Hello Gents!

I hope I'm not going against general rules on the forum as it is my first time posting.
So if I am, just give me a nudge in the right direction and I'll be off.

Anyways, I've got a decision on my hands and was wondering if any of you cool fellas have any insight on it.
I've got a 68' 3 speed with a 2.73 rear ratio.
I'm doing a manual transmission conversion with a TKO 500 (3.27 1.97 1.34 1.00 0.68)
I've also got another 68' Rear with a 3.73 Ratio.
Not sure which Rear would go better with the TKO.
I heard recently that someone had a rear ratio similar to the 3.73 and almost never used 1st gear because it was so short.
In contrast, it would also be rough if the gears were too tall with the 2.73.
Therein lies the rub.

Anyone have any experience or thoughts on the two choices?

 

The 3.73 is the choice of those two with that trans....but read through this.

You will not like 5th gear with a .68 O/D and 2.73's. 65 MPH would be about 1500 RPM...and unless you have a REAL torquey low RPM combo tuned to perfection..it's going to be a slug.

First gear will be a total of 12.19. A 4.88 and a wide ratio Muncie equal 12.29. So...for drag racing that's a great gear combo. Also real handy with a low RPM engine to get it going easily. It's going to feel real short geared in first.....5000 RPM is only about 33 MPH......but you just short shift into 2nd and away you go. It will FEEL strong as heck!

A much better combo would be a 3.36 or 3.08's. That would get you about 70 MPH at 2000 RPM...but still only about 36 MPH at 5000 RPM in 1st.

If we drop to a 3.08...we get 76 MPH at 2000 RPM and a 10.10 overall first gear.. just a little less than a 4.10 and a wide ratio Muncie would be and better than a close ratio.

If you haven't bought trans yet...you might re-look at the various ratios available. Might be able to get a .80 O/D ratio that gives you a little better combo for the lower numerically gears. The 2.73's with the 3.28 first would be pretty nice to drive. In between a set of 3.73's with a either a wide or close ratio Muncie.

This trans combo is very similar to a late 80's Mustang 5.0. They had a 3.35 1st gear and a .68 O/D. I have one and have had 2.73's, 3.08's and 3.55's in it. The 3.08's were a huge improvement and the 3.55's are even better overall. That's a LOW rpm engine that is happy at below 5500 RPM and makes good TQ for its size.

You didn't describe motor combo but I'm guessing a mild 327?

JIM

 

427 got it right. .080 for the overdrive and 3.55 rear gear. If you have the 3.73 already run them, it will be a ton of fun to drive and great on the freeway. T

 

 

theres 2 tko 500 first gear ratios available depending on the 5 speed shops. 3.27 or 2.87, the tko 600 is 2.87 and has 2 different overdrives available, 0.64 or 0.82. I've been doing a lot of research trying to find the best ratios for the 3.55 rearend i have. I was hoping to switch to a 3.70 rear and keep my muncie M21 stock but the ratios just dont work. I'm rebuilding my M21 to a M20 to get a lower first gear. Calculating your gearing is easy. I would use the 3.73 and the 2.87 first gear. The final overdrive gear you should figure out what rpm/ speed you want to cruise at. Most of the driving on US 495 around here is 75 to 80 for prius and minvans, I get passed by them all the time

(first gear ratio) x (rear end ratio) = close to 10:1 for a good overall launch and stretch off the line and then the same for the high gear. Look around online and you will get rpm calculators that will tell you speed and rpm for a specific gear. dont go below 9:1 in first or you will be riding your clutch of the line with daily driving and it sucks, I know. Math for the 3.73 x 2.87 = 10.7:1 Thats good stuff

 

Are you sure you have 273 gears? Those were not common by any means and don't recall them for early C3's. Tom used to sell them but he told me several year ago he discontinued them do to lack of interest. I think I setup one new set in all the years building vette diff's. Not saying you don't have them but they would not be original to the car. 308 was the standard ratio with the 3 spd automatics and 336 for the 3 spd manuals- not very many of those were made. Could also be an open diff.

I would say 95% of the diff's I built for OT cars used 373 as the ratio of choice. Some 411 and some 355 but most were 373.

I had to check and yes 273 were an option for BB automatics. I wonder how many were actually built?

http://corvettec3.ca/axle.htm

 

I've never heard of an early C3 with a 2.73 rear gear.

 

There was a recent thread that showed there was a year that 2.73 rear gears was an option.
I don't remember the exact year, but there were several members that confirmed their C3's had the option.

 

Early C3's or late C3's?

 

I believe it was early C3's, 68-70 BB automatics.

 

68 AND 69 HAD A 2:73 OPTION. , 427 /390 AUTO

I HAVE THE 5 SPEED with a 3:55 and love the combo

 

what 5 speed gearing choice are you running,( first gear and overdrive ) ?

