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From 3.23 to 3.55 Hwy mileage drop?

Posted By: ragtop

From 3.23 to 3.55 Hwy mileage drop? - 05/22/11 08:39 PM

What percenct hwy. mileage drop going from a 3.23 rear to a 3.55?
The increase in RPM is exactly 10%. Then, will that translate to a 10% drop in hwy mileage? Or are there other factors that I am not aware of. Thanks.
Posted By: dOrk !

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/22/11 08:42 PM

IF you don't have a big-torque motor ... it could actually HELP mpg.
Posted By: ragtop

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/22/11 08:46 PM

Quote:

IF you don't have a big-torque motor ... it could actually HELP mpg.



Would that be because of the increased intake velocity from the higher RPM?
Posted By: dOrk !

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/22/11 08:50 PM

GOOD A/F speed(that helps atomization) is always a plus.

What is your combo ?
Posted By: gch

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/22/11 08:52 PM

If you are lugging the motor bringing it farther up into the torque band would increase the efficiency.Will it increase mileage
Maybe at lower 55mph but doubtful at 65-70.
Posted By: stumpy

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/22/11 09:04 PM

Here's something to play with to find what difference the gears will make. I really don't think it will be much. http://vexer.com/automotive-tools/speed-rpm-calculator
Posted By: 360view

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/23/11 12:42 PM

When my pinion bearing began to howl at 46,000 miles,
I swapped in a $75 set of NOS Chrysler 3.21 diff gears in place of stock ratio 3.55 in a 1995 Ram 360 Magnum 2wd 518 auto where the truck weight was 5400 lbs.

Highway mileage at 70 mph
over the exact same I40 trip from
Durham to Wilmington NC and back
went up by
1 MPG,
and stayed that way for multiple trips.

One other thing I was not expecting:
2nd gear of a 518 with 3.21 ratio diff
is actually much better
than 2nd gear with 3.55
especially when towing up long grades.
Posted By: ragtop

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/23/11 08:19 PM

Quote:

When my pinion bearing began to howl at 46,000 miles,
I swapped in a $75 set of NOS Chrysler 3.21 diff gears in place of stock ratio 3.55 in a 1995 Ram 360 Magnum 2wd 518 auto where the truck weight was 5400 lbs.

Highway mileage at 70 mph
over the exact same I40 trip from
Durham to Wilmington NC and back
went up by
1 MPG,

Thanks. What was your hwy mileage, just to see what percent, your 1 MPG is?
Posted By: TC@HP2

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/23/11 08:37 PM

Quote:

Or are there other factors that I am not aware of. Thanks.



Compression ratio and cam profile.

The volumetric efficiency point of an engine exactly matches the torque peak. If this torque peak occurs lower enough in the power band and you have a high enough compression ratio to squeeze it without detonation and this new higher rpm is closer to the torque peak, your mileage may go up.

eg; my 74 D200 gets better mileage at 75mph than at 55 mph. At 75 mph the cruise rpm is closer to the torque peak and it produces more useable power with less throttle, so the engine is working more efficiently.
Posted By: feets

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/23/11 08:47 PM

I really wish people would forget they every heard about torque peak.
That is measured at wide open throttle. Who cruises on the highway at WOT?

Due to the laws of physics and fluid dynamics things change quite a bit at part throttle. Port velocity, vacuum signal to the carb, and a host of other things all change at part throttle.
What happens at full throttle is completely different.
Posted By: forphorty

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/24/11 02:24 AM

Quote:

I really wish people would forget they every heard about torque peak.
That is measured at wide open throttle. Who cruises on the highway at WOT?

Due to the laws of physics and fluid dynamics things change quite a bit at part throttle. Port velocity, vacuum signal to the carb, and a host of other things all change at part throttle.
What happens at full throttle is completely different.

Posted By: BSB67

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/24/11 03:12 AM

An engine spinning at 3000 rpm at an a/r of 15:1 will burn more fuel than that same engine with the same a/r at 2500 cruising on a flat highway road. If you get the same or better mpg with a numerically higher gear, it's probably because you have too rich of an a/r in the transition or idle circuits. Torque curve and efficiency have little to do with it unless you're towing or driving in hilly condition. Why do you think today's cars have over drive transmissions? My late model van turns 2000 rpm on the freeway at 70 mph, no where near it peak torque.
Posted By: 360view

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/24/11 01:15 PM

Quote:


Thanks. What was your hwy mileage, just to see what percent, your 1 MPG is?




Depending on wind and air temperature,
after the 3.21 diff gears were installed
MPG at a steady 70 mph over this route
varied from 18.7 to 19.2

This was with 87 octane Exxon gasoline,
which back then did not have 10% ethanol in it.

