gearing vs mpg

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skycuda67

skycuda67

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Columbida gorge, Oregon
so this summer im kicking around the idea of a cross country roadtrip to go see some family. Either need to fly or drive but i'd rather drive my barracuda. For being a 2500 mile trip(one way) I was wondering if it would be smart to swap in some taller gears to improve gas mileage just for the trip, right now i have 3.55's coming off a 318/727 combo. as is im running 3000rpm at 65mph. if i did somthing like 2.93's in the rear it'd cut me down to under 2500rpm's. has anyone tried this? better alternatives?
 
our 5th avenues do great with 2.26's. the smaller the better. 25 mpg HW with the 318/904's. our 70 RR 383 cruises around 65 mph at 27 - 2800 and its got 3.23's
 
2 things...

1st... With the gear change you also need to change the way you drive. The tendency is to use more throttle to maintain the same "feel". More throttle = more gas. You could easily wipe out any gains.

2nd... Do you have a 2.xx third member? If you need to buy one it will wipe out some of what you would save.
 
I have a 3.23 rear section that im trying to sell becasue I want a 4.10 posi
 
ya i realize it'll cost money to buy and the need for change in driving habits. but if i buy them once i'll have em, and im sure there will be more occaisions to use em while i own the car. know where i can find em for cheapest? done some searching and havent found any for sale yet :/
 
For Bullrun we swapped to a 2.73:1 rear end, and a 28" tall tire.... with the 833 4 speed.... we were getting 14mpg and we were cruising exactly at 70mph at 2000rpm
 
i put 2.94 gears in my dart when i put the stock 360 in the car. love them.. i actually get 16 mpg and can cruise very well now. if i was going to do 2500 miles i think i would change the gear to a 2.76.


DSCN0498.jpg
 
I'd recommend 2.76's or something around there. Cruising speed on the highway around here is about 80 mph and in my '70 Duster that's around 2700 RPM with those gears. I'd also recommend getting your ignition timing set just right (especially total+vacuum advance at cruising RPM) and the carb primary jetting a little on the lean side.
 
What was done to the engine? What torque convertor? What carb? And what ignition setup. be specific...
 
If you're interested, I have a set of coarse-splined 2.76 gears and yoke I've been trying to sell. They came out of a 742 case. PM if you'd like to know more.

Also, perhaps try to find a gauge that alerts you to how much vacuum you have while at different operating levels and tune with that. There's an older one called the "Motor Minder" by Stewart Warner. Not sure if anyone makes something like that anymore. They used them in mileage runs back in the '50's and '60's. See pic below.

aig5.jpg
 
Just remember, depending on the specs of the cam your running, your engine may now not be operate efficiently at the new lower RPM. Turning a bigger cam motor at a lower than optimum RPM means it's running at the very bottom of it's torque curve and requires more fuel to make power. Other than the possibility of a little less engine wear and a reduction in ambient noise, the saving in fuel may not be worth the hassle of the gear change. Your call. 8)

Steve / MOhorsePAR
 
MOhorsePAR said:
Just remember, depending on the specs of the cam your running, your engine may now not be operate efficiently at the new lower RPM. Turning a bigger cam motor at a lower than optimum RPM means it's running at the very bottom of it's torque curve and requires more fuel to make power. Other than the possibility of a little less engine wear and a reduction in ambient noise, the saving in fuel may not be worth the hassle of the gear change. Your call. 8)

Steve / MOhorsePAR
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That's a very good point. If your cam is too big to make torque at low cruising RPM's then you won't see much of an improvement by cruising at those lower speeds.

Vacuum gauge is also a great idea. In fact I have an AutoMeter 2-5/8" 0-30 in./Hg vacuum gauge in my car and it is very interesting to watch. You can really see the difference it makes in accelerating slowly and steadily vs. hammering the gas.
 
the cam im runnin is a 284/484 purple power range 2200-6400 so ya i cant really get too low there :/. like i said 2.93's will cut me down to 2500 and so 2.76's would take me down to...carry the one... about 2300...eeek
 
2.93's would probably be better then. That cam has quite a bit of duration, especially compared to the cam I'm putting in my 318 which only has 256/262* advertised duration.
 
i never even thought of this before for my 5 speed swap...im running the same purple shaft 284 duration/ 484 lift cam in my 340, and in 5th gear i was planning to cruise at 65mph at 2100 rpm (with 3.91 gears and 28 inch tall tire)...from what i understand, the overdrive will be useless here because my cam isnt in its power range and thus still sucking back lots of gas in order to constantly cruise at that rpm?
 