 

I have that same TKO with a 3.27 1st gear and 0.68 OD behind a 454. At first I had a 3.08 diff and didn't like it. City streets had me in between 3rd and 4th. Also felt like I had to slip the clutch too much taking off.

Now I have a 3.73 diff and love it. 4th is good down to 35 mph and up to 50+. 1st feels perfect, I don't feel like I'm short shifting at all. 5th is good at about 55 and up.

 

427, Thank you for the detailed response!

Trans with the ratios I told about above are locked in and are on their way already.
I've got a 350 in her with some top end work incoming to come to about 340 hp.
Not sure if that changes your opinion on 3.55 being a better overall ratio.

Rogers thanks for the Rescue!

So my tko 500 with the 3.27 first gear and .68 OD are locked in.
So the first gears would be 3.73 x 3.27 = 12.2:1 or 2.73 x 3.27 = 8.9:1
With neither of those being close to the 10.7:1 you calc'd. So maybe it is better to gun for a 3.36 x 3.27 = 11.0:1

Thoughts?

I'll have to double check the VIN but I'll get back to ya on it. But am fairly confident it is a 2.73.

Sweet Zwede!

Good to hear that ya nailed your gearing!
I'm starting to think it is going to be a trail and error just like your 3.08 to 3.73.
Maybe it is just something I won't know till shes out on the road and I can get a feel for it.

Not sure if it changes your thoughts on my ratios choices if I have a 350 in her at 340 hp +/-.
Did you ever consider a more middle of the road ratio like the 3.36 or the 3.55?

Cheers

 

Like you figured out. Throw in the 3.73 and give it a couple of drives, you can always have the hearing changed. If your 2nd rear end is a positive. You could change out the hearing in it then just swap them. Which ever one you dont use could be sold off, especially the 3.73. They're pretty popular

 

I think Zwede gave you some good real world feedback. And if it's a 350 with a true 340 HP...it's got a little thump to it and will like the added gearing.

That .68 O/D is made for steep rear gears and/or an engine that is happy to lug down on the highway at 1500 or less. That's why I was mentioning the .80 range as being a little better with the highway gears.

My car has a 3.07 rear gear.....and my 5 speed is non O/D. My first gear is a 3.49 for a total of 10.71. I would like a little more but that's as far as I can go in my trans. On the highway I can cruise with the 3.07's fine...but I would love to have a small O/D to knock it down a little. Oh yeah.....I've also got 1350 HP to help pull it along! LOL

I ran a Doug Nash/Richmond 5 speed for years in my car. Also Non- O/D. I used 3.36's most of the time with a 3.27 first gear and it did very well with my 427. Just made it a heck of a fun car overall...but a slight O/D would have been nice on the highway.

JIM

 

With a 350 that has a bit of cam in it you want the 3.73. Anything less and you'll find 5th useless.

 

I have a 1968 Corvette roadster with a 427 engine in it. The engine is making plenty of power naturally aspirated and has fairly strong low and mid range torque. I love the combination of the factory gearing that my car has, along with my 4 speed it launches like a battleship. My car came with a 3.36 rear and has no trouble on the highway and still has impressive acceleration.

The other time you would really appreciate the 3.36 gears is on road trips with cars getting single digit mileage. After my experience I don't think I would go much higher like the 3.73 on any Corvette I own, I like my sanity. Try a few hours in a side-pipe equipped Convertible with a big block and 4.11 gears and see how you feel. I have done that once, never again!

 

ctmccloskey We're discussing 5-speed overdrives. With 3.73's I'm doing 2,300 rpm @75 mph.

 

Thank you zwede, I sincerely apologize for disturbing you. This is an interesting post to "follow and learn from" as there is a lot of practical experience talking here on this particular post.

I am just here trying to learn all I can to be able to get the specs right for when I make the swap to a five or six speed. The wonderful people at Comp Camps told me that the 3.36 rear would be a major limiting factor in the cam selection, they said I needed a higher rear end, at least a 3.73 is what they told me.

I want to know how to select the best gearing combination for the next transmission I buy. I have a (500 hp+) BB and it allows me to go 25 mph in fourth without bucking or any ill actions, the car will accelerate a bit faster in other gears but I love the torque and power.

Recently we bought our daughter a new Camaro with the six cylinder and six speed combination, at first it felt like "two gears too many". After driving it for a couple weeks I really learned to "like" those extra gears, GM designed this car with four speeds for around town and the extra gears get the mileage up to 30 mpg in a Camaro on the highway. Using it as a four speed around town was great and then on the highway I slipped it into 5th and 6th and was very pleased. I could get used to having five or six gears really easy if set up like that transmission is. GM seems to know what they are doing on that Camaro, it is a nice car.

Who makes a good transmission that can handle the power without breaking the bank? Is there anything different about the gearing when using a BB?

Thanks for sharing all this with us, I have learned a good deal from this forum! Thanks to all of you!