[Brad_bb]

I've been working directly with another forum member the last few days. It originally
started when I decided to sell an original 64 posi unit that had 3.90 gears. In talking
to him, it was clear that we needed to discuss the variables and way to determine
what rear end ratio he needed to choose. (the reason for my earlier post -
http://forums.maxperformanceinc.com/...d.php?t=748283 ).
In truth, I needed to got through it myself for my own 64. Here is what we came up with.

The purpose of the following data calculation is to compare an original M20
in a 64 GTO to a newer overdrive Tremec transmission, while evaluating a
number of different ring gear ratios. You can also use this data to match or adjust
1st gear performance, if that is what is desired, while seeing what your RPMs would
run at highway speeds. These calculations will help drive the decision
towards which transmission to choose, and which ring gear ratio to choose. It is
important to note that Tire diameter choice is important and you want to come as
close as possible to the true tire diameter. If possible, determine the actual tire
that will be used, so that you have that variable(diameter) locked in when creating
the data.

(rpm x tire diameter in inches) / (Transmission gearx rear end gear x 336)= MPH

Example: 2200 RPM, 28 inch tire, 2.56 first gear, 3.90 rear end

(2200x28inches)/ (2.56x3.90x336) = (61,600)/(3354.624)= 18.36 MPH

For the data generated below, I am assuming 5000 RPM as the hard acceleration
shift point from 1st to 2nd gear. For the last gear for the M20 and Tremec, I'm
assuming a cruising rpm of 2000. I chose a tire diameter that is typical among early
GTO's and will keep it constant for the comparison. Of course if you choose
the tire you are actually going to run, you would use it's correct diamter in a new set
of calculations to help you choose the ring and pinon.

FYI, you can change the RPM number and it will calculate the speed. So if you want to
find out what RPM it would take to run 70 MPH in last gear, change the rpm until you
get the desired speed.


*I have these calculations as an excel spreadsheet so that it's easy to change variable and
recalculate.

Looking at this data you can see that a Tremec with a 3.23 gear is going to give nearly
the same performance in 1st gear as the M20 with a 3:55 gear. This we know from
Pontiac experience is a good choice for 1st gear. At the same time the Tremec(.64) will
deliver 82 MPH at 2000 RPM in 5th gear. The Pontiac Motor can probably handle that,
but it is a bit long. The Tremec .87 overdrive might be more optimal, but I'm not positive
that is available for this application without asking Tremec. With the .87, you'd be running
2350 RPMs to cruise at 70 MPH. That is pretty good.

The Tremec with 3.55 ring and pinon and the .87 overdrive gear would be even better
to launch, and highway speed would still be decent at 2500 RPM for 70 MPH.

If you use the M20 and 3.23 gears, you'd have to get your RPMs up more to start out in
1st gear. In 4th gear you'd have to run at 2700 RPM to cruise at 70MPH. That's a bit
higher RPM than I'd want for cruising.

If you use the M20 and 3:55 gears, you'd have a much nicer 1st gear launch. But you'd
have to run almost 3000 RPM to cruise at 70 MPH.

If you really want to launch, like to race, the 3.90 would be even better, but your
highway speed would suck, running 3250 RPM to hit 70 MPH.

Note: these conclusions could change if the actual tire size changes significantly. This
is just a sample set of data. I would not make a decision until I had the actual tire size
to use in the calculation.


How to calculate Tire diameter from
http://www.tirerack.com/tires/tirete...e.jsp?techid=7

Example - 275/60-15



275= the section width in millimeters when mounted on the tire manufacturer
recommended width wheel.
60= aspect ratio(actually .60), which is the tire's aspect ratio or profile. This is a ratio of sidewall
height to section width.

Section Height = Section width x aspect ratio = 275 x.60 = 165mm
converting to inches = 165/25.4 = 6.496 inches

Total tire diameter = Section height in inches x 2 plus wheel diameter in inches
= (6.496 x 2) + 15
= 27.992 inches

WARNING:*These calculations (and those provided by all online tire dimension calculators) only
reflect nominal tire dimensions, which are often subtly or substantially different than the tire's
actual physical size and/or the dimensions provided by the tire manufactures.

Drivers working with close fitment tolerances or trying to maintain the original tire diameter of
staggered size fitments (similarities or differences) should use the tire manufacturer's
specifications for the exact tire(s) they are considering.

Actual Tire Section Width is Dependent on Wheel Width

All tire sizes are assigned specific rim widths upon which they are measured (measuring rim), as
well as can often be mounted on slightly narrower or wider wheels (rim width range). Therefore,
it is important to note that actual tire section width will depend on the wheel width the tire is
mounted on. The rule of thumb is that tire section width changes by 0.2" for every 0.5" change in
rim width, being reduced if mounted on narrower then measuring wheel and increased when
mounted on wider wheels.

