What is the best Rear end ratio for Fuel economy?

[ChryslerCruiser]

318, auto, interstate driving, road trip vehicle.. 70 MPH absolute max speed..

I have a 2.2 rear gear right now. Would a 2.7 or 3.23 be better rear gear?

Would it be the same answer for a 360?

 

[Duke5A]

For the factory setup? You've got the best ratio for highway driving. Final should be in the low 2's with the off-idle torque curve of the factory cam'd 318. If you wanted to increase city mileage without sacrificing highway an OD transmission like the 500 and rear axle with a 3.23 is where you want to be.

 

[Lightning II]

But........not all of us have Overdrive though. :C

 

[Camtron]

the gearing you have combined with a solid ignition system and well tuned carb is going to be about all you can do for MPG performance on the stock set up.

 

[69-]

Yes, keep the 2.2 + properly tuned, you wont get better. A 3.23 w/o OD will give you like 3k+ RPM at 70mph, hence more gas, as more RPM...

And keep yourself light footed

 

[ChryslerCruiser]

Thanks Guys. I remember hearing years ago about the mustang GT guys going to a higher rear gear, looking for better street/strip performance, and got better fuel economy in the process. I did not pay any attention to the specifics, because I did not have a RWD V8 powered car at the time.... and at the time I never understood why the factory did not gear for best acceleration if it came with better fuel economy as well.. So that is where the question came from..

 

[Camtron]

Maybe with a 4-5 speed and/or overdrive. Less time spent under acceleration, more time spent at cruising speeds in the higher gears and/or OD. My trans has a lock up converter that drops cruising RPMs noticeably in 3rd gear. If you have a non lock up trans, that could be a worth while swap since you’re looking to use the car for a highway cruiser; then again, you may already have a lock up trans, I don’t know. But, if you were to do that, it’s worth it to go a step further and do that, A500 swap Duke mentioned...or just keep her cherry and stock.

 

[ChryslerCruiser]

A904, which I believe has lock up.

 

[BudW]

Chrysler came out with lockup mid-year 1978 (across the board). All but special order cars built after that date came with lockup.
BudW

 

[69-]

Except Trucks, I assume (79 MB400 TF727 has no lockup).

 

[BudW]

I think 1 ton trucks didn't get lockup for a few years afterwards, as well. The 1/2 ton pickups did get lockup, though, in mid-year '78.

 

[Mikes5thAve]

I think lock up in trucks also depended on application and could also be ordered without it. I've seen quite a few trucks into the 90s that had non lockup on the options sticker.

 

[Aspen500]

My '81 D150 with a 318 didn't have lock up. If it was the original trans though, I can't say for sure. My '78 Cordoba 360 did have lock up and was annoying around 35 mph. Lock, unlock, lock, unlock.................It got kind of old after a while, lol. Come to think of it, my '96 Dakota does the exact same thing, AAAARRRRRRGHH

 

[Lightning II]

If only there was a way to slave it to the Cruise Control switch.

 

[AJ/FormS]

Would it be the same answer for a 360?

Yes and no
Generally
the bigger the bore, the more fuel it will burn.
But not always.
The secrets to fuel economy start with;
a lightweight free-rolling chassis, Engine locked to driveshaft at cruising speed.
Then the smallest engine with the fewest number of cylinders , running efficiently, at the slowest possible rpm that will still cruise at the target roadspeed.
That's a big ideal.
And cannot normally be achieved, Normally Aspirated, because said engine still has to overtake other slower cars, climb hills, and actually get up to that cruising speed in a reasonable period of time and distance.

A good target to do all those things is about one cubic inch per ten pounds of loaded chassis. So that means at 3600 pounds, the 360 is a good choice. The 318 comes in at 11.3 pounds per cube, and is better when fast acceleration is not important. The Slanty is 16 pounds per cube, and struggles in all categories except actually cruising.

Once the engine size is chosen, then you have to wade thru all the parameters to bias the build in whatever direction you want to take it. And yes, it is possible to custom build a 360 that in point to point fuel economy, can totally obliterate a factory stock 318 , or even a factory stock 225.
And the reason is because those older factory stock engines were severely handicapped by government regulations to never achieve their true potential. And that is an on-going political battle.
Having said that
when fuel-economy is the first priority
then the best idea is still to start with the smallest sized pistons, in a custom built engine. The slanty is an ideal candidate having the right architecture to get it done. Small bore pistons, a long stroke, and low-rpm torque capability, that cannot be matched by any other Mopar factory engine. The problem is all in how the factory absolutely killed it with the tune. The low compression, the retarded ignition timing, the EGR camshaft, and the less than stellar carburation, all conspire against serious economy in stock form.
But those are all things that YOU can change.

