[PontiacMatt72]

After developing a noise in my 70 GTO M20 transmission, I discovered a broken gear tooth on first gear. Time to upgrade to a TKX (been wanting to for a few years now)

Car currently equipped with a 10-bolt with 2.93 gear. Have a 12 bolt rear ready to build.

I’m trying to decide whether to purchase the close ratio TKX (2.87 first, .68 fifth) or the wide ratio (3.27 first, .72 fifth).

If I go with the close ratio, I’d likely build the 12 bolt with a 3.55 or 3.73 gear. If I go with the wide ratio, I’d likely go with a 3.08 gear for the 12 bolt.

Car has a ”stock-spec” 400 (2802 cam, everything else 1970-correct). It’s a cruiser, not a race car. Might play around with some occasional autocross events, but it’s 98% street-driven cruise-in car.

So, wide ratio with 3.08 or 3.36 rear gear, or close ratio with 3.55 or 3.73 rear gear?

[steve25]

What size rear tires, as in how tall in inches?

Does the car weigh in at over 3600 lbs?

[PontiacMatt72]

Rear tire is about 27” iirc. I ran some rough numbers, and between 27.5” and 26.5” makes pretty insignificant rpm difference for cruising speeds (like 70mph). I haven’t weighed the car. But it’s got pretty much everything it would have had from the factory plus vintage air ac (which I imagine is significantly less weight than factory ac). I bet 3600 lbs is pretty close.

[Formulas]

Build 3.55 12B Buy new wide ratio Muncie gear set and associated parts for rebuild of 4speed..

with the prior cam / cubes / trans / rear gear it wasn't much fun at very low speed or clutch release i bet

[The Champ]

You're used to driving your GTO with a 2.92 rear and an M20 with a 2.52 first gear. Effective gear in first being 7.38.

Going to a 3.08 with a WR 3.27 first gives you an effective 10.07 gear. Now contemplate the 3.08 with a CR 2.87 first and you have an effective 8.84.

Either option will give you a dramatic improvement pulling away from a standing start. The difference between a .68 and a .72 final drive gear is minimal.

FWIW - you're second option in your poll is for a 3.36 rear. Not a ratio available in a 12 bolt. After 3.08, the next option available is 3.31, than 3.42 I believe.

If I were you, I'd go with the 3.08 and then decide whether you want to do an easy smoke show from a standing start (WR) or have the upshifts closer together (CR) for the occasional times you autocross and keeping the car closer to it's power band while cruising.

This is coming from someone that has run his '64 GTO with the original M20 (2.56 1st gear in '64), with the original 3.23, a 3.36 and after replacing the original 389 with a 455 with 3.42's and now a set of 3.08's (effective 1st gear is currently 7.88). I'm looking at the TKX also and think I'd be happy with the CR with my 3.08's cruising at a little over 1800 RPM at 70 MPH.

[steve25]

The 2802 cam in a 9.5 to 1 400 will start to get into its peak torque band at like 3500 rpm at full throttle.
I think setting things up for a 70 mph cruise at 2200 will overall be liked better.

[tom s]

AS no racing I would get the wide and a tall rear.I ran the 3.27 with a 3.31 and LOVED it!Tom

[Formulajones]

Doesn't really matter what you pick, all of your choices are going to be better than a 2.93 with an M20 lol.

Me personally, and actually all the installs I've done for others, always go for the 2.87. Primarily because it's the only version that comes with the most overdrive, the TKX being a .68 The 3.27 1st gear versions doesn't offer it.

I prefer to set up the car as a total package which includes changing the rear gear to suit the setup, so the 2.87 trans has always been the go to.

