This is what I've come to realize by staying with the 4.10s. The rpm would go down at speed with the 3.54s, but the gain would be minimal and a trade off at that. Don't know what the rpm is at 70 but it doesn't seem to be taxing the engine (tires are 245/75r16 30" tall)
And the van will scoot.

This.

My 64 300 came with 3.23's stock. It's get a solid 12 mpg regardless. Swapped in 4.10's and started getting 15. The 413 was warmed up a bit and I suspect the mild gears put it out of it's sweet spot whereas the steeper gears was more to it's liking.

This is true Dave. Another factor is that if your cam/engine makes peak torque (best mileage) right in the "sweet spot" of your highway rpm & you drop the gear ratio, then you may not be in that "sweet spot" anymore....but the math is still good for a ballpark idea of mileage, rmp at highway speed, etc.

I like to think more of displacement per mile than RPM per mile. For example you could have a 512 stroker turning 2000 revolutions per mile (a gear/tire combo that gave you 2000 RPM @ 60 MPH for example) and displace the same air and have the same basic internal friction as a 383 turning 2674 revolution per mile (or RPM @ 60MPH). The two should get very similar MPG and put similar TQ to the tires with identicle top ends, cams...

Also the whole thing about keeping the vacuume as high as possible is sort of bologna, yes you want to keep it as high as possible within a given combo but keeping that number down through gearing is not really by it self a bad thing. You will always get higher vacuume in 1st gear than 2nd 3rd 4th... but you will absolutly not get your best MPG driving everywere in 1st gear. The reason you were getting better MPG watching that gauge is because you were not doing as much work accelerating and then slowing down if you keep the gauge steady with a particular load if you were trying to keep the vacuume up. Also it is a good indicator of your engines tune state. I always got my best MPG with my 318 magnum 5 speed dakota by running a much higher gear selection than most would ever consider running and then accelerating with WOT at a low RPM. I could get near 20 MPG in town all the time driving this way, you will not get anywhere in a hurry but it saves a butt load of gas. If I ran it up to 2500-3000 to shift and accelerated at part throttle is would get maybe 12MPG in town.

I've been driving and digitally ignition tuning a 8.6:1cr 318ci in my dailydriven Dart for well over 5 years and hardly got the mileage to improve noticably (10-11mpg).

I've now got a 11.3:1cr 360ci installed in the car which has significant more power more but the mileage is still the same.
The 360ci has a 208-210 cranking psi with its CompCams XE256H cam.
A518OD / 3.55 gears.

Last year I had installed residual pressure valves in the brake system. 10psi for the rear drums and 2psi for the front discs.
The front disc pressure valve was good for 1 mpg loss in mileage. Noticing hard to rotate front tires lately, I removed front res.valves recently and promptly got the lost 1 mpg back.

Somethings not right if you are only getting 10-11 mpg in a 318 Dart. You should be getting a lot better than that.

Tell me about it.
With all the attempts I've done at improving it I could write a book of things that seemingly DON'T work.

Had hopes the high cr 360 engine now in the car would be more efficient, but I'm still not seeing it happening.

I'm currently kinda blaming the transmission (A518) and the perhaps in-efficient / old design(?) '60s stall convertor (non-lockup) I've used in it for the crappy mileage the car is getting.

I have a 42RH (OD+LU) needing to be rebuild. It has a matching '90s convertor which is also 1" smaller in diameter (less weight / more efficient?).
Hope this trans will be able to 'fix' the mileage issue with the car.

Gas mileage is affected by many factors - the largest factor is rolling resistance. I have owned wicked cars that got good gas milage
One is particular: E Body 416 six pak with 4sp OD and 355's
16 mpg going to and from Carlisle, back in forth to Watkins Glen etc. Top speed 140+ ET 12.20 to 12.80 with 11.90 trap speeds 400+hp at the rear wheels

Place car on level pavement in neutral
Can you push it backwards without effort??
Probably not. This is the test, if you can't do this there will be no gas mileage.

Synthetic fluid in the diff, run level 1/4 inch down.
Rear wheel bearings greased with synthetic grease, end play 0.005-0.007
Front wheel bearings in synthetic grease, bearing preload about 50 inch pounds.
Proper wheel alignment
Manual Trans? Passon gear lube
Auto Trans? Mopar ATF4 or better yet Amzoil synthetic

Engine oil, 10-40 or 10-30 synthetic, 5-30 if your engine will tolerate it

Proper engine tune up and distributor curve - here is the other major mileage factor. Lots of timing advance under part throttle needed.

K&N air filter

60% distilled water, 40% antifreeze, bottle of coolant enhancer.

195 degree thermostat

External Trans cooler if AT with internal radiator connection bypassed

In some posts people complain that on their next trip after a mofification the MPG change they were expecting did not happen or went the opposite way.

One thing I learned doing MPG tests from Durham NC to Wilmington over a mostly flat I40 in a Ram was that
side winds,
Air temperature, and especially
pavement changes from blacktop to concrete
changed fuel economy WAY MORE than I first “guesstimated.”

To really do meaningful tests on real world interstate highways to nail down small changes on the order of 5% you need two vehicles travelling convoy style.

As a miner I know there are many miles of big tunnels in Limestone quarries and Salt mines. I once joked with a friend of my father’s who got out of the coal business and into a Limestone quarry that I wanted him to pave ten miles of it for me to do MPG tests.

That's a lot of turd polishing there.

A stock, no polished turds, 318 Dart should have no problem approaching 20 mpg highway.