But why bother when you can just drop an EFI 5.2Magnum in there, for way less money. That will instantly match or better whatever you can get out of the 225, and makes double the power........................ that design is pretty hard to beat as an all-round powerplant.
But if you sacrifice the EFI and that "keg" intake, then you are left with a basic, "hi-compression" 318.
So then if you already have an LA318, then you might as well start by building it up

 

[M_Body_Coupe]

Hmm...is this a pure fuel-economy goal, or are you looking for a little bit of that "gitty up and go" as well?

I remember going to a station wagon 2.94 (??? I think) ratio from the factory 2.4x (this one I can't remember for sure). It was a significant difference in terms of how 'faster' the car felt. Mind you, no rocket by any means, but the motor was just that much happier and certainly picked up the pace quicker with very little change in fuel economy!

That was a 318 engine with a cam slightly hotter than the stock 340 cams and the 904 2.74 1st gear wide-ratio setup, so that helped the off-the-line feel.

Anyways, my point being: the motor has to work harder at lower RPMs to pull/accelerate same vehicle mass. The numerically higher ratio allows the engine to labour a tad less, and we all know that ICE are simply more efficient at higher RPMs anyways. So in theory at least there is that sweet-spot of an opportunity to go faster and use less fuel getting there.

 

[ChryslerCruiser]

Hmm...is this a pure fuel-economy goal, or are you looking for a little bit of that "gitty up and go" as well?

I remember going to a station wagon 2.94 (??? I think) ratio from the factory 2.4x (this one I can't remember for sure). It was a significant difference in terms of how 'faster' the car felt. Mind you, no rocket by any means, but the motor was just that much happier and certainly picked up the pace quicker with very little change in fuel economy!

That was a 318 engine with a cam slightly hotter than the stock 340 cams and the 904 2.74 1st gear wide-ratio setup, so that helped the off-the-line feel.

Anyways, my point being: the motor has to work harder at lower RPMs to pull/accelerate same vehicle mass. The numerically higher ratio allows the engine to labour a tad less, and we all know that ICE are simply more efficient at higher RPMs anyways. So in theory at least there is that sweet-spot of an opportunity to go faster and use less fuel getting there.

I am thinking you have a very valid and relevant point.. Which helps explain what I mentioned in an earlier post and towards what the original question was about. I remember reading 20ish year ago that the mustang guys were swapping in lower gears to help in the acceleration department.. and they discovered a fuel economy benefit as a result. Because lower gears allows the engine's average effort to be in the happy place more of the time... Yet overdrive transmissions allowed the engine to operate at a relaxed RPM on the interstate.

Maybe if one was to drive with a vacuum gauge, and 100% focus on economy, the highway gears (904 transmission) could do "OK". Just as the factory intend when they set them up for the cooperate fuel economy rating... BUT when driven in mixed driving, and trying to merge on the on ramp with out being creamed by those traveling at 80MPH, the average goes down the drain.

Gear up the rear end, and let the engine operate in it's happy place more of the time, rather than laboring. Maybe upgrading the cam a bit to have the "happy place" of the operating range coincide with the cruise RPM.. That is to say have the cruise RPM be slightly below the torque peak RPM of the engine.. so one is in a high vacuum state more of the time..

 

[M_Body_Coupe]

...Maybe if one was to drive with a vacuum gauge, and 100% focus on economy, the highway gears (904 transmission) could do "OK". Just as the factory intend when they set them up for the cooperate fuel economy rating...

Yes, but keep in mind that the wider 904 gear is a 1st gear only situation. The 2nd and 3rd gears have exactly the same ratios as the other transmissions (3 speed autos that is). So for help with everything else you need a different rear-end ratio.

You are right though about the vacum gauge approach. In fact even on my somewhat rugged edge coupe I still tune for cruise using the vacum gauge (and O2 sensor and data logging). That is primarily about getting the max advance pulled in so I can get every little bit of that fuel economy I can chase after. Umm, it is a 4k stall coverter with a 4.10 gear, so fuel economy may sound a little funny, but hey, higher-vacum = higher-efficiency = better fuel economy!

