shmoky

My 68 has 3.70 rear gears. With the M21 tranny and a first gear ratio of 2.20 that calculates to 8.14 final ratio. Didn't realize that it was that low. Maybe that's why it launches so well. The Tremec TKO first gear ratio is 2.87 and that comes out to 10.62 final ratio.

How much of a difference is that going to make? I'm running a BB with 490HP/525TQ. Will the TKO still be driveable in first gear or should I be looking for a tranny with a lower ratio? I was led to believe that you want to be as close to "10" as possible -- is this true?

Thanx,

shmoky

Belgian1979vette

10 is ideal.

My guess would be that with a 8+ ratio your car would launch a bit slower, although maybe it more personel feeling than anything else.

midyearvette

my 66 has a 336 with the 287 you speak of and it is plenty short with my454 but not as short as you will be. i would install anyway and change the rear gear if needed ....you will like the tremec......

ED69ray

You probably have enough off idle torque to push the car with a 3.55 rear

My SB '69 had a 3.70/M-21 combo - I found it ok on the street.

Unless you need the OD tranny, the M-20 wide ratio is better off the line with its 2.52 first gear and 4th gear on the M-20 is same as the M-21 at 1.00.

Good Luck, Ed

shmoky

Thanx for all the replies.

Ed: The MAIN reason for the 5 speed is the 5th gear. As at 60MPH I am at 3200RPM and need that lowered. I was just concerned (and trying to get more opinions) about the 1st gear final ratio with the TKO.

shmoky

Gordonm

You have a ton of torque so 1st gear is going to be tire smoke if you nail it. You will have to learn to roll into the throttle. Once you get this down it the car will launch faster. The M21 never was a good street transmission, it was designed to be a road race trans. You will definitly like the OD of the Tremec!

shmoky

Thanx Gordon, I was waiting for your reply. Your description of my 1st gear is and yes I will have to practice it . So with that and the 5th gear I should have the best of BOTH worlds:reddevi l

shmoky

PS I'm will be putting in PS in the next couple of weeks and then start saving for the TKO. ($5000.00 Canadian after taxes etc.)

63mako

The taxes are a killer. Seems weird, I sell quite a few parts and when I ship to Canada they charge import taxes on used parts. Like 30%! Most of our auto manufacurers buy lots of parts from Canada and there is no tax, same with Mexico. No tax on imported parts to the US. Can you bring it over the border yourself without paying tax? Or can a US citizen bring it across without paying tax? Or is that smuggling. You will like the TKO 600. The TKO RR might be better. Think it is a .80 overdrive instead of a .63 which is a big drop on the 4th to 5th upshift. With it you would be @ 2560 RPM @ 60

Young69Owner

Why do you want as close to 10 as possible? He's going to spin tires easily with that much torque and an 8. Doesn't it depend on the engine? Funny though, I have 2.95 * 3.36 = 9.9! I imagine it is hard to get much closer than that! I go sideways off the line. How do we roll the throttle? I want to time my 0 to 60's...

TheSkunkWorks

If you go with a TKO, I'd go with the 600 RR for the reasons 63mako stated, but while you're still evaluating your decision it's worth considering the Richmond SS/OD 5-speed, as well.

TKO 600
2.87 - 1.89 - 1.28 - 1.00 - 0.64

TKO 600 RR
2.87 - 1.89 - 1.28 - 1.00 - 0.82

Richmond SS 5-speed
2.89 - 1.85 - 1.31 - 1.00 - 0.77

M21
2.20 - 1.64 - 1.27 - 1.00 - SOL


TSW

7T1vette

There's a lot of difference on comfort and mileage at hiway speeds between the 600 and 600RR. If you put a bunch of miles on at hiway speed, I'd stay with the 600...and if you can't manage wheelspin in 1st, back down on the rear gear a bit. If the 3.71 still works for you, you can always start off in 2nd gear around town...you'll have no problem doing that.

gerry72

That rpm at that speed calculates closer to a 4.11 rear. You'd be around 2,700 rpm at 60 with a stock (or close to it) tire diameter and a 3.70 gear.

Gordonm

Unless you are road racing I don't think the big drop from 4th to 5th is a big deal. How many of us are really looking to accelerate at 120 mph when shifting into 5th gear. Most and I bet about 99% of TKO owners want the 5th gear as a good OD to keep the rpms downs. You can use the first 4 gears for racing or what ever and use 5th for cruising. I have never found this to be an issue. I autoX and drag race my car and never have come close to using 5th gear for any of that. There are a few here that need the RR version but not many. If you are running a 3.55 or deeper gears the .68, 500 or .64 600 trans will work fine.

shmoky

Thanx for the input. I won't worry about it anymore. The TKO with .64 5th gear sounds like it will work great. (I need a smiley that shows burning rubber )

shmoky

Belgian1979vette

It ultimatly depends on your engine an its operating band. M21 close ratio was usually for engine with a higher rpm operating band. They were used together with a higher numerical rear gear to get the engine in its power band faster and compensate for the lower tq of the engine. If there is enough torque low down, then you can get away with a lower numerical rear end and the same trans.

Young69Owner

My thinking on the rear end is similar, it really only sets up first gear and your last gear. The others will fall into place. If you can already spin the tires, there is no need to go higher numerically...

63mako

Some of us enjoy roasting the tires at will.

Young69Owner

I can too, so how much more do you need? At this point, you can set it up so that you can shifter later... :

ED69ray

Understood, my car had the M-21/3.70 combo - although 70mph for me = around 3200rpm.

With your hp/tq numbers, I would suggest trying a 3.55 or even 3.36 rear end and keep the 4 speed.

I looked at the Richmond Street 5 speed for my build - I have both 3.08and 3.70 rear ends for my 4 speed car - The 3.08/Richmond combo looks pretty nice as it has a 1-1 5th gear ratio.

Some of the 5 speeds have very tall 5th gears - IMO, if you gotta run 80mph to use 5th youre overgeared.

I decided to go M-20/3.70 instead for nostalgia, cost, and hopefully not spending too much time slabbing along at 70+mph

Good Luck, Ed