The 2wd truck also had a ARE brand tonneau on the 6.5 foot long cargo box.

With a girl friend working at the Blockade Runner Hotel, and me living in Durham and going to graduate school during the week,
I did this trip frequently,
and also tested
running with the 1995 MP PCM engine controller on 93 octane,
running without the factory front air dam,
running without a radiator fan,
running without the air intake tube that goes from passenger side fender to air cleaner box so that the throttle bodies got hotter air,
running with/without KN washable drop in air filter,
running with 180, 195 and 205 degree F thermostats,
running with/without AC on,
running with aerodynamic vortex generators magnetically attached to rear of roof,
and a few other mods.

First main thing I learned:
wind direction and speed has more effect than most vehicle owners realize,
and every test run needs to be adjusted to take that into account if you are testing with a single vehicle.

Second main thing I learned:
Claude Travis,
known as 'Mr MPG' in the 18 wheel truck testing world,
knows a lot about how to properly test trucks for modifications, and incorporated his wisdom into the TMC/SAE Type IV test procedure.

http://www.landlinemag.com/Archives/2004/May2004/Bottom_Line/Does_it_work.htm
Posted By: TC@HP2

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/24/11 04:12 PM

Quote:

I really wish people would forget they every heard about torque peak.
That is measured at wide open throttle. Who cruises on the highway at WOT?

Due to the laws of physics and fluid dynamics things change quite a bit at part throttle. Port velocity, vacuum signal to the carb, and a host of other things all change at part throttle.
What happens at full throttle is completely different.



Yes, things change considerably, however, to totally dismiss the power curves of an engine is akin to just taking shots in the dark. The power curves at least give a frame of reference to start with. Time on a chassis dyno, or seat tie on extended drives, can then be used to refine the results, if desired. I also think that since the stock mopar big block heads were undersized to feed a 440 from the get go, stock heads are going to be producing high velocity at lower rpm levels than most aftermarket or other make heads.

Quote:

Torque curve and efficiency have little to do with it unless you're towing or driving in hilly condition. Why do you think today's cars have over drive transmissions? My late model van turns 2000 rpm on the freeway at 70 mph, no where near it peak torque.



My reply stated my experience was with a truck, and since I live in Colorado, all my usage with towing is in hilly conditions. Some obviously bigger than others. I also had the chance to test out this operation on a trip from Denver to Phoenix and back. Not exactly flat there either.

I also think we would all agree that factory epa mileage estimates, warranty longevity vs customer claims, and driver perceptions mean that the OEMs may very well set cars up to cruise in a method that is not entirely optimal. Does that mean cruise rpm should all be cranked back up to 3500rpm by the OEMs, no. But perhaps engines should be built to produce more torque lower in the power band. To the average car buyer, torque means nothing. Except for heavier duty truck marketing (3/4 ton+), nearly all advertisements talk horsepower and/or mileage.

Posted By: 360view

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/25/11 12:08 PM

"Power Band"
Run near the peak torque rpm,
higher diff gears 'so the engine does not work as hard'
.... are all signs that our high schools don't teach a subject (engines) that many high school students yearn to know.

There are supposed to be 'gateway drugs'

Learning about engines is a great 'gateway drug' to learning science.

They say a picture is worth a thousand words,
and if we could somehow teach a few million teenage boys what the picture in the attachment has buried inside,
my bet would be that it would drop the gasoline fuel consumption of the USA at least a few tenths of a MPG.... which is billions of dollars.

Attached picture 6649869-BigBSFCvsPistonSpeed.jpg
Posted By: Dodgem

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/25/11 03:33 PM

You will hardly notice the change. the main factors determining highway mileage are weight, rolling resistance and air resistnce. none of these change at 70 mph your rpm changes slightly but with the increased gear mechanical advantage increases therefor less cylinder pressure is needed to maintain 70 mph. But it is spinning slightly faster but you will not need to give it 10% more throttle to get there.

Your in town mileage will not change to any degree at all.

Posted By: rtplumcrazy1

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/25/11 06:15 PM

back when i had my 340 Duster i swiched from 3:23 to 3:55-and noticed no diference in mileage-UNLESS i drove the car harder. Then the mileage dropped about 1.5 miles per gallon. My 340 engine was prety much stock-except for headers-carb-and intake manifold. If i had it to do again-i would not change from the 3:23 gears-car ran aout the same with both sets of gears. But when i went to 3:91 gears-big drop in mileage.
Posted By: Kern Dog

Re: From 3.23 to 3.55 Hwy mileage drop? - 05/25/11 08:22 PM

Nice link, Stumpy.... Thank you!
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