Hey guys I want to get everyone's opinion on this one. I have a '69 RR that I am currently restoring and I am trying to decide what gear ratio to use in the rear end (currently a 3.23). I want to be able to use it on the street for cruising, but want to be able to light up the tires when I feel the need. The obvious choices are 3.55, 3.73, or 3.91 for the set up but I have only driven a 3.91 other than my 3.23. I am afraid a 3.91 might be too low and give me too high of rpm's with whining going down the highway. SO I am thinking that narrows it down to 3.55 or 3.73. Any opinions on which is better or is there even a noticeable difference between the two? Thanks for helping out!!

Whats wrong with the 3:23? I had that in my cuda and
it would light the tires up just fine plus would top
it out at around 140 MPH.

I have a 2:76 in my dart thats a 4 speed that roasts the tires off. Sell me your 3:23 if you don't like it.

3.55.

roadrunners where light to light terrors.. NOT hiway cruizers.. so stick some 4.30's in there.

Again,

I'll take the 3:23's off your hands.

I'm gonna go against the grain here. I run 3.91's in my Satellite, and love them. But, My car is used around town only. It rarely sees freeway miles, and when it does, I'll admit its not exactly the hot ticket! If you do any freeway miles, I'd go with 3.73's, just so you can peek at 60 MPH below 3g. If its just a Saturday Night Special, and you're in it for the fun factor.....3.91's are fun. Heck... 4:10's!!!

I have a 2.76 in my 318 cuda and it will lite em up real good. It is great on the highway unless I plan to break a speed limit at all, then I wish I had even higher. I would use the 3.23 or higher for cruising and especially considering MPG these days.

8-3/4 is a pretty quick swap, enough practice and you can change gears in 20 min. I have 3.91 in my runner and like it, but change to 2.76 for the highway. If I had 4.88 I would probably use that for bursts about town. I do like 3.55 as an all around gear.

Also depends on what size tires you have in the rear. I like my 3:55's with a 28" tire. With a 28" tire I also would not be afraid of cruising the highway with 3:73's either. I had 3:91's for a couple years and it was fun, but the highway was a bit much for trips over 1.5 hours. I'm not a sissy, but the rpm's and noise get annoying.

depends on how much highway driving you are going to do. What are the engine specs..and on and on. 3.91's are a good gear for a b-body. With a set of 275/60/15's you can cruise at 70 no problem. 3.55's are ok but they are dog gears compared to 3.91's...as are 3.23's. Try the 3.73's they are pretty new to mopars. Let us know how they run.

i drove with 4.30's on the hiway with a 28" tall tire at 60-65mph.. wasn't all that bad.

yeah 3.91's and even 4.10's aren't THAT big of a deal. I often drove 50+ miles one way to race w/ 3.91's and 4.10's. When you build one of these cars MPGs should be the last thing on your mind. Choose the gear that will BEST work with you set-up.

With a short tire (something less than 27-28") I would take the 3.55. 28 and above, I would opt for the 3.73.
But this is a question of preference, there really is no right or wrong answer.

Not to high jack but would a swap to a 3.73 or 3.91 from a 3.23 be noticable and how much will it affect the top end?

3.55 (or 3.54 in danas) are the best all around ratio for a street driven Mopar. Especially with the price of fuel. Any lower (higher numerically) is just crazy.

355's in my 69 roadrunner is just right

Again, I would love a 3:23 in a 742 if anyone wants to part with one.

if you plan on doing any freeway driving, I'd go 2.94's or 3.23's w/28" tires.

my car with 3.55's and 27" tires was turning 3k RPM at 70mph without the OD tranny, and it got tiring after about 30 minutes at that speed....now I cruise at ~2200 RPM at 70 with my A500...

Someone on this post made this calculator which is quite cool I might add. Will tell you exactly what you will get.

http://vexer.com/68rt/speed.html

When put behind 600lbft. of torque 3:55's are NOT a dog gear.

because lots of street drivers have 600 lb-ft of torque on tap, right!????

I reccommend 355 with a tire size of 25 or 26 inches...that roadrunner will accept some meaty shoes and what I would do is run a drag/street tire of 30 inches hieght and run 391 or 410.Either way you will light them and have a very nice top end

My "street driver" has 600 ftlbs.

