Better gas milage or waste of time?

[MountainMoparRobin]

if you have manual transmission sounds like not many like the highway gears, automatics multiply torque I guess thats why the 318 in my 83 does ok with the 2:95

[Caleb]

i've got a '74 la 318 in my 66 d 100. has 3.55 gears and 3-spd tranny. gets 10- mpg, but it has a 4-bbl edelbrock and a crane 272 degree cam. .454" lift. funny thing is, i had the old slant six in it and it also only got ten, so i guess its not too bad

[MountainMoparRobin]

Caleb,
that /6 should get alot more than the 10 mpg, and your 318 with the crane cam would get better with a A833 with overdrive you've been searching the archives

[bluefury]

I have a spare 8 3/4 third member ready to go with 2.76 gears, the truck has 3.91 right now is this a good swap for a MPG increase or waste of time?

[b

A 3:23 is going to be your best choice for the power train combo you have and type of driving you do. You will not be happy with the 2:76 set. Milage will be a tad better with an open diff as opposed to a sure grip, but perhaps not really measureable.


Will.][/b]

[Russ]

I would try the 2:76 gear and see how it works. I had an 87 Dodge Diplomat and an 87 Chrysler 5th Avenue with a similar rear gear and with a 318, 904 with a lockup converter, they would get better than 20 mpg on the road. I usually got high teens around town if I kept my foot out of it. I wouldn't do it with a straight shift truck but since you have an automatic I'd give it a try. The truck definitely won't be as snappy as it is now but with gas prices like they are it might be worth the difference.

[tom-adc]

This is going to happen but we've been so busy travelling I haven't had a chance to do a thing.
I also need to change out the axle seals so I'll kill two birds with one stone.
Had a wheel cylinder leaking back there, changed it out but now need to do the axles also.
Got a bum leg right now, I found out if you are 66 you shouldn't jump into a pool with only 3 1/2 feet of water missed the 5 foot marker by this much

[HEtkind]

the 07 Ram 1500 has the six with a six speed, 3.21 gears and 245/70R17 tires and gets 21 mpg at highway speeds with a overall tires height 31.6", but you have to downshift to climb a speed bump. Higher engine speeds give much better torque, however

the 68 utiline with a the 318/727/3.91 suregrip and LT235/75R15 tires with an overall height of 28.7" is getting about 14 mpg. This truck has been getting most of the use lately

the 62 Town Wagon with the /6 motor, np420 gear box and 3.55 suregrip and LT225/75R16D with a tire height of 29.35 is also about 14 mpg. Will do just fine at 4k rpm...

The 68 with headers always sounds like it could use another gear, but the Town Wagon tows GREAT...

[moparbob71]

I did that online calc thing and plugged in all the info- taching at 2000 with my tires and at 70 mph it said to use 2.6 something. I have a set of 2.76 SG out of a 69 Newport I was going to use since I want to drive this regularly. I will have to tow the Dart or Cuda occassionally so I was thinking about going w/ the 3.23's, but that would push the 70 mph rpm's to 2300 if I remember. Mileage or towing? Next on Oprah.

[dodgegrrrrrrl]

RULE #1: One of the big problems with these trucks is aerodynamics, or the lack of. That is why your gas mileage drops to about nothing when you're pushing a headwind. The engine RPM is the same, road speed is the same but you're pushing the gas pedal through the floor to overcome the wind drag. Get a vacuum gauge and drive by the thing. The higher the vacuum the better your mileage. For example, I added a cap over the bed on our 66 W200 and got two more inches of vacuum AND 1-2 MPG going down the highway.

RULE #2: The easiest way to change your final drive/engine RPM@MPH is taller or shorter tires. Tall skinny tires give better MPG that wider tires of the same diameter. More air pressure in the tires helps too.

RULE #3: Vehicles hypothetically get their best mileage running @ 70% of the horsepower curve, HOWEVER, see rule #1. The theory is that as you double your speed, your wind drag squares, so if that 70% is at 90 MPH the gas you'd be saving in a Prius is going out the tailpipe on your Dodge, burned to overcome wind drag.

