Cruising rpms with T56 and 3.42's? - Chevy Impala SS Forum
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Old 01-28-2013, 12:03 PM
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Default Cruising rpms with T56 and 3.42's?

Hey Guys,

Yes, I know I've had a couple t56 cars and have always preached at least 4.10's but preferably 4.56's... But has anyone done it with 3.42's? I'm looking for cruising rpms at highway speed. I run a 26" tire. I have plenty of motor, and this will be a pleasure use vehicle, with much commuting duty, and it already has recent 3.42's. I'm assuming 6th will be useless under 65mph, and that's fine with me. I'm hoping the highway rpms from 70-80mph will be less than 2K rpms so I can get some really decent mileage out of it.

Also, how was it getting it going from a stop? I've driven 3.08 and 2.93 cars with the t56 and it felt fine to me.

I know someone had put together a spreadsheet with the differsnt trannies and gear ratios and tire diameter to find rpm, but I can't seem to find it now. Anyone still have it, or a link to it?

Thanks guys! Catcha later.
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Last edited by toomanytoyz; 01-28-2013 at 12:05 PM.
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Old 01-28-2013, 12:47 PM
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I think the stock gears for F bodies with T56 was 3.42. You'll be fine.
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Old 01-28-2013, 12:55 PM
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3:42 may work. the F bodys came with 3:42 with the T56

with that said the B-body weighs quite a bit more than a f body

my experience was with 3:73. It had a slight "dog" feeling off the line and even more so starting uphill so went 4:10

I suspect the 3:42's even more doggish off the line and would require more clutch slip to get going. Guy out here in SoCal did T56 swap years ago with 3:08's, burned up his clutch in a year or so of normal driving

smokemup.com has a calculator

if 3:42 you would never see 6th gear under 80 mph without lugging the car. with 4:10 at 80 mph I tach 2k rpm with a 27" tire so I leave it in 5th at 65 mph and tach about 2100

at the track I run a 26" tire so gear ratio then is 4:xx? (lower ratio than 4:10)
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Old 01-28-2013, 01:20 PM
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Yeah, I knew the fbodies came with the 3.42's and a 26" tire, but as stated, I'm about 1200 pounds heavier than those. And with 4.10's, I would use 6th gears as low as 50mph, with a 26 or 27 inch tire, with no issues for cruising.

Thanks for the calculator link. I'll check it out when I get home.
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Old 01-28-2013, 02:49 PM
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Here's the one I found on this forum: http://www.impalassforum.com/vBullet...d.php?t=233667
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Old 01-28-2013, 05:13 PM
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What ya puttin it in?
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Old 01-28-2013, 05:46 PM
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http://craig.backfire.ca/pages/autos/speed-calculator

26" Tire, 3.42, T56 with 0.74 & 0.50 OverDrives

RoadMpH 5th: 2.53 6th: 1.71
46MpH 1503 995
54MpH 1765 1169
61MpH 1994 1320
69MpH 2255 1493
77MpH 2516 1666
84MpH 2745 1818
92MpH 3007 1991

Fwiw:
4L60E * 2.56
3rd: 2.56 vs your 5th: 2.53
4th: 1.78 vs your 6th: 1.71

If my [enervating] experience with 2.56 is any indication, your 6th gear will provide negligible highway acceleration without the assistance of a tailwind, a draft, or a downhill or
more torque in the 1000-2000 RpM range than our LT1.

IMO, with a T56'd B-car, 3.73 would be the bare minimum, unless highway MpGs were the ultimate goal, or I had a more torquey engine than the mid-90s ironhead LT1.

With either 3.73 or 4.10, the extra gear could be mitigated with tires as tall as 29", which would add more rubber to grip the road.
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Last edited by Marky Dissod; 02-08-2013 at 10:37 PM.
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Old 01-28-2013, 06:22 PM
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I drove my 3.08's to nationals. It wasn't ideal but they worked, for the month I had before all the rear end parts arrived. I would guess 3.42's would be doable if you don't mind slipping it a fair bit. With 3 08's and 6th gear at 75 on a stock motor if I remember correctly I pulled down something like 28-29 mpg.

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Old 01-28-2013, 06:35 PM
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IMHO, if the T56 is going in the long roof car....4:10's. The "cruise & hwy" will be way happy with 5th or 6th gear but more important is the car will not bog off the line or be a bitch pulling from a dead stop up a hill or driveway

the 3:42's will "work" in terms of just being able to drive the car but the 1200 lb weight over the f-body weight will reveal the fact you need more gear......and your clutch will last longer
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Old 01-28-2013, 06:57 PM
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OK, that's all about what I figured. Thanks guys.

I know that I really want 4.56's, but was curious if the 3.42's would be do-able for mileage concerns. My motor is NOT stock, so if I had any shot at increasing the mileage, I was gonna take it.

I've got plenty of time to think on it...
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95 WB4 -The Toasted Marshmallow- Quick and Fat, sorta like it's owner...
00 Sierra 2500 ECSB -The Big Slut- H2's & 33's, TB crank, CAI, Magnaflow, Cowl'd, etc...
94 Fleetwood -The Ill Brougham- Praise the Lowered! Satin Black, red wheels, whitewalls, glasspacks. 'Nuff said.
American Ricer
www.badassofne.net
"Ooooh! Look! Something shiny!"

LOWER IT!!!
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