YES

Sorry caps lock was on, yes

My friends car has 800 horsepower and one of those 318 HEMI six-pack super commandoes.

3.23 here w/ a 26" tall tire

16 mpg @75 mpg turning 3400 rpm AND 12.56 @106 in the quarter...

best of both worlds for me!

Quote:

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what I would do is run a drag/street tire of 30 inches hieght and run 391 or 410.

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There's about 3 rpms difference at 70 mph when comparing 26" tires with 3.55's and 30" tires with 4.10's (this happens to be the combo on the GTX) Here's a decent online calculator which makes it easy to compare options...

http://golenengineservice.com/calc/calcrpm.htm

I run 4.30's with a 29" Hoosier QTP tire and I love it. I drive the freeway at 60 mph and turn about 3200 rpm. It dont bother me at all and yes I dont have to go any faster then 60 as I just run the slow lane when I need to. I have driven 120 miles one day all on the freeway to a show. But the 4.30's more then make up for it at low speeds and at the track. I love a hard launch and good low end. I will put up with 3200 rpm on the freeway to have my 4.30's as I did it all the time back in the 70's. Ron

amen to that Ron.

I currently run 3.91. I also have a 4.30 pumpkin. If your big block is putting out good tourqe you do not need a lot of gear. With a good tourqe converter 2800 or so, 2.94 and or 3.23 are great gears. I ran a 14.65 @ 96mph with 2.94, 11 inch converter. The 3.91 gears just get me down the track quicker 14.018 @ 95mph.

Tooling down the highway turning 3200 is actually fun. It's probably not so much fun on an extended road trip but for a weekend cruiser...

3.73's in my 70 Hemi Challenger w/ 275/60x15's & a 4 speed and just did a 1300 mile cruise. 70-72 mph at 3500 rpm gets old fast! And trying to "go w/ the flo" at 75-80 means cruising at 4000+ RPM. Go to pass someone & you're at 5000 RPM. Too much wear on the motor, IMO.

I may switch to 3.23's as did my friend w/ his T/A. He likes the hwy ratio much better than the 3.91's & its got great acceleration, too. I tend to agree. At 70 in 4th gear, I was busting the tires loose at sea level w/ the Hemi & 3.73's & think the motor's got enought torque to smoke the tires w/ 3.23's.
I'd think your choice depends on the power band & torque curve of your motor. But if its a street car w/ only an occassional street race, go w/ the 3.55's. That way you can enjoy the freeway as well as a burnout or two.

(here's an attempt to figure out gear ratio for highway cruise in a scientific way, based on piston speed and intake manifold vacuum. comments welcome)
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Now lets discuss gearing and fuel economy.

In city driving gearing doesn't matter nearly as much as not having a
'heavy foot' or hauling a lot of weight in the bed of the truck.

A vehicle that is either overgeared or undergeared will lose MPG at highway
cruise.

An engine is numerically overgeared if it is not cruising at about
60-75% open throttle.

An engine is undergeared and "too weak for the job"
if it has to operate with its pistons going faster than an average of about
1200 feet per minute when at 75% throttle.

Note that you could reduce engine weakness by turbocharging
as well as the much more common thought to just make the engine larger in
cubic inches. This is the reason nearly all diesels have become turbocharged
in the last 20 years.

How do you calculate this average piston speed?

Multiply the stroke of the engine in inches by two (because the piston goes
both up and down on one rpm) and then divide by 12 to convert inches to
feet. Multiply this by the rpm.

So where does 1200 feet per minute end up for various engines?
Here some examples:

For the 3.58 stroke of 3.7, 5.7, 5.9 Dodge engines:
2011 rpm = 1200 ft/min /( 2 x 3.58 inches/12 inches per ft)

For the 3.405 stroke of the Dodge 4.7V8
2114 rpm = 1200/(2x3.405/12)

For the 3.00 stroke of the Ford 5.0L V8
2400 rpm = 1200/(2x3.00/12)

For the 4.72 stroke of the Cummins Inline 6 diesel
1525 rpm = 1200/(2x4.72/12)

Is it possible for an engine to be made to get its best fuel economy at a
piston speed above the typical 1200 feet per minute?
Yes.
Very thin piston rings, using less than the normal 3 rings,
slippery coatings on piston skirts, extremely hard but slick coatings
on bore walls like "NikoSil" and keeping the bore walls very hot
so that the oil there will be thin and less viscous can all allow the
'best economy' piston speed to be raised but even 'state of the art'
giant marine diesel where nearly every trick is used seldom exceed
1500 feet per minute.