Unless you live in Fla., Ok. or Louisiana (FLAT!) you'll learn to hate gearing under 3.0. You'll find you spend way too much time rowing the gearbox or standing on the gas because you're at the bottom end of your power band. (see RULE #3) These engines are just big air pumps. The more revs your turn the more air/fuel mix you pump, with one caveat; the enrichment circuit on the carb. The lower your manifold vacuum the more gas the carb adds to the air/fuel mix and that is why uphill or pushing headwinds burns more gas at the same road speed and/or RPM. (This can be RULE #3a) Higher gearing that puts your RPM below your power band @ highway speed will require larger throttle openings, for example, when starting over an overpass or pushing a slight headwind. This will lower manifold vacuum and engage the enrichment circuit on your carb, putting more gas into the fuel/air mix and cause a drop in mileage, even though turning fewer RPMs. Also, when you have to use 3rd gear to go up every hill, you've just lost your savings you got by lower RPM on flats.

The trick to all of this is finding the right match of RPM@MPH vs. drag vs. topography and load. The easiest way to play with this is with several sets of rear tires.

At one point in time I had a SCOUT with 3.73 gears. I had 33's on it and went to 35's with the idea that taller tires would give me better mileage and better off-road performance. It got horrid mileage with 35's because driving down the freeway at 55 MPH it was turning 2100 (48% of power band) RPM and I had to downshift to accelerate, giving me @ 11 MPG overall. When I went to 31's with that same ratio , my down-the-road RPM moved UP to 2750 (max HP was rated @ 4400RPM), @ 62.5% of rated HP RPM, and I got up to 16 MPG even with that 3.73 ratio. Same axles, same road speed, same wind drag, same carb, same topography, same weight, LOWER RPM, gave me worse mileage with the big donuts. Why? Had to downshift bunches and that pulled both "air pump" rotation up AND the lower manifold vacuum causing the enrichment circuit to be open too much.

To HEtkind, a FI engine gives a different set of problems and solutions than a carbureted engine. For example an O2 sensor will tell the computer to lean out the mix, preventing that over-enrichment that low vacuum gives us with a carbureted engine. Ideally, we'd all switch to the injected 5.2 & 5.7 roller motors to up our mileage .

[hemithekid]

probably not, the motor will rev higher and work harder to get you to speed.

[MountainMoparRobin]

spend the $1400.00 and get overdrive: http://www.phoenixtransmission.com
If you didn't see my latest post driving at 75mph with 440/727 3:23's is 2800 rpm and 80 is 2950 or 3000rpms
my transmission came out of a car, Monaco with 2:95 gears so the tranny is geared low to have good take offs, works well with the 3:23's have hauled 1000 lbs and didn't even phase the 440

[HEtkind]

Every time gas gets too high, we start thinking about gas milage and numerically lower gears. But these are trucks, and they need to be able to haul and tow, and low gears won't do that for you!

Everything is a tradeoff, but in my book, a 3.21 is about as low as I want to go on a truck, it makes 6th gear on my 07 ram unusable at normal highway speeds or towing even a light trailer, and I am wishing I left the 3.90 gears in the town wagon instead of going to 3.55 gears, but then again the 3.55 was the flange axle and a sure grip!

But if the vehicle doesn't need to haul or tow, then use whatever gears work for you...and a 440 has enough torque where gearing probably won't matter too much. With a /6 motor and a 4,400 lb town wagon, gearing DOES count.

Howard

[Rodger]

Hola Class

Why spend big hard earned cash for an transmisson from Phoenix when the five speed manual transmission from any Dakota, Ram 1500 or the like from 1987 until this morning will fit to any 273-318-340 or 360 engine. The fith gear has Over-Drive.

Those below three ratio rear axle's are fine if you only go down-hill around here in an box shaped truck.

The 440 may have size to pull the below three rear axled truck's, but the engine needs to be cammed to "act right" at the correct hwy speeds. What is the fuel cost to have and 2.76 ( or the like ) ratio and an cam that starts pulling after 2,000 RPM's. ( ... but it pulls great if I keep it in "2". )

Rodger & Gabby
COS