This is for a 64 Slant truck. Totaly stock. The data plate says it has a 3.5 ratio rear axle. It also says it has a 4 speed floor shift but it's a 3 speed on the column... It is low geared. It's hard to downshift without the tires barking.

There are a couple of ways to check the ratio of your rearend from what I've read. I seem to have lost them all. I'd like to check mine and see if I can get better MPG with a different rearend. Also, can I just change the gears easily without changing the entire axle?

I ran two tanks of gas and checked my mileage - 14.5 per gallon. NOT what I was hoping for even though it's a truck. Much of it is highway mileage but some is back-roads and in town.

I also just switched to a 2 barrel intake and carb. I tried to adjust it with a vacuum guage but I couldn't find a place to plug the guage that showed any vacuum at all so I just did it by the RPMs.

It runs great. Choke is a little slow to warm up but works pretty good. Is there anything else I can adjust for mileage?

What two barrel carb? Is the motor otherwise stock?

You can tap into the vacuum line for the choke pulloff diaphragm to get a vacuum source.

14.5 MPG for a truck is actually pretty good. There should be a tag affixed to one of the differential cover bolts on the rear axle, but that may have gone missing. Jack one rear tire up,take the truck out of gear, spin the tire and count how many times the tire turns when you turn the driveshaft one revolution. Compare the two numbers and there is your rear gear ratio.

Better MPG can come from many things. Use synthetic 5W-30 oil or thinner, switch your thermostatic choke pulloff system to one of the faster operating electric kits from CarbsOnly (see HERE), run skinny low rolling resistance tires, fine tune your distributor spark advance curve, make sure the advance mechanism in the distributor is lubed and moving freely and the vacuum advance is also working, switch to HEI electronic ignition, run a free-flowing exhaust, install a cold air intake system so your motor isn't sucking hot air out of the engine compartment, lighten your truck as much as possible, and (my personal favorite) make the truck more aerodynamic by adding air dams and streamlining the body:



There are probably more ways and I am sure someone else will chime in with things I have forgotten.

_________________
Casually looking for a Clifford hyperpak intake for cheap.

14.5 mpg out of that truck is extremely good mileage. You are pushing a heavy brick through the wind; your mileage will not get much better.

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

FWIW, you do not down-shift into first, the 3 speeds do not have a synchronized first gear.

_________________

When I first got my truck I got 18 mostly city when it was all stock. But with many mods, a big diamond plate tool box and 14,000 miles later I get between 11 to 14. I have a vac gauge in the cab so I can keep my foot out of it as much as I can while cruising at a steady speed.

_________________
Old truck: 82 D150 225, 2bbl, dutra duals, 2.5" straight pipe to dual 2.5" tail pipes. Mud tires, 4spd, old forest service truck.
Current truck: 98.5 Ram 2500 ccsb, sport, 24v cummins, 5spd, 4.5" lift, 35's, 5" straight pipe.

The "original" choke for your truck will never work right with the BBD carburetor. I strongly recommend you get the CarbsOnly kit.

If you are serious about improving your fuel economy, then you must install at the very least a vacuum gauge and a tachometer. You will also have to determine basics like your rear axle gear ratio, what kind of transmission you have (three or four speed, etc...) If you are trying to keep the truck looking like an original vintage 60s truck, then you can track down period correct or reproduction of period correct gauges. Without proper instrumentation you will never be able to accurately document what is going on with your motor and make improvements.

As far as the tires go, wider tires cause more rolling resistance. Skinnier tires give better fuel economy. If you do go back to skinnier tires, I strongly recommend you get light truck tires. They have stiffer sidewalls that will help with the "wishy-washy" feeling.

Jacking up both wheels will never be the right way to find out the rear gear ratio since the wheels will just spin freely. Since you need to learn what kind of rear end you have anyway, I recommend either pulling the rear differential cover or dropping the center carrier and counting the gears.

