Helo, I;am very frustrated. a friend passed away and left me his Dodge D 100 slant 6 225. After doing a compression check, all cylinders were below 90 psi, repair manual reccomended a rebuild if under 100psi. The truck had little power and only got 10 mpg so I had the motor reduild back to stock. I now have a little more power but the mileage is still crap at 10 mpg and I have to go up all steep hills in second gear. It has four speed manual transmission. I spent all the money I had saved up on the rebuild and now have the worst of both worlds. Crappy power and mileage. Any advice on how to improve mileage and or power will be appreciative. It has a 2 barrel carberuter on it. Thank you.

Hello and welcome. First, we would love to see pictures of your truck!

Second, can you give us a little more detail about the truck? What year is the truck? Do you know what rear axle gear ratio and what four speed manual transmission the truck has? A high (numerically) rear axle gear ratio and/or a four speed with the "granny gear" first gear might be great for towing slowly, but get horrible fuel economy when driven on the highway.

Have you adjusted the valves? How about base timing? Have you verified that the vibration dampener accurately indicates TDC? Have you verified that the vacuum and mechanical advance mechanisms in the distributor are working? If the truck is from the 1980s, is it equipped with a lean burn ignition or carburetion system?

Where are you located? Thee may be a board member nearby who can take a look at the truck for you.

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Casually looking for a Clifford hyperpak intake for cheap.

Thank you for the reply.

Its a 1975 and I do not have any pictures at the moment.

The 1st gear is very low and rarely use.

I live in north idaho close to spokane wa. I would be very grateful to get help from a local member.

How can I find out the rear axel ratio?
How can you make sure the harmonic balancer is functionng properly.
I had the entire motor rebuilt and machined. The head and the lower end. A shop built the motor but a friend installed it for me.

I do not believe the timming is 100% Does anyone know what it should be timed at? I found 0 degrees but read other sugestions. It's a 225.

I do not believe the hoses for the vacum are hooked up properly anywhere and some lines are plugged. Does anyone have the diagram.

I will find out what transmission is in it but it might be a few weeks until I can get to it.
Thank you for your help.

Aha. I have a 76 D100, but I live over in Tacoma. Not exactly driving distance.

If you have a very numerically high first gear that you never use, then you do not have an overdrive. I don't do manual transmissions so somebody who knows more about them will have to tell you more.

To check the vibration damper, you have to get a piston stop tool and verify that the timing mark on the damper is still indicating TDC correctly for cylinder #1 at TDC on the compression stroke.

Since your truck is a 75 it should have very few vacuum hoses. The main one you should be concerned with right now is the distributor vacuum advance hose that runs from the carburetor to the distributor. You also need to verify the the vacuum advance pod on the distributor holds a vacuum and advances the timing when vacuum is applied.

Base timing may very well be TDC from the factory. I know the base timing setting on my 76 D100 is TDC (or zero). However, if you know what you are doing, you can change that setting for more power and economy.

There should be a tag on your rear axle that tells you the rear axle gear ratio. I bet it is 3.5 or 3.9. The granny gear manual trans with a 3.5 or 3.9 rear axle gear ratio is not a recipe for fuel economy, but it should have plenty of useable power.

Have you adjusted the valves lately?

You best bet for vacuum diagrams and timing settings is the factory service manual. Until then, carburetor diagrams and repair manuals are available HERE for free download.

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Casually looking for a Clifford hyperpak intake for cheap.

hello Korndeezy,

I own a 1982 D150, with a NP435 granny gear transmission on a slant 6 with 3.5 rear.

I have done just about everything to this truck. (I learned mechanics on this truck with the help of the guys on this forum)

I have never been able to get better mileage that 12-13 mpg city driving. I live in Tijuana Mexico, lots of stop signs, stop lights, traffic, and hills.

it does excellent on highway, don,t know the miles.. never done the math.

when I first got this truck, it barely ran,, and because I only had one speed, (slow) mileage was pretty good.

I did notice that the better my truck ran, the faster I drove, the less MPG i get..

I could never help myself, every time i did something to the truck, i would get so excited that I had to test it.. how did i test it?? dive fast!!

I even ,at one point, put in a A8033 OD trans.. I did not like it, so I went back to my granny gear 4 speed, (its really a 3 speed with a low gear)

I know that if I drove slow I would get a lot better mileage, and,, if i did not have the short 12 to 20 minute commute in traffic,, well you know.

My truck runs like a dream, but the MPG are always very low teens.

I don't mind, I take lots of pride in my truck, mainly because I have done all the work my self.

