druckle

I have a 74 with 410 hp 350, TKO500 tranny and 3.36 gears. Rear end broke and now thinking if I should change gear ratio to 3.55 or 3.73. I have access to a 4.11 and think that is too low!!! Thoughts?

Cool bean

It all depends on your application. I'm in the middle of my 74 build/restomod and have similar configuration as you (400+/- hp, 355, aggressive cam. Mine's an auto: 700r4.) The rear is something I did more research on than many of the other mods.

I landed on a 3.73 because my primary application was street/spirited driving/road trips/stoplight-to-stoplight/occasional trips to the track. Because of the road-trips, i wanted overdrive. With the final drive lockup of 0.7:1, 27" tires, I could do approx 2300 rpm at around 70mph (if my math is off, its not by much).

My decision was between 3.55 and 3.73. I went with 3.73 because my track goal was a 12 second car. For no other reason than "just because."

Right now the original 3.08 is on the car, but it's comin' out as soon as Mike (traccdog2) delivers the new HD-10 with 3.73 gears.

THere's lot's of threads that I researched about the rear gears. Depending on your application, 4.10 might be too high or just right. I like that TKO you have! Maybe something like that is in my future....

73racevette

A lot depends on the final gear ratio of your TKO500. They come in 2 different configurations. If it is a 0.63 final ratio, a 3.73 or maybe even shorter gear is probably a good bet. A 3.36 is a pretty tall gear for a 0.63 final ratio trans.


If it is a 0.82 final ratio, you can probably get a way with a taller gear.

c3_dk

It looks like a TKO 500 has 3.27 in first gear ration, so with a 4.11 it will give you a drive line @ 13.43 (this is VERY high)
With a 3.73 it will be 12.19
3.55 @ 11.60
3.36 @ 10.98

A good street drive line is 10

Just my 2 cents.

73racevette

You are correct, I can't edit my above post for whatever reason. That post was for a TKO600. The TKO 500 only comes in a single configuration as far as I know with a 3.27 first and a .68 5th gear.


I agree a good first gear overall ratio is around a 10. Another consideration is what RPM you will be at on the highway. I think you will find the TKO500 has an interesting gear configuration with a VERY low first and a pretty high 5th. You need to decide which is more important, a good 1st gear or a good 5th gear. Does not seem you get both with that trans. For the 5th gear a 3.55 or even a 3.73 is likely a good match, but those are going to be a really low 1st gear.

OMF

I've done the change from 336 to a 355 with a TKO600 trans with the .63 OD.
I do a fair amount of highway driving, and can say that I wouldn't want to go with any lower of gear for my usual driving conditions. I also think that if you went to a 3.70 rear gear, first gear will pretty much be useless.

So in a nutshell I think 3.55 is best all around.....

gkull

If he really has 410 hp out of a 350 ci it would have a higher duration cam.

You have to have the correct OD final gear ratio correct. First gear just falls where it falls. I drove around with 4.11 rear for over 20 years and it was really nice with a TKO 600. 2.87 X 4.11 = 11.80 that 10 first gear ratio is some dreamers gear ratio with a BBC and 40 pound flywheel

I could take off in first from an idle or really blast off through and intersection.


So setup your rear gear with driving in OD in mind.

zwede

I've always believed the "1st gear overall ratio of 10:1", but these days I don't agree with it at all. Modern cars with manuals have a much steeper 1st gear than 10:1. I had a 3.08 diff with my TKO (3.27 1st, 0.63 OD), so overall ratio in 1st was right at the magic 10:1 and I hated it. Way too much slipping the clutch in traffic, driving in the alley behind the house etc.

I now have a 3.73 diff and I think all the gear ratios are perfect.

Btw, I have a 454 so there's no lack of low end torque.