The cold and hot air intake systems can and were used together, and both are good to have. The heated air intake operates only when the ambient temperature is very cold- typically below freezing but there was some variation. There is a sensor mounted inside the air cleaner that senses the temperature of the air entering the air cleaner. This sensor controls a vacuum switch. When the air is cold enough, vacuum from the carb is applied to an actuator in the air cleaner snorkel which opens the flap in the floor of the snorkel and allows hot air drawn from around the exhaust pipe to be drawn into the intake. This helps with cold-weather driveability and with cold-startups. Once the ambient air in the air cleaner gets above a certain temperature, the sensor blocks vacuum flow to the flap in the snorkel and the door closes, blocking the hot air from entering.

A cold air intake is nothing more than a hose from the end of the air cleaner snorkel out to some spot behind the grille or in one of the fender wells which allows the carburetor to suck colder air from outside the engine compartment. It is a long-knwon and well documented fact that motors give better performance and fuel economy when they draw in cooler air (the caveat to this is when the motor is first starting up in cold weather, which is why heated air intake systems were used). On older cars and trucks (basically mid 70s and older, except for some exceptions), stock air cleaners were designed to draw the air for the engine from the engine bay. This meant that the carb was constantly fed hot air. Adding a cold air intake feeds the carb cooler denser air and improved fuel economy and performance.

Both heated and cold air intake systems were used by all manufacturers by the early 80s, some even earlier. Chrysler cars got them starting around 76 or 77, and trucks got them by the early 80s. I highly recommend you add a cold air intake to your truck.

You fuel economy will be greatly impacted by the gears in your truck as well as the ignition system, timing advance curve, and driving style.

_________________
Casually looking for a Clifford hyperpak intake for cheap.

I have the np435 4speed, 4th gear is 1:1. My rear gear is 3.55 in the original chrysler 9.25" rear. At 65mph my engine is at about 2800rpm. For me I would go higher and id like to put an nv4500 in it. When I weighed my truck with my plastic truck box it was at 4150lbs. Take everything off I had put on ,it probably weighed around 3700 to 3800lbs but thats just a guess. For a stock truck I wouldn't really want a lower rear gear cuz like dan said its like driving a brick, the a bodies are more aerodynamic than our trucks so they can get away with lower gears but I think itd be too slugish if you put a lower gear in it. If you did that youd probably have your foot in it more because of the sluggishness from going to a lower numerical gear which would probably result in worse fuel economy. do reeds suggestions and you could probably hit 18.

_________________
Old truck: 82 D150 225, 2bbl, dutra duals, 2.5" straight pipe to dual 2.5" tail pipes. Mud tires, 4spd, old forest service truck.
Current truck: 98.5 Ram 2500 ccsb, sport, 24v cummins, 5spd, 4.5" lift, 35's, 5" straight pipe.

For what it is worth, my brother's 83 Dodge shorty van has a 225/904 combination and a 3.2 rear gear. It gets about 15 MPG on the highway if he keeps the speed down.

_________________
Casually looking for a Clifford hyperpak intake for cheap.

Where you have 3 people you will have 6 opinions

Do you know what the truck weighs? My 85 shortbed weighs 3320 if my memory serve correctly. It has a 2 barrel and 3.21 rear and automatic, and on the trip to Elk creek it got 18.7 MPG, and we were loaded to the hilt and ran 65 the whole way.

Depending what type of driving you do (lots of moutains, hills..etc) I would not be afraid of a higher ratio. If you have a 8 3/4 rear a 2.94 ration would drop right in and can be had reasonably in a non-suregrip.

My old truck had a 3.54 rear and OD tranny and that puts your final ratio at about 2.50. It wasn't a powerhouse but ran fine and it only had a single barrel carb. It could get 20 MPG out on the road

Make sure of what you got first. And make sure your Speedometer is right. You may be getting more than you think.