[elefantrider]

Decide first what tires and trans you want to run, then gear accordingly.

I wouldn't put a Tremec in my GTO if you gave it to me but that's just me.

M20 + 3.73 or 3.90 rear gears + GV overdrive to bring final drive down to 2.91 to 3.08.
Best of both worlds and you can keep your current 3.90 installed.

[Brad_bb]

I was not aware of the Gear Vendors Overdrive. What surprises me is that it's listed at $2895 on their website, several hundred dollars more than a Tremec- an entire transmission!

One downside to the Tremec is that you'd have to modify your trans tunnel(body) to fit. Would the GV Overdrive unit require any such modification?

There's some interesting reading about the GV Overdrive on their website.
http://www.gearvendors.com/hrgm4sm.html

[tom s]

Although IMO they dont shift as nice look into the Richmond 5 speed OD trans,dont believe you have to mod your tunnel with it.Also if you car is not a super rare numbers matching special car the tunnel mod is really not that big of deal.The TKO was and is the best mod that I have EVER done to my cars in the 50 some years I have been playing in this pontiac hobby.If mine I would start with a TKO 500 3.27 first and a 3.55 in the back and the .67 5th.Tom

[dld]

I have a few comments to add.

you may want to chassis dyno the car to get a base line on the Torque the motor makes . I' thinking a new style 389 with low compression my not be near factory spec. your optima 2000 cruise may be a little short.

For comparison first gen CTS v with 400 lbs torque and a tremec with ratios of
2.97:1 2.07:1 1.43:1 1.00:1 0.84:1 0.56:1
a 3.73 stock gear and 1" shorter tire than you example . hardly can handle 6th gear any slower 70 . (Cruise gear only) car is also near the same weight as 64 65 GTO but more aero than the Goat.

admitted first gear is a little low in the V . you can use 2nd for dead stop takeoffs with no problem. I generally use 5th more than 6th at highway driving. But i'm a little aggressive on the highway. LOL

Another thought is the lower rear gear the less load on drivetrain in any gear. a car with a lower rear gear will also out pull a car with a higher gear. even when matched combined final ratio.

[Brad_bb]

There's a lot to think about, advantages and disadvantages. Tom S., I'm not afraid of a tunnel mod. I would only have to repaint/rework what I damage in the modification (the underside of my floor is bodyworked really nice and painted semigloss black).

I updated my spreadsheet to include the GV overdrive final gear if attached to a Muncie (1:1 trans gear X .78:1 overdrive rear = .78). I also adjusted the RPMs to know what each combo would be running at about 65MPH. This gives you a good idea whe RPM you'd be at cruising at highway speed. Remember that this chart is just for this specific comparison and would need to be re-run with diameter for the actual tire used and all gear combinations considered.



Although I just leaned about the GV overdrive today, so far I like that you probably wouldn't need any tunnel mods, that it allows the use of the original Muncie, Splits the gear ratios evenly and essentially turns your 4 speed into any combination of 8 gear speeds you desire, gives you a good last gear overdrive, is high torque/strength rated, allows 1 or 2 possible clutch-less shifts, they make a look alike Muncie white ball for the shifter with built in control button. They actually have a myriad of different shifter balls and handles to choose from with the button.


Drawbacks are price (about $2850), and the need for a shortened driveshaft.

Unknowns: How tricky is it to shift with the button? How comfortable is the overdrive shifting? Is it easy to miss a shift, or what can go wrong while shifting? Is there any added stress to the case? In the info thy provide, they actually say that stress on bearings etc is less because you are splitting the shifts and thus reducing the stress.

[Brad_bb]

If I decide to use my original M20 or an M22 I have(if I use the GV overdrive) , who should I have rebuild it?

[chinquapalian]

458, t400 3.08 200 =70, 300 - 100 get a posi well thats my experience m22 under workbench

[Brad_bb]

While considering what I was going to use for transmission gearing, and considering using the M20 with a GV overdrive, I'd still need the M20 rebuilt. I know that all of the internals are now available and most are redesigned with improvements. I decided to talk to Paul at www.5speeds.com to see if 1)will he rebuild Muncies? and 2)what did he think of the GV overdrive?

I talked to him this morning. He offers 100% brand new Muncie M20's and M22's with 100% new parts. He would build a Muncie for a customer using the customers original case, but using all new internals. He would have the case re-machined to accept the stronger larger diameter countershafts that are used in the new Muncie's. So essentially you'd have a brand new everything except the cases. This could cost a little more than than the brand new Muncie due to the machining and disassembly required.