Next project, turn it into a towing truck, I bought a small 18 foot fifth wheel.. you know i will be on this forum with lots of questions....

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

Does the truck have a catalytic converter? If it does and it's plugged up or melted down you will have a lot of pressure in the exhaust. Power and economy will suffer.

Many carburetors are calibrated quite lean. If lean enough to induce misfire power and economy will suffer. A wide-band air/fuel ratio meter will tell the story.

If the timing is late power and economy will suffer. Distributor work is pretty economical.

A compression increase will help power and economy, but that requires cylinder head removal. Bringing this up now is a little like closing the barn door after the horses are out, but...

I had a 1974 D100 club cab with a 225, 1bbl. Even though the engine was in good shape after I had the head rebuilt it was always a slow truck and didn't get very good economy. I drive a lot lately and wanted another pickup so I got myself a new 4 cylinder 5-speed Frontier in January and it gets about 24 MPG when driven nicely.

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Joshua

Ditto what he said about the catalytic converter. When mine clogged up I could barely top 35 mph. The local muffler shop checked it for free and said it was definitely clogged. $200 later and it ran like a scalded ape and mileage returned to "adequate" instead of pathetic.

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If it was easy, everybody would be doing it.

Thank all of you for the great advice. I do not believe there is a catalytic converter but will have to check.

my first gear is a granny gear.
I will try a fuel ratio meter and check the timing and vacum line to the distributer.

The vibration damper should be good since I had the motor completely rebuilt. I will look into it.
The head was rebuilt as well so I would hope the valves would be adjusted correctly. The builder has a good reputation were I live but a friend installed it for me since I did not have a picker, time or complete confidence to do it myself.
Mr. Lopez, how slow do you drive on the highway and or in town. Do you keep it in fourth gear in town? I can put the gear into fourth if im driving 25 mpg hoping that will help with mileage.

Thank you all again.

I have two slant six '84 D-series. The better of the two is a longbed with the NP435 (granny four speed) and a 9.25" open rear with 3.2 gears. It's no speed demon but the engine is fairly healthy. Highway driving, all under 60 mph, I can see mid-to-high teens. However, in town I am consistently around 12-13 mpg just as Enrique Lopez. It's basically a stock engine with a remanufactured Holley 1945 and HEI. Stock 2" exhaust with cat delete and a crummy worn out glass pack from the previous owner.

The mileage is a bit frustrating, but from everything I'm reading these trucks never did great around town, even compared to the 318 and 360 it seems.

It has been wonderfully reliable, though, for a 30 year old truck that has spent a lot of time sitting.

Around 25 mph, I'm usually in third gear. Fourth gear is around 35, depending on road conditions (hills) and traffic. High way, usually stay below 60 mph.

Not a very good city truck, but an excellent work truck. I have loaded it with rock, wet sand, gravel. Way over a ton of weight. It never had any problems.. start in granny gear, second gear,, third,, and cruse... I never put it into fourth gear while very heavily loaded..

I have read many threads on this forum, every one with d100's and d150's seem to get around the same mileage.

But there is a lot of very knowledgeable people on this forum, I sure that you can improve mpg with their help..

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

Thank you Enrique. I have not touched the truck yet. Theres a few feeet of snow surrounding it at the moment.

adjusting the valves (lash) isn't something your engine builder can do very well, since the motor needs to be warmed up and running.

most slants have solid lifters so as the cam lobe and lifter face wear (along with pushrod cups, rocker tips, valve steams to a lesser extent) the valve opens less and less. every so often, you need to tighten everything to account for this.

Thanks NEVjr. I will check out the valve adjustment.

Hi,,
I took a long road trip,, (450 miles one way, 900 miles round trip), in my D150 slant 6 - NP1435.
I have not done all the math yet, but, I am happy to announce that for the first 147 miles, my truck averaged 21 mpg on highway going 55 to 60 mph..
I am impressed.. I have to also mention that this included driving though a mountain range.

It was very flat after this, so I know my mileage improved even more..

So yes,, even the D150, D100 are capable of great mileage. Even with no overdrive....

I will post my total mileage later, after I do all the math.

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

People have given you a lot of easier things to check, but it may be good to know if the garage degreed the cam in with a degree wheel. Or if he just lined up the dots. Many of the timing sets do not have the dots stamped in the right place and if installed like that it can make driving and mileage terrible.

Rick

You should be able to do much better than 10. I got 10 with my race truck, towing the race car and trailer, and 6 WOT dragstrip runs to boot.

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