...Gear up the rear end, and let the engine operate in it's happy place more of the time, rather than laboring. Maybe upgrading the cam a bit to have the "happy place" of the operating range coincide with the cruise RPM.. That is to say have the cruise RPM be slightly below the torque peak RPM of the engine.. so one is in a high vacuum state more of the time..

Ha ha...now it's getting into the dangerous "hot rod" teritorry though, right? We all love to tweak things, but keep your goals in mind, which is why I asked that orginal question: fuel economy or???

 

[ChryslerCruiser]

Ha ha...now it's getting into the dangerous "hot rod" teritorry thoug, right? We all love to tweak things, but keep your goals in mind, which is why I asked that orginal question: fuel economy or???

Yeah, "beware of the demon tweek"

I suspect based upon what you have stated above that a 3:1 rear ratio would generally do better than a 2:2 rear, b/c the engine is not lugging as frequently.. but the factory needed to pass CAFE tests, and so geared the cars towards that... Real world operation is frequently different, except when driving with the vacuum gauge, and how often does that happen other than the occasional hot rodder who wants the best of both worlds?

My only comment about camming up the engine is based upon the perception that Max fuel economy is found at or below peak torque RPM, and pure fuel economy suffers increasingly as the engine is operated as speeds above peak torque... so camming the engine so it is at, or below peak torque RPM at cruise, rather than being above peak torque when on the interstate.. would produce better BSHP #'s and thus better fuel economy...

 

[AJ/FormS]

318, auto, interstate driving, road trip vehicle.. 70 MPH absolute max speed..

I have a 2.2 rear gear right now. Would a 2.7 or 3.23 be better rear gear?

Would it be the same answer for a 360?

Ima guessing your 2.2 equipped 318 might have say 205/75-15 tires on it, which have a roll out of about 84.8 inches. Then 70= about 1918 in loc-up mode.
For straight hiway use, this is a pretty hard to beat starting point.
How much fuel economy it gets will depend on the current state of tune. Or maybe how good a tuner you are.


Tuning with a vacuum gauge is as good as hopeless. Rather, it takes forever to see results.And you will never be 100% sure if it's better or not.
What you need an on-board adjustable timing control.
Then get her up to temperature and speed on a hard, flat, level hiway, and run her for a mile or two. Then retard the timing 3 degrees and watching the speed-O, wait. If the car loses speed, then whack the 3 degrees back in, and watching the speed-O, wait. If the speed recovers, whack 3 more degrees in, and watching the Speed-O, wait. If the car picks up speed, reduce your speed back to the chosen cruise-speed, and let her settle there for a mile or two. If you have a vacuum gauge installed, note the difference in vacuum, from the starting point to the new, more closed, throttle position. I bet you cannot see a difference. But if you do, it's only because you saw it move just a tiny bit. Had you not seen it, you would never have known that it moved.
Ok so keep driving at the new throttle position, at the chosen roadspeed and let the engine settle into it. Then whack 3 more degrees into it, and wait. If the car slows down, retard it 3 degrees to the previous number and call it done. But if the car picked up speed, reduce the throttle back to the chosen roadspeed and drive again, until the car is settled, then whack 3 more degrees of advance into it, and wait. Repeat, as many times as necessary, until the car slows down, then back up the timing 3 degrees and call it done.
Now, you need a tach to see at what rpm your chosen roadspeed occured at.
Say it was 1900@70 mph
Stop the car.
Rev it up to 1900 and read the timing. What you read there, is your best cruise timing for 70mph @1900rpm. Now you just gotta make it happen. Butum, if your chosen roadspeed is not 70@1900, then use the numbers you recorded.
Butum,
if you change ANY parameter; like engine temp, inlet air temp, elevation, cam-timing, compression ratio, I mean any parameter; then you gotta start over. So leave that timing control installed.
In this way, you can determine your best cruise timing in a matter of a couple of hours, instead of weeks and months.

To make this go even faster, before you even start;
Just rev it up to 1900 and set the (cruise) timing to 45 degrees advanced with the vacuum advance can fully advanced. Then go cruise at 70mph and work the Dash-mounted, dial-back, timing retard box, as already described. Maybe you'll get it in two or three steps.

Now you can start to work on the carb..........
and after you get the cruise circuit leaned out, then you gotta redo the timing again.
Pretty soon, she will have the cruise tune to 85/90% of perfection, maybe you'll stop there...... calling it good enough.