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Old 11-05-2008, 07:39 PM
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Need some help sorting out tranny ratios and rear end ratios

I have several options available

Rear end ratios
3.55
3.79
4.11

Tranny ratios (T-56)
1ST 2ND 3RD 4TH 5TH 6TH
2.98 1.99 1.35 1-1 .86 .73 Transmission 1
2.71 1.79 1.30 1-1 .89 .75
2.62 1.60 1.21 1-1 .85 .76
2.29 1.60 1.21 1-1 .85 .76 Transmission 2

Rear tire diameter of 27.0"

I have played around with some programs that give you RPM with different combination's of gears.

Here is their output.

Transmission One
6600 RPM Redline
Gear
Ratio Max MPH
1st Gear 46.92MPH
2nd Gear 70.27MPH
3rd Gear 103.58MPH
4th Gear 139.83MPH
5th Gear 162.6MPH
6th Gear 191.55MPH
Final Drive 3.79:1

Transmission Two
6600 RPM Redline
Gear
Ratio Max MPH
1st Gear 61.06MPH
2nd Gear 87.4MPH
3rd Gear 115.56MPH
4th Gear 139.83MPH
5th Gear 164.51MPH
6th Gear 183.99MPH
Final Drive 3.79:1


now changing the final drive gear to 4.11

Transmission One
6600 RPM Redline
Gear
Ratio Max MPH
1st Gear 43.27MPH
2nd Gear 64.8MPH
3rd Gear 95.51MPH
4th Gear 128.95MPH
5th Gear 149.94MPH
6th Gear 176.64MPH
Final Drive 4.11



Transmission Two

6600 RPM Redline
Gear
Ratio Max MPH
1st Gear 56.31MPH
2nd Gear 80.59MPH
3rd Gear 106.57MPH
4th Gear 128.95MPH
5th Gear 151.7MPH
6th Gear 169.66MPH
Final Drive 4.11

I like the combination of 3.79 and Transmission 2, third gear becomes a great combination for a "pass now" gear (drop into third and pass anything on the road!)

The T56 I am looking at is a Rockland T-56 Son of Tranzilla good for up to 800HP.

Am I missing something in the evaluation of the gear ratios for both the tranny and final drive ratio?

I would appreciate comments here because I am probably missing something or don't see other issues.

regards, Herb

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Old 11-05-2008, 08:35 PM
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Umm, looks like you got it sorted out. Make your choice based on your wallet. I mean, what are you asking?

How technical shall we get?

Last edited by C-10; 11-05-2008 at 10:02 PM. Reason: forgot the r in 'your'
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Old 11-06-2008, 05:10 AM
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Quote:
Originally Posted by C-10
Umm, looks like you got it sorted out. Make your choice based on your wallet. I mean, what are you asking?

How technical shall we get?
as techincal as you can is fine.....I want to make sure that I have thought this out enough........
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Old 11-06-2008, 07:39 AM
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unless this is a 100% (road?) race only application,,,looking at mph at the HP "peak" rpms is kinda' useless....

do your calculations at normal shift rpms and based on the "actual" available power to weight ratio at those rpms....
with high power/low weight and a six speed what happens often is you do only use 3-4 gears on the street which is fine...
(ex: my daughters Porsche first gear is useless, winds up way to quick just driving normal)
you "can" downshift to pass....

what you really do want to know is how "quick" will the motor recover Hg and wind up to "reach" the HP peak to go onto the next gear....
simplest formula:
force on the tires= actual flywheel TQ at that rpms x tranny ratio x rear ratio

more force on the tires per lb of weight=less load on the motor so it will wind up quicker....

illustration for for off the line:
300ft/lbs at 2,000rpm x (tranny1) 2.98 first x 4.11 rear =3674 ft/lbs on the tires
300 ft/lbs at 2k x (tranny2) 2.29 first x 4.11 rear=2823 ft/lbs on the tires

tranny1, 3674ft/lbs divided by 3674lbs car weight=1ft/lb per 1lb of car weight....
tranny2, 2823ft/lbs divided by 3674lbs= only .76ft/lb of force per lb....
tranny 1 with taller 3.55 rear gears=3173 ft/lbs which darn close to the same performance as tranny2 at 3674lbs of weight

the 5th and sixth OD gears determine your hwy speed rpms for mpg ...
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