_________________
2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Thanks for all the ideas. It looks like almost everyone is getting better MPG than me with close to the same set-up - even with automatic transmissions. Maybe I only need to adjust and tune EVERYTHING and see what I get. I've always wanted to check my advance curve since I read about it but it's an all original truck with the original distributor. I really doubt it's off much unless I change something.

Reed: My "original" choke was manual. I bought an "original" choke for a BBD when I changed it over. It really is working pretty good and doesn't stay closed too long. The cold air intake wouldn't be hard to fabricate and I will try to rig something up for that.

Would shaving my head and doing some porting and polishing help?

I agree about the guages and will get them but I really don't drive very fast. It will keep up with traffic on the highway but it really only wants to drive at about 50MPH. It seems strained or constricted but if you put your foot in it it will go 80. It doesn't like it but it will. It may just be the brick hitting the wind.

Would changing to a BBD change my timing curve needs?

One of the next things I need is a 2-1/4 pipe from the manifold to the muffler.

I would really love an auxilary overdrive for highway driving. I have one but I'm not sure it would work on this truck. I bought it for a Jeep. It's also 100 years old and I'd have to check into finding out if all the switches and servos work.

These trucks look awful with the original skinny tires. I don't see me changing them anytime soon.

I'm still thinking about the rearend ratio. It's a bear to drive already but I can pull out in traffic and get up to speed pretty easily. It will take off from a rolling stop in 2nd gear since I opened up the exhaust manifold compared to the old one - I guess the BBD might help with that also but I really like that. It has a non synchronized first so you have to almost stop to downshift into it but I'm learning more how to do it as I drive it more.

Right now I have a bent tailgate to try and fix. Removed the rear bumper for sandblasting about 3 days too soon. Big ole' tree just jumped up behind me - can't steer the damn thing or see anything out of the porthole they call a back window.

I'll start some of the other ideas as soon as I can.

Thanks,
Danny

If it only wants to go 80 I suspect you have either a low gear ratio rear axle (high numeric ratio) or your timing isn't advancing right. You need to determine what your rear axle gear ratio is and you need to open up your distributor and make sure the mechanical and vacuum advance mechanisms are working correctly. Getting a tachometer installed so you can see what your highway RPM is will also tell you a lot.

_________________
Casually looking for a Clifford hyperpak intake for cheap.

How to calculate rear axle ratio:

http://www.slantsix.org/forum/viewtopic.php?t=36418

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I have an '85 D100 shortbed 2wd with original 1bbl carb. Everything stock.

The rear end is pretty low geared I think, but I do not know for sure what the ratio is. I know 1st gear seems like a granny gear. If I want to, I can shift into OD (4th) at 40 mph and old slant doesn't mind. Seem I stay in OD most of the time except in stop and go traffic.

I get 18-20 mpg. 4 speed manual, the truck really comes alive above 65-70 mph. Top speed is almost 90, not sure though since speedometer doesn't read above 80.

Oh, tires are stock skinny tires on front, the rears are wider but I'm not sure what width. 235/75/15 mud tires on rear 215/75/15 car tires on front. I plan to change that soon.

_________________
1985 Dodge D100 with Slant Six, Standard Cab Short Bed

Me, my dad, and my brother, have all owned 2wd slant six trucks. All of them routinely would do at least 20 mpg on the highway. All but one of them had 3.9 gears, and the last one was 3.55's.

The 78, with a two barrel, bigger cam, truck tires, and not going over 60 would achieve better than 22 on a trip. The other three trucks would do right about 19 to 21, depending on whether it was warm or cold weather, on a trip.

With the 4bbl ( Carter 625 AFB) the 78 would do 18-21 mpg when carefully driven on the highway.

The first thing I would start with is to upgrade your ignition, then to make sure the vacuum and centrifugal advances are working and timing set right, and then get the carb set right.

Any slant six that's not totally worn out, or has burnt valves should not have any difficulty getting 18 to 21 in a 2wd pickup - assuming you're not trying to go to fast.

_________________
'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.