not a whole lot better than that; you're hauling a big, heavy brick through the wind with an overburdened, underpowered engine.

You can create the illusion of better fuel economy in the usual way: put on smaller tires or install a smaller speedometer drive pinion so the odometer runs fast, racking up more (pretend) miles between fill-ups. (LOLROFL, etc)

In all seriousness, you aren't going to be able to do too much better than that without spending more money (exotic transmission swap, fuel injection retrofit, etc) than you'll ever make back in fuel savings unless you're using this truck all day, every day.

But the EPA fuel economy rating for an '84 D100 or D150 was 14/17 (automatic) or 15/19 (4-speed), according to the current, more realistic test system, and there's almost no difference between an '81 and an '84, so you might get a couple more MPGs out of it. Beyond a good tune-up, you can and should also check for exhaust restriction -- catalytic converters tend not to last long on carbureted vehicles. Also check for brake drag, engine not reaching full operating temperature, etc.

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

My '84 W100 with a NP435 4 speed is getting 12.5 mpg all city driving (Dallas). I believe my lean burn ignition is not working properly. I am converting it to HEI as soon as I can. I hope for a little better mpg after the swap.

What transmission do you have? Do you still have the lean burn ingnition and carb?

_________________
1976 Feather Duster /6 4sp
1984 W100 318 727 np241
1972 'Cuda 340 4sp
1985 D250 360 46RH

13 is about average for a truck or van. Like Dan said- you are pushing around a big heavy brick. You can get better if you work on things like the ignition advance curve, cutting weight, the ignition system, and do the usual efficiency increasing steps like a clutch or electric fan, skinny low rolling resistance tires, synthetic oil, free flowing exhaust, cold air intake, etc...

The best economy I ever got with a slant six was about 18 MPG in my brother's 83 Dodge shorty van. This was steady highway driving at 55-60 afte I had just done a full tuneup on the motor. It has a 225 with a 904 and 3.2 rear gears. If your rear gears are numerically higher than 3.5 you really shouldn't expect better than 13 MPG unless you have an overdrive transmission.

If I were you, I would be happy with 13 as a baseline and focus on improving efficiency of the vehicle in other small ways.

I have basically the same truck. With a 2 bbl carb, HEI conversion, dual Dutra manifolds and no catalytic converter, manual transmission and 3.55 gears I can eke out 16 mpg highway. You're not going to get a whole lot of fuel economy out of these trucks.

_________________
If it was easy, everybody would be doing it.

The above being said, board members have reported that when forced induction is used it is possible to get high 20s MPG out of a slant powered truck. But this is a fairly involved modification that will almost certainly cost more than the cost of the fuel saved and should not been seen as a worthwhile modification for fuel eocnomy purposes alone.

Thanks guys for all the info. I don't have any brake drag, I do have 215-70's on it. A few weeks back I bought a oem distributer and plan on converting to HEI but not for the MPG's. I'll consider bonus if the mpgs improve after hei.

I'm not sure of the trans but it's a auto w/lockout. Lean burn is disabled and previous owner put points dist in.

i got 21 mpg on my 1982 D150, long road trip.

Went though some mountains, some hills, some flats.

Single barrel 1941 Holley, NP435 transmission, 3.5 rear, HEI conversion, going 55 to 60 mph.

Biggest improvement happened after the HEI install.

I would get 13 mpg prior to the HEI..

Get the HEI....

_________________
Slant power!!!
At least I try...

Looks like I'll try an get this HEI swapped over soon

Do you have an EI distributor with vacuum advance to work with the HEI?

yes I do. I have one with a advance curve for a truck, brand new.

Make sure it isn't the one with double pigtails. That one won't work.

_________________
If it was easy, everybody would be doing it.

You can't get a distributor with dual pickup/pigtails with vacuum advance...
the function is mutually exclusive...

But then again I have enough spare junk parts I could probably make one and see who picks up on the fugazi....LOL