However lately I've used G-Force T5 with aftermarket case and G-Force gear sets.
I wasn't a fan of Tremec knocking 5th gear up to a .68 or .72 when it used to be .64 It really starts to lose it's appeal. The G-Force T-5's have optional gear ratios and the ability to choose different 5th gear ratios without changing 1-4, another annoying thing about Tremec that you can't do.
To be honest the 2.87 TKO/TKX that Tremec claims to be their close ratio really isn't. The gears are spread pretty wide, the only close gear is 3-4, that's it.
With the T5 using the 2.95 gear set, 1.93/1.34 for 2nd and 3rd, it actually acts more like a close ratio than any of the Tremec offerings, and the G-Force version is rated to the same torque as the Tremec, on top of that it's a couple hundred dollars cheaper and a smaller package to boot. I also like the fact that the T-5 doesn't have a 3 rail shift system.

Just some food for thought. I'm sure the Tremec will treat you well, they are excellent upgrades for any muscle car.

[jhein]

The main question for me is how do you drive it? Lots of interstate hwy driving or mainly lower speed roads? Then use a calculator to figure out your RPM at the various speeds you may drive. FWIW, I voted for the close ratio with a 3.73. I have the close ratio with a 3.31, but, I also got the .81 OD, which is perfect for my driving habits.

https://purperformance.com/p-29669-rpm-calculator.html

[soupman]

I agree with jhein.

I have the close ratio with a 3:27 BW 9 bolt rear. My car is light, around 3200 lbs. I feel this is perfect combo for me. I cruise in 4th gear a lot since my engine is turning from 1600rpm up to 2400rpm, which is equivalent to 35-55mph. Most of my cruising around town is usually 45-50mph, this way I am not downshifting from 5th to 4th all the time, just leave it in 4th most of the time.

[Formulajones]

Shucks I use 5th in mine all the time as slow as 35 mph around town and especially 45+ and dont have to downshift at all to gain any speed needed. I use the .64 overdrive with 3.73's. I don't have any real reason to cruise around in 4th at higher rpms creating more noise and wear when I have another gear to use. And it's a pretty rambunctious high strung little 302, not a big torque 400 or 455 you guys have.

[AG]

If you target a 70 mph cruise at 2000 rpm in 5th gear then you will need a 3.38 rear gear, or 3.31 and 3.42 for a 12-bolt. if you want a lower cruise rpm then get a higher ratio, if you want a higher cruise rpm then go with a lower ratio like 3.55 or 3.73. If also depends on the size of your cam on how low a rpm you want to cruise at.

[chuckies76ta]

This topic comes up lots when people decide to upgrade there drive train. This right here from post #9 is what one has to answer. The main question for me is how do you drive it? How do you want to drive the car and where will you have the most fun with it. Mine has 28" tires, TKX with .81 OD and 3.73 gears. My engine likes to cruise at around 2200. I've had back and forth conversation with Formulajones on cruise rpm and mph and gear selection. I gave it a try one time I had the car out and yes I can cruise below 2200 rpm no problem in forth gear but for me it's just no fun. Most of my driving is around the city with occasional highway driving. I can buss it to 80 mph no problem, but that's just for fun. I usually cruise at 60-70 mph out on the highway. I could care less about fuel consumption. That's just me. The car is for fun not a daily driver. If your gonna drive on the Interstate at 80 mph you might want use a lower numerical gear with the 27" tire. There has been times when I though another O/D like a 6 speed transmission would be fun, but our speed limits are only 60 mph most places. And I rally enjoy my setup now and don't want to mess with it. I could see changing rear gear to 3.55 but it's probably not worth the effort. Gotta figure out how you want to drive the car. It's really that simple. What works for one doesn't work for the next person. I love down shifting and keeping the rpm up and listening to the roar of the engine and exhaust system. After about 1 hour of that I've had enough till my next drive.

[VCho455]

Here's a great gear ratio calculator to help you. It gives visual graphs of shift points and also allows you to compare transmission ratios.

One thing many folks forget about when making these choices is your overall ratio in first gear at idle. My LT1 T56 Blazer is a good example. With a cam and other bolt on parts it makes about 100 hp over stock. But with the upgraded clutch it's got a fairly stiff pedal (compared to new vehicles).

