[Hamman]

I'm thinking if I run back wheel 295/45/20 the overall diameter will be 30.45 inches.
Ratios in box 4:1, 2.36:1, 1.53:1, 1.15:1, 0.85:1, 0.67:1
my rough calcs are that the rear end ratio should be 4.11.
is that right? Optimum? Drive like?

I worked out that in 6th rpm would be 18-1900 at 60 mph.

 

[racsan2448]

Go to this website and plug the numbers into their speed analyzer.
www.keislerauto.com

 

[55mike]

I'm thinking if I run back wheel 295/45/20 the overall diameter will be 30.45 inches.
Ratios in box 4:1, 2.36:1, 1.53:1, 1.15:1, 0.85:1, 0.67:1
my rough calcs are that the rear end ratio should be 4.11.
is that right? Optimum? Drive like?

I worked out that in 6th rpm would be 18-1900 at 60 mph.

According to the Richmond gear calculator you are looking at about 1823 RPM's at 60 mph with that set up. Check out this link for other combo's http://www.richmondgear.com/101032.html

 

[Tuned55]

I'm running 4.10 gears with 27'' tires. In 6th gear (T56) @75 mph I'm right at 2,000 RPMs

 

[midnightrider]

I think I would be a bit more concerned about your first gear....with that 4:1 first gear and a 4:11 rear, you're gonna be thru first in a flash. Unless you're engine is built up and designed for real high RPM shifts it looks like first gear ratio is too steep. I'm running a t-56 with a 3:36 first (early z-28 trans), 28" rear tire, motor with power range of 2000-6000..started with a 4:11 rear. With that set-up first gear was pretty much useless....I mean great burnouts but not a good match for overall performance. I knocked the rear down to 3:73 and it is better, but still wish I opted for the 2:97 first gear equipped t-56. I don't know what it is but I believe there is a number/guide out there where you can match your engine to your first gear ratio x rear end ratio, of course taking the rear tire height into consideration also. Mine is currently 3:36 x 3:73= 12:53, which I believe is still on the high side for my 6000 or so RPM shift point.

 

[kbrace]

Ron has a good point 4.1 x 4.11 = 16.85 which is very steep. Even with his 3.36 x 3.73 = 12.53 is probably pushing the limit. I know you are running 30.45" tires which will bring this down a little. I think a 3:55 or even 3:36 would be ideal. I am running 2:88 first gear and a 3.70 for a factor of 10.66 and with smaller 26" tires the car still launches very strong off the line.
Do some more research and good luck.
ken 57

 

[55mike]

I think I would be a bit more concerned about your first gear....with that 4:1 first gear and a 4:11 rear, you're gonna be thru first in a flash. Unless you're engine is built up and designed for real high RPM shifts it looks like first gear ratio is too steep. I'm running a t-56 with a 3:36 first (early z-28 trans), 28" rear tire, motor with power range of 2000-6000..started with a 4:11 rear. With that set-up first gear was pretty much useless....I mean great burnouts but not a good match for overall performance. I knocked the rear down to 3:73 and it is better, but still wish I opted for the 2:97 first gear equipped t-56. I don't know what it is but I believe there is a number/guide out there where you can match your engine to your first gear ratio x rear end ratio, of course taking the rear tire height into consideration also. Mine is currently 3:36 x 3:73= 12:53, which I believe is still on the high side for my 6000 or so RPM shift point.

Excellent point. I know with my Tremec, the 3.27 first gear coupled with the 3.50 rear gears is about right..

 

[Hamman]

I think I would be a bit more concerned about your first gear....with that 4:1 first gear and a 4:11 rear, you're gonna be thru first in a flash.

Yeah I, struggling with that - there must be a formula somewhere..... did I mention its an auto?

 

[Hamman]

Ron has a good point 4.1 x 4.11 = 16.85 which is very steep. Even with his 3.36 x 3.73 = 12.53 is probably pushing the limit. I know you are running 30.45" tires which will bring this down a little. I think a 3:55 or even 3:36 would be ideal. I am running 2:88 first gear and a 3.70 for a factor of 10.66 and with smaller 26" tires the car still launches very strong off the line.
Do some more research and good luck.
ken 57

Using the "Richmond Street Performance Calculator" www.richmondgear.com
I get:

using 3.36 rear gear
30 mph in 1st is 4560 rpm
50 mph in 3rd is 2836 rpm
70 mph in 6th is 1739 rpm

using 4.11 rear gear
30 mph in 1st is 5592 rpm
50 mph in 3rd is 3469 rpm
70 mph in 6th is 2127 rpm

 

[roger1]

The Corvettes with that transmission come with a choice of a 2.56 or 2.73 ratio.

Why not even consider a 3.08 for your car?

Not sure what you are after or what kind of a torque curve your engine has. But, if it were me with the Ram Jet 350 that I have, I would choose the 3.08.

In any case, I think you would be sorry if you chose anything lower than the 3.36. First gear would become just about useless.

This transmission was obviously designed for high rear end ratios.

Edit:
When you use those calculators, don't forget to use a smaller number for the tire diameter due to the load on the tire.
The wider the tire, the less the tire will flex. But still the loaded radius times 2 should be what you use for diameter.
With your tire, I would estimate the loaded radius to be about 3/4" less than unloaded. If that were true, you would use 28.95" in your calculator for diameter. You can measure your loaded radius directly fairly easy.