With regards to the GV overdrive, he advises against it for mainly 2 reasons. The first is cost. You will end up spending nearly double what an appropriate 5 speed would cost to get that overdrive gear. Secondly, he says that with replacing the tail housing and the added length and how it's supported, you weaken the tailshaft assembly. He doesn't much care for the GV when it comes to the Muncie.

After discussing my vehicle and current rear end gear, he recommended I consider the Legend LGT-700 5 speed transmission. It would NOT require any floor tunnel modification to the car, but the trans mount would be moved further back. Here is a Brochure on it-FYI it looks much better when printed out.
http://www.5speeds.com/legend/LGT-Brochure.pdf
Here is a little more info:
http://www.legendgt.com/

The close ratio version is 2.66, 1.78, 1.30, 1.00, .69/.80 (choice of overdrive gear)
There is also a wide ratio available.
Paul recommended the .69 OD gear for me.
Plugging in 1st gear and the overdrive ratios to my original spread sheet, here are some numbers. I adjusted the RPM to get 65MPH so I could see what RPM I'd be crusing at 65MPH.



1st gear if very close to the original M20 2.56 first gear.
At 65MPH I'd be crusing at about 2150RPM with the .69 OD gear

Cost: Currently the Legend LGT-700 is $3495, plus another $150 for the mid-mount shifter location to fit the GTO correctly, plus $55 for the correct mechanical speedo gear connector/gear). The trans also has back up light capability built in. You just need to add the correct plug/wires to your back up light harness. He also sells clutch and pressure plate to match (I think he said centerforce), which I would need for my car as well.

Cost-wise this is a better option than the GV+Muncie, and would be more compact/simpler. A Muncie by itself would be $1700-$2200, but you would not have an overdrive gear. The overdrive gear is worth it to me as I know I will take some road trips.

[Brad_bb]

[elefantrider]

Will it be available with other first and second gear ratios?

[Brad_bb]

The close ratio 1st and second is 2.66, 1.78 respectively
The wide ratio is 2.97, 2.07 respectively.

[The Champ]

Will this work with the factory console?

[elefantrider]

I'd take the wide ratio.

[Brad_bb]

Can you elaborate? Why? With a 3.90 rear end, you are not going to spend long in first gear, or perhaps skip it entirely. Maybe that is your idea?

Regarding the console, I don't know for sure. I do not plan I using my original console (and no it's not for sale either). When I install the trans, I mock up my console and see.

[Old Man Taylor]

I would definitely take the close ratio. I already went from 4.33's to 3.73's, so mine would be ideal with the 2.66 first gear.

[FrankieT/A]

I could be wrong but I don't think anybody in their right mind is going to split gears with Gear Vendors. I can't believe they even advertise that. Unless of course maybe you are towing a 30 foot travel trailer uphill with your muscle car!

[Tom Vaught]

Originally Posted by Old Man Taylor

I would definitely take the close ratio. I already went from 4.33's to 3.73's, so mine would be ideal with the 2.66 first gear.

X2

Hard to beat a engine rpm of 2250 rpm at 80 mph with $2.75-$3.00 per gallon gas out there today IF you like to actually drive your vehicle somewhere.

Tom Vaught

[elefantrider]

Originally Posted by Brad_bb

Can you elaborate? Why? With a 3.90 rear end, you are not going to spend long in first gear, or perhaps skip it entirely. Maybe that is your idea?

Regarding the console, I don't know for sure. I do not plan I using my original console (and no it's not for sale either). When I install the trans, I mock up my console and see.

Forgot that you don't want to change the rear gear.

Close ratios are good for if you like to shift all the time. Wide ratio is good at street, cruising and drag race.

And why cruise at 2200 at 65mph when you can cruise at 2000rpm or less with a higher rear gear.
Nobody cruises at 65MPH around here anymore.... more like 75-80mph just to keep up.

[geeteeohguy]

Originally Posted by Tom Vaught

X2

Hard to beat a engine rpm of 2250 rpm at 80 mph with $2.75-$3.00 per gallon gas out there today IF you like to actually drive your vehicle somewhere.

Tom Vaught

Gas in California hasn't seen $3.00 per gallon in at least 6 years. It has been $3.55-$4.10, with the exception of 2008, when it was $4.80. It is currently $4.10 for regular. Drove the '67 yesterday to the Madera Pontiac show and didn't use too much gas at 80mph with the 2.56 rear gear. Gas isn't going to get cheaper long term.