Oh DO NOT PRELOAD THE BEARINGS use the fsm method.

one time, a million years ago in the mid 70's, i had a 74 dodge 4x4 pickup with a 318/727/3.91 gear 35" tires that got right around 5mpg pulling a super bed load of junk to the scrapper. it was tuned to the max, but to be fair, i had added a "super" bed made entirely of 1/8" plate steel, with 2" oak floor, and a 3" roll bar using 1/4" tube gas pipe. [and that was a real "treat" to bend !] it had 6"&8" C channel cross members, as well as a piece of 6" well casing between the rear frame horns. up front, there was a fabbed 2" push bar on top of some 4" well casing for a front bumper. to say this thing was heavy would be an understatement ! i built a 440 8.5cr with a torque/rv cam, TQ carb/factory hi stall converter, and swapped it for the 318. mileage went instantly to almost 14 ! the 318 was foot to the floor all the time, while the 440 was just loafing. i wished i never sold that truck. it could sure haul the loads, plus push/pull almost anything out of the road !

In town mileage is hurt, cause we are trying to get 4000 ids up to speed, on the highway, it rolling resistance, AND wind resistance. Usually, less filling of each cylinder per mile is the answer, if everything is set up right. If our engines were turbo charged, then lowering the RPM, and increasing the manifold pressure would do the trick. Read what Charles Lindberg did during WWII to help the P-38 guys save fuel.
Two cents.

Dilbert,
I tried the car-push a number of times.
Some of my other cars move real easy, but the Dart takes some effort to get rolling.
I've always found cars are a bit harder to push backwards. Probably because more internal transmission parts are being rotated then I think.
The 3.55 gears make it a tad harder too, but that should work opposite when power is coming from the driveshaft.

Transmission has ATF-4, external cooler in front of radiator. But transmission oil never gets really 'hot' so installing thermostat would be better I think.

I ran with 0w20 synthetic oil in the 318 during the last year or two.
Besides easier acceleration during morning starts it had maybe a very slight noticable effect otherwise.

Thought about putting synthetic oil in the (Auburn SG) diff. Or maybe even a thinner oil, but I that would focussing on .1 of mileage numbers I would think.

Car has cold air intake.

Ignition timing is digital (MegaSquirt) and I have been tuning the timing map for quite a while now. Timing is increasing the moment there is little to no load on the engine.

I guess an app for Android or iOS could now be written
that could generate a BSFC map for a vehicle/engine combo in real time
by driving in different gears at different speeds.

In addition to the data from the OBD-II port,
GPS and accelerometer data could calculate power independent of OBD estimates.
It would be simpler if the roads were level, but GPS could factor in elevation changes and estimate uphill and downhill grades. Google and Apple maps mostly now contain this grade data.

I tried to test the rotating-resistance of two 727 transmissions and a A500/42RH I got laying around by grabbing the inputshaft with 2 fingers and rotate it clockwise;

Both the 727's rotated fairly easy but the A500 took way more effort and required a firm grip to rotate.

Something is not right with the a-500 or the 727s, too tight of clutch clearance or end play or... when a 727 and a500 are properly assembled the a500 shaft is always easier to turn. At the other end of the trans is a different story as you will have an extra set of clutches and more weight on the a500 output shaft but the extra drag there is more than offset by the lower RPM in %99 of cases (for example a 4cyl might not have enough spare power at a low RPM to overcome the extra friction/weight).

All 3 transmissions were used ones. Both 727s are working transmissions and I only know from the previous owner the A500 has 2nd gear 'gone'.
I've also got used A518 (4x4) unit to try out.

Remove the drive shaft and push the car backwards on a level surface.
See how much force it take- put synthetic lube in the rear, reconnect then try it again. You will be so surprised...


The suggestions to correct the tune up & reduce rolling resistance need to be followed...or you are wasting your time.

As far a mpg and rear end ratios...the rpm of the motor at it's cruising rpm [say 70 mph with an automatic] is where the converter's stall speed
should be just below this rpm. If you are always under the stall speed of the converter you are loosing efficiency and gas mileage.

There is an interesting USA Patent for an improved method of estimating the aerodynamic coefficient of drag of a real vehicle out on roads.

Near the end of their description of their aero method, they also describe a neat way to better estimate transmission and differential friction and “stored rotational energy” in driveshafts and wheels.

They jack up and put the vehicle on jackstands, start the engine, engage the 1:1 gear and bring the drivetrain up to a speed. They measure the rpms of the wheels, then take the vehicle out of gear and let the drivetrain coast down to a lower rpm ( but not zero). After gathering this data, they remove the wheels, and re-run the coastdown again without the wheels.

With the two sets of data, they could calculate both drivetrain friction and stored rotational energy if they just knew how much rotational energy the wheel and tires have.

They devised a neat way of measuring this.

Think of Chrysler torsion rod suspension.

These inventors made a “torsional pendulem” by fixing a small diameter steel shaft to a ceiling and welding on a plate to the other lower end of the steel shaft with studs to match the bolt pattern of the wheels.

They attach a wheel/tire to the plate, then twist the tire on the hanging pendulem, and let it go.

The wheel/tire will twist back and forth in a cycle whose twists per minute depends only on the rotational inertia of the wheel/tire/mounting plate, plus the “spring rate” of the steel shaft. It is a easy to build “ I of a wheel” measuring instrument, where I is the symbol scientists use for “Rotation Inertia” of a rotating mass.

Old fashioned mechanical wrist watches have a similar torsional pendulem inside them.

https://patents.google.com/patent/US5686651A/en?oq=5686651

US Patent 5,686,651