The problem was with 3.42 gears, 25 inch high tires and the atrocious 2.66 first gear that F-body T56's have stop and go traffic was tough as the car wouldn't go any slower than 5 mph without feathering the clutch. By changing the rear end ratio to 3.73's means the car to go 3 mph without constantly feathering the clutch making a big improvement in driveablity. The 3.73's also made 0.50 6th gear a usable option at 60 mph where before it was too much.

If your not racing the car and need to base your ratios on that then driveablity should be your goal. GM makes the ratio choices based on emissions and MPG as their foremost considerations with driveablity as the next consideration hence the 2.66 ratio in the F-body compared to the 2.97 ratio in the later C5's. (Keeping the CAFE average up helps them keep the cost of new Vettes down.)

Just my opinion which along with a couple sawbucks will get you a steaming hot cup of Joe just about anywhere.

https://www.blocklayer.com/rpm-gear

[jhein]

Originally Posted by VCho455

Here's a great gear ratio calculator to help you.

https://www.blocklayer.com/rpm-gear

That is a great calculator. Thanks for posting it.

[VCho455]

You're welcome.

If your more of a numbers person here's a different one. It's nice as you can print the results out for later reference.

https://weddleindustries.com/gear-calculator

[track73]

'since I don't race my 79 T/A and didn't want the hassle of a Tremac, I installed a 3.43 first gear ST-10 (cast iron case) and a 2.73 rear gear. It's almost like a 3 speed with overdrive. The 461 has so much low end torque I can put it in 4th around town with out any problem.

[AG]

Originally Posted by Formulajones

Shucks I use 5th in mine all the time as slow as 35 mph around town and especially 45+ and dont have to downshift at all to gain any speed needed. I use the .64 overdrive with 3.73's. I don't have any real reason to cruise around in 4th at higher rpms creating more noise and wear when I have another gear to use. And it's a pretty rambunctious high strung little 302, not a big torque 400 or 455 you guys have.

What rpm are you at in 5th gear going 35 mph and what is the duration @0.050" of your cam?

[65madgoat]

Best compromise here is 3:42 rear gears and .81 OD TKX with 2.87 first gear. Ideal. Trust me.

[PontiacMatt72]

A lot of great feedback, thanks everyone! It’s funny though, there have been 22 “votes” cast in the poll for this thread as of 8:45am Oct 31. I’ve attached a screenshot of the current results. (Three of the choices each have an equal 6 votes, and the fourth option isn’t far behind with 4).

Obviously any of these choices are a drastic improvement to what I’ve dealt with as far as the first gear situation. The upside to the M20 and 2.93 rear gear is I can keep right up with interstate/highway traffic effortlessly. But around town is tricky.

How do I drive the car is the question that everyone keeps going back to, and I understand why. But that’s tough, because I drive it in a lot of scenarios. From occasional parades to hot rod power tour (this past year). HRPT consists of pretty much all driving conditions - stop and go traffic to interstate routes. And, I’ve been equipping the car with suspension and braking components that would support occasional autocross. I’ve not competed in any autocross events ever, just considering trying it out. But this will not be a dedicated autocross car.

Can’t have it all, I’m fully aware there will have to be sacrifice somewhere.

I think I’ve narrowed it down though to two options: the wide ratio TKX (.72 OD) with the 3.31 gear, or the close ratio TKX (.68 OD) with the 3.73 gear.

Effective first gear ratios:
Wide/3.31 = 3.27 x 3.31 = 10.82
Close/3.73 = 2.87 x 3.73 = 10.70

Cruise speed RPMs:
60mph
Wide/3.31 = 1,807 rpm
Close/3.73 = 1,923 rpm

75mph
Wide/3.31 = 2,259 rpm
Close/3.73 = 2,404 rpm

I can easily deal with cruising rpms up to around 2,500. That’s a non-issue for me.

Is there any secret formula to decide on between the wide or close ratio trans?