skycuda67 said:
the cam im runnin is a 284/484 purple power range 2200-6400 so ya i cant really get too low there :/. like i said 2.93's will cut me down to 2500 and so 2.76's would take me down to...carry the one... about 2300...eeek
Click to expand...

mshred said:
i never even thought of this before for my 5 speed swap...im running the same purple shaft 284 duration/ 484 lift cam in my 340, and in 5th gear i was planning to cruise at 65mph at 2100 rpm (with 3.91 gears and 28 inch tall tire)...from what i understand, the overdrive will be useless here because my cam isnt in its power range and thus still sucking back lots of gas in order to constantly cruise at that rpm?
Click to expand...



wheres that cam cleaning up for you guys? the 509/292 i was running didn't clean up until just after 2200rpm or so. i was running 3.91 gears. dropped it to 3.55 and it was just ok around town but better on the highway. was gonna do the 2.94's but did the math and it would have sucked around town. cool as hell on the highway but totally undriveable at lower speeds.
 
mshred said:
i never even thought of this before for my 5 speed swap...im running the same purple shaft 284 duration/ 484 lift cam in my 340, and in 5th gear i was planning to cruise at 65mph at 2100 rpm (with 3.91 gears and 28 inch tall tire)...from what i understand, the overdrive will be useless here because my cam isnt in its power range and thus still sucking back lots of gas in order to constantly cruise at that rpm?
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Where's the problem? You just need to drive faster when you're in high gear. :toothy10:
 
I like the 2.94 on the highway. I have a low gear set in the 904(ptc converter) also. Ran 12.8 et (360, 500 lift 230 dur. 9:1 comp) at the strip and freeway cruising is alot more tolerable than the 3.55's.
 
abodyjoe said:
wheres that cam cleaning up for you guys? the 509/292 i was running didn't clean up until just after 2200rpm or so. i was running 3.91 gears. dropped it to 3.55 and it was just ok around town but better on the highway. was gonna do the 2.94's but did the math and it would have sucked around town. cool as hell on the highway but totally undriveable at lower speeds.
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hey joe, i dont understand what you mean exactly by the cam cleaning up? i havent installed my motor yet, but that is the cam thats in it and i will be using it...i think the powerband starts at 2100 or 2300 rpm...I have 3.91's with a 28 inch tall tire, and the final drive ratio in my tranny is a .68
 
just doesn't run real smooth, its real choppy and doesn't really have any power below that rpm. with 3.91 gear and the 9 1/2 dynamic converter the car was great on the street. the 3.55 gears were ok but you could tell the difference when pulling away from a light. the 2.94 would have been fine on the highway because the rpm would have been fine. but around town with that cam with 2.94's it would have been miserable to drive.
 
so should i be okay then with the cam im using and the 3.91 gear around town and cruising in 5th at highway speeds?
 
i would think. i can't get to the calculator site that i usually use here at work. but yea if you use the 1 to 1 gear around town you'll love it. then on the highway the 5th gear should be ok. you should really be able to fly..lol. any idea what your rpm will be at say 65 mph?
 
I think the cam will determine it. If the engine is built properly, and if the cam comes in at 2200 (Which I have my doubts about), then the gearing has to keep the engine above that number. Not at it. That's where the power band starts. For decent milage, you need to be IN the range, not just barely touching it. This is one of the reasons to avoid the MP stuff. Was the cam degreed? I wouldnt go with anything higher than 3.23s knowing that cam is in there. You'll get better milage by turning the engine in it's efficiency range rather than just turning it slower and less efficient.
 
Hey Joe,
You can actually change that final gear ratio on there to a .68 (thats what mine is)...I figure in town i will be fine with the 1:1, its the highway im more concered about...at 65 mph i will be turning 2100rpm...how does the sound? and moper, the cam is not degreed...its been in there for a long time, im just going fire up the motor to make sure it runs (after basic inspection) and then stick it in the car...probably wont see another cam until a rebuild i suppose...With this cam you say you have doubts about it coming in at 2200..when do you actually think it hits? also, i understand that being in the efficiency range will net me better mileage, but what about guys who run real high horsepower cars on the highway...their powerbands kick in at around 3k lets say, but with an overdrive they are turning rpms 2500 and under (some alot more under that)...how do they get away with it? I'd rather not change my gears from a 3.91 to anything higher...i already 3.23's in teh pig right now, but i want the added oomph of the 3.91's that i have
 
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