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Old 08-20-2008, 09:40 AM
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Comment on this ratio?

The transmission is a T56 manual with 0,75 overdrive in 5th and 0,5 overdrive in 6th gear.

The rear tires are 295/50-16 (diameter = 27,6") and the rear axle ratio is 3,54:1

The car is a '34 Highboy roadster and the engine is a mainly stock CSB but with airgap intake, QFT 650 cfm carb, headers and 2x2,5" dual exhaust.

Maybe a ratio around 4.00:1 would have been better without ruining the mileage?

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Last edited by staleg; 08-20-2008 at 01:57 PM.
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Old 08-20-2008, 10:41 AM
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i say a 4.56 gear or lower.
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Old 08-20-2008, 12:48 PM
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Depends on what you want out of it. Like it is now, the car is light and should move pretty quick plus the double OD will get you some good mileage. If you want more "scoot", go to a lower gear as stated. Personally, I'd run it as-is and see (you've more than enough motor to pull it)...

Russ
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Old 08-20-2008, 01:26 PM
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numbers just to illustrate:

a stock Caprice(?) is:
350ft/lbs x 2.46 first gear x 3.0 rear gear=2583 ftlbs of force on the tires divided by 3500lbs car weight=.73 lbs of force per lb of wt...
your car is:
350ft/lbs of flywheel TQ x 2.46 first gear ratio x 3.54 rear gears=3047 ft-lbs of force twisting on the tires divided by 2400lbs of car wt =1.26 lbs of force per lb accelerating

350ft/lbs x 2.46 x 4.11=3538/2400=1.47 lbs of force per lb= darn hard to get it to hook up and not just burn the hides

food for thought:
really deep gears in a light car on a wet road are not fun....
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Old 08-20-2008, 01:54 PM
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Thank you for very constructive inputs guys!
I leave the rear axle as is for the moment.

I guess I'm in a "change-what's-already-done-once" mood these days.

Not always so vice. Probably better to use the energy on the work that's still left.
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Old 08-20-2008, 02:19 PM
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your welcome and thank's for saying thanks...

not every day that I get to give input on a all steel body 34' in (beautiful!) Norway!!!!

did you notice that a picture of your car is on the HR main page?
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Old 08-20-2008, 02:38 PM
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No I didn't until you said so!

But I'm afarid I deleted that photo from my journal because I forgot to downsize it before I put it in there.
I will add it again pluss some more, when I've downsized them a bit.
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Old 08-20-2008, 10:42 PM
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Lets think about his guys.....

295/50/16 tire, rated at 753.3 revolutions per mile.

3.54 diff with 0.5 overdrive is like having a 1.77 diff with 1:1 top gear.

fifth gear 3.54 x 0.75 x 753.3 = 2,000 rpm at 60mph.... not too bad
sixth gear 3.54 x 0.5 x 753.3 = 1,333 rpm at 60mph..... far to tall !!!!

I think something like 4.0 would be better
fifth gear 4.0 x 0.75 x 753.3 = 2,260 rpm at 60mph
sixth gear 4.0 x 0.5 x 753.3 = 1,567 rpm at 60mph

Personally with THAT gearbox I would be going for something more like 4.5
fifth gear 4.5 x 0.75 x 753.3 = 2,542 rpm at 60mph
sixth gear 4.5 x 0.5 x 753.3 = 1,695 rpm at 60 mph still plenty tall enough !
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Old 08-21-2008, 12:17 AM
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Yes I have done that math too, and agree.
But I just leave it as it is for now and drive it like this from the start. Or if a Jaguar diff with a better ratio shows up I buy it.
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Old 08-21-2008, 12:29 AM
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You are right.

Get it on the road and see how it goes. The lowest available jag diff is 4.09 and they are pretty scarce. Your best bet might be some Dana 44 gears, maybe 4.11 or something.
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Old 08-21-2008, 07:02 AM
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As I've found out so far (it might be wrong) is that the 4,09:1 differentials are from 6 cylindered Jaguars. Probably more rare in The US than in Europe, since you always have had lower gas prices than us.
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Old 08-21-2008, 08:54 AM
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counterpoint?:

for a 2400lb car to be able to "maintain" 60mph you need (maybe with a strong head wind?) 50FWHP/150FWTQ available....

a bone stock 350 makes approx 90HP/290TQ at 1500rpms......

your motor will be "loafing" with about 18Hg for a lean mix at 60mph with 3.54 gears at 1500...

(my wife's stock 3.8L/3900lb(!!!)/AOD Jimmy is 1500 at 60mph, even with the air flow of a brick wall it does drive fine with very little pedal at 60mph...but.... Florida is basically FLAT

if road testing does show that you want/need better pedal response in 6th with the 4.09 gears....
fair to very good chance the motor will wind up so quick just driving normal in 1st gear that you can't/won't use 1st gear on the street from a stop sign (which is OK,)
(so you might as well have left the 3.54 gears in it and not used 6th gear to be able to use 1st)

stelag,
are you sure you do have the .50 version, not the .62 version?

http://www.sallee-chevrolet.com/Chev...ions/gmt56.cfm

makes a big difference for figuring the desired first gear performance because they are different ratio (my 2.46 example was way off) and cruise rpms....

Last edited by red65mustang; 08-21-2008 at 09:14 AM.
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Old 08-21-2008, 12:05 PM
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I have tried but not been able to fully decode my transmission.
The dependeable guy I bought from, told me the transmission was from a LT1 equipped Camaro, and the trans do fit straight on my L05 1992 model 1 piece rear main seal first gen small block.
So then we can eliminate LS, Ford and Viper transmissions.

My trans has a tag that says 12550647 and I haven't been able to find this number on the web anywhere. The closest I find is 12558016 which have the 0,5 sixth overdrive gear.

On a webpage somewhere I read that the 1255 prefix was used on the F body 94-97 cars and that all of them have the 0,5 sixth gear. The 93 LT1 T 56 trans have the 0,62, but they are not as strong as the 94-97 models.
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Old 08-21-2008, 01:03 PM
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start another thread and ask?
title it something like: "help decoding a T56 tag"

I really don't know...
"export only" number? calibrated for Km? modified to get around european patents? factory special order/build number?
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Old 08-21-2008, 02:52 PM
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No. The transmission is imported from The US.

Just now I have my hands full of making a left and a right roadster rear fender out of 1,5 left sedan fenders...
And after that I'll make a new hood inspired from Chip Foose's P-32.

And after that it's time to show the project to the authorities and God knows what they "discover". Just to showing me that they have more than a clue, you know...

So I will probably leave my mystery transmission for now. At least I know it fits physically.
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