Honda has announced that over the next 5 year period that the major
part of their engine research $ will be spent on ways to reduce internal
engine friction, and this is from a company that NASCAR engine builders
already admit has the best rod and crankshaft bearing material for sale.

Which is more important: having the rpms near this 1200 foot per minute
speed for the pistons, or having the throttle in the 65-75% open range?
Answer: it is more important to be in the 70% throttle range and you should
slow down the rpms to get there. There is not much change in efficiency
when piston speed drops from 1200 down to 800 ft/minute but there is a big
change in efficiency when the throttle goes from 70% open to 40% open.

Why?

Imagine that you have a disassembled engine in front of you. Put a loose
piston in a bore and pull it down against the friction of the rings. It won't
be very hard.

Now imagine that same piston in the bore has the top of the
bore sealed off and has a vacuum sucked of 12 inches of Mercury.
12 inches of vacuum is about 6 psi of negative pressure. If the piston is
4.00 inches in diameter it has an area of about 12.5 square inches
(remember pi R squared but most cherry pies are round?)
6 pounds per square inch times 12.5 square inches = 75 pounds would be
required to pull the piston down - you probably couldn't do it with the grip
of just a couple fingers! An engine running with a high vacuum in the intake
manifold has to do just this, and for eight cylinders!

Engineers refer to this as one of the two parts of "Pumping Losses". The
other part of Pumping Losses is due to pushing the exhaust out the tailpipe.
Most engine owners easily imagine that exhaust loss, accept it, and spend
a lot of time and money on mufflers, headers, Y pipes etc. These same
engine owners would do well to remember the "Pumping Loss" on the intake side
and try to keep it low during highway cruise conditions.

{Note to engineers: Yea, I know, 'negative pressure' does not really exist and
what actually happened is 14.7 psi of air pressure is pushing the underside
of the piston and there was just less pressure on the top of the piston to
create a delta P - but the general public thinks in 'vacuum' terms.
And it is not just the general public - talk to the biologists who think
-100 psi or more of 'negative' pressure pulls water up capillaries to tree
tops}

Setting Gear Ratios to suit the Customer's Expectations

Most auto manufacturers intentionally overgear their vehicles because they
know from experience that customers will bitterly complain about a vehicle
that downshifts to climb a hill.

Engineers know that to give their customers the best possible highway MPG
the transmission would downshift on EVERY hill, but the sales department
reminds them that that every employee at the automaker (including engineers)
will lose their jobs if customers {even ignorant ones} get mad
and buy someone else's vehicle that meets their expectation.

So vehicles are overgeared to 'feel strong' during the 20% of the time they
are climbing hills in Overdrive, even if it means this hurts MPG a bit during
the 80% of the time the vehicle is on level roads or going downhill.

An engine is numerically undergeared if it is trying to cruise in the
'Power Enrichment' zone above about 80% throtttle opening where the
PCM computer greatly enriches the air to fuel ratio from 14.7 to 11.
On most vehicles with automatic transmissions it is not possible to cruise
in OD on level highways in the power enrichment zone because the
transmission will downshift to a lower gear long before the accelerator
is pressed that far.

For the same reason anyone who presses the accelerator down all the way near
the floor whether they are driving in the city, highway, climbing a hill, or
pulling a trailer is going to get about 25% worse MPG than someone who
presses right up to about 70% throttle but never goes over it.

This is why the old advice to install a vacuum gauge
can certainly help MPG if the driver watches it out of the corner of his eye.

When the vacuum gauge reads about 1 to 6 inches of mercury
you are in the fuel guzzling 'Power Enrichment' zone that you want to avoid
except when you want the highest horsepower and accept the higher fuel usage.

When the vacuum gauge is showing about 7 to 9 inches of mercury vacuum
your engine is converting fuel into horsepower at its most efficient level.

Each engine goes into Power Enrichment at a slightly different MAP sensor
level, but if you listen closely to the engine sound you can hear it happen
as the engine gets a bit smoother with the very rich mixture.