Kacyc3

First things first define broke. Broke teeth off, broke carrier, broke saddles. 2nd what were you doing when it broke? Depending on what is broken and what you were doing, you may want to see what it would take to put a 12 bolt in, gears for them start at 370 from what I have read.

druckle

I have run the 3.36 with this tranny for 2 years and at 70mph it turns slightly less than 2000rpm. The 350 has a LT4 hot cam with 218/228 duration at .050, .520 lift. Dyno sheet says 410hp and 429tq at flywheel. No racing, just a street car with a driver that likes to get up to speed quickly!!! Sounds like the 3.55 might be a better choice. I don't know what is broke yet, I will look into it next week. I'm just trying to cross bridges before I get to them.

jb78L-82

FWIW-I have a similar motor as your year one 350 but a 355 with 10.2:1 compression, AFR aluminum heads, and a Howards roller cam with similar lift .525, LSA 110 and duration of 219/225 with LTH of 1 3/4 inch primaries-I figure about 425+ Gross HP. If I had a manual tranny that turned around 2,000 RPM's at 70 mph, I would be tickled pink (I have the OEM super T-10 with 3.70 rear). Just changing the rear to 3.55 from 3.36 is not going to buy you much more acceleration and on the other hand will not hurt the cruising RPM much either....not sure I would spend the money for that change in ratio. Just my 2 cents.

v2racing

Automatics in modern cars are much lower (higher numerically) geared too. My Cadi CTS Coupe has 3.73 gears with a 4.065 low gear for a 15.16 overall low gear. It has a 7000 shift point in full auto at WOT and with a 8.84 overall second gear, it yanks pretty hard at the shift. Manual shifting is hard to keep up with, but it does it on it's own quite well, especially after I tuned the trans with HP Tuners software.

I'm yet to finish the 75 Vert with it's 4.11 gears and 700R4. I'm sure it will be touchy trying to shift manually, but the auto shift should not be an issue. Traction will be though. I had this motor and tranny in my 80 Vette and with 3.08 gears and traction was already iffy. Cruising with 3.08 gears and a .67 overdrive was asking for a ticket because it was doing about 85 before the engine was comfortable.

Mike

gkull

Mike, I did not have a detent for 1st gear with my 700R4. The governor is set at 5000 rpm. So you put it in second detent. Then you floor it and it does and auto 1-2 and you can do the manual 2-3 up around 90 mph or something. I tried the 6500 rpm gov and even with slicks it tended to blow the rear tires off after the 1-2 shift.

people don't seem to understand that if you have a good motor calculated top speeds can be over 200 mph with OD. Also super low rpm does not make the highest cruise MPG. It is cruising at an rpm where the engine is most efficient and that might be 2500 rpm

v2racing

George,
I agree completely. I have a couple different governors and several different weights and springs to tune it in. I do have the detent for the 4 speed overdrive I installed in my shifter.

With careful tuning my 80 got 21 MPG at 85 to 90 mph, it was turning about 2100 to 2400 rpm. Any slower and the rpm wasn't in it's cruise range with my ports and cam and gas mileage would fall off. It would cruise at 1500 rpm without bucking but it just felt and sounded like it wasn't happy there.

With 4.11 gears and 27" tires the RPM would be close to 7000 at 200 MPH. With a .67 overdrive that puts the driveshaft speed over 10,000 rpm. That's a little scary!

Mike

druckle

I see where the 3.55 is called a performance rear gear and 3.36 and 3.08 is not. Is there any difference in the strength of the units because of thicker teeth or something or is it just a difference in just the ratio?

F4Gary

With a stock 4-speed, for every rear gear change you gain or lose about 200 rpm. 3.08 to 3.36, 3.36 to 3.55, etc.

gkull

That is why I installed drive shaft safety loops.

Is there any difference in the strength of the units because of thicker teeth or something or is it just a difference in just the ratio?



the ring gear is the same and pinion goes down in the number of teeth have relatively the same strength because the teeth are the same size. Properly setup the teeth are not the weak link in a vette rear end.

druckle

Thanks for all the reply's. Once I get it out and see what broke I will know whether to stay with the 3.36. If I have to replace it I will probably go with the 3.55. Thanks again for the input.

fishslayer143

I ve tried 2.87 , 3.31 and 3.73 ...with overdrive, the 3.73 is JUST RIGHT in my opinion

c3_dk

I agree.
My 69 convertible has a 3.70 from the factory (L46 car), soon the m20 will be replaced with a T5WC (1352-176) with 2.95 in 1th gear and 0.63 in 5th. Perfect in my mind.

My 69 Coupe has a TH2004R with 3.70 rear, again perfect in my mind.