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Cruising rpms with T56 and 3.42's?

8K views 28 replies 14 participants last post by  Blue Wail 
#1 · (Edited)
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. :)
 
#3 ·
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)
 
#13 ·
No offense, but that guy with the 3:08's that burned up his cltch in a year either needs an automatic or needs to learn that with a lees than ideal setup (stock gearing t56'd b-body) you can't pound on it 24/7 like ou can with the proper setup and expect it to last.

I have over 22k on my swap with the stock 2:93's and the stock 97 trans am clutch/flywheel/trans that had 70k on it to begin with. I use 6th gear anytime I can including on flat ground at 50mph (just over 1000rpm) and on the highway and while you do have to downshift if you want to accelerate with any type of quickness, it has been fine. I have averaged 28-30mpg highway with is as well.

I have also towed a 2000 pound trailer with the car loaded down to the gills as well for a 750 mile one way trip (1500miles round trip) and still have no issues with the clutch and averaged 20-22mpgs with the trailer.

That being said, I am myself debating between 3:73's and 4:10's when money permits. I have also thrown around the idea of 3:42's so if the OP does do the swap with those gears, please fill me in on your real world mileage and wheter you used 6th gear al the time or just at highway speeds, as no-one Ive ased to date know their mileage or never used 6th even with 3:73's.
 
#4 ·
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. :)
 
#5 ·
#7 · (Edited)
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.
 
#8 ·
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.

-Brian
 
#9 ·
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
 
#10 ·
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... :D
 
#11 ·
Three months after swapping my T56 in, I drove my SS to Ft Lauderdale and back. 30 hour drive each way. With 3.08s. Drove fine, and I drove fast. Clutch survived, and is still fine ten years later. The 4.10s went in the next year.

Run with the 3.42s. With your rear tires, it will be ok. If you hate them, you can start saving for the 4.10s (yes, 4.10s, the right gear with 295 35 18s).
 
#12 ·
I am with everyone, I am sure that they will be fine, but I am not sure what benefit you will have as far as gas mileage goes.
My car for instance, using 4.10 gear out back, 26.7 inch tall tire, average 70-75 mph, 2100-2200 rpm, I would get 25-26 mpg regularly when I drove it daily in the summer last year.
Motor isn't built by any means, just typical bolt-ons, headers, not cats, x pipe, etc. and 93 octane tune.

Just to give you a comparison.

But as Jay stated use the 3.42's I am sure it will be fine
 
#14 ·
I converted my 94 to a T56 in about 2003, ran it every day of the year, and never did take the 3.08's out. I never used 6th gear unless I was over about 85 mph. When I did the conversion, the whole clutch, trans etc had 94,000 miles on it and it seems to me I put about another 70,000 miles on it. I never beat on it or I likely would have ripped the clutch out of it though.

Point being, you CAN run a T56 with 3.08 and a stock clutch unless you want to wail on your car a lot.
 
#16 ·
as noted some have and do run a T56 with 3:08 and even 2:93 in a B-body. I know Ed aka Autocrosser on this forum prefers 3:73 for autox. The F-bodys came with 3:42's

Like torque converter stalls the rear gear and clutch combo selection is viewed diffrently by many people as driving style and acceptable engagement techniques differ.

I will add that if using a higher gear ratio like 3:08 and having any clutch other than organic surface would be a PIA as slipping more aggressive friction material clutches (puc type) would suck

Given the cost of any gear ratio and reprogramming PCM to accomadate is the same I would stay in the 3:73 - 4:10 lower gear ratio range. Will other (higher) gear ratios work, yes. 3:42 is as high as I would go and only with a organic surface (stock) clutch disc

I also don't feel lugging the motor under 1500 rpm at crusing is good on the motor, but that is just me.
 
#17 ·
I will say though, I always had the feeling that if I dumped the clutch at 5000 RPM I likely would have been looking at a pile of rubble on the highway, so I just never did it!
 
#18 ·
I have plenty of motor,
As in any more cam than stock?

My experience when I had an LT1 with even a baby 304 is it made less power than a stock engine down low and did not really like running in the lower teens RPM wise.
Sure it was smooth, just no pull at all.

Even with 3.73, it was pretty lackluster in 6th at legal speeds.

So, while the bone stock cars might be able to pull the lower numerical gears, you might find if you have a cam that has any lope is going to push your best ratio up.
 
#19 · (Edited)
I have the Comp 305 and fairly flowly heads, comparable to the AI's.

And ideally, I would want 4.10's for this car. But I *JUST* built the rear end last year, when I was going a different route with the car, and do not want to crack back into it at this point. There was a lot of money spent there and I don't care to spend more on it! ;)

I know 3.42's are not ideal, and that they will work. I was just wondering if anyone had actually done it. I'm not looking for any kind of performance or quickness in 6th gear. That'd be strictly for cruising on the highway. I would expect to downshift out of 6th, no matter what the rear end ratio, if I wanted to go anywhere in a hurry. ;) I had used 6th gear as low as 50-55mph in my 4.10 car regularly with no issues, so I assume I could easily use 6th with 3.42's cruising at 60+mph. But my idea of just cruising may be different than yours? If I want to accelerate, I'll downshift. I don't mind. It's why I installed a manual tranny. :p

I've driven 3.73 cars and thought they were fine, but not as snappy as my 4.10 or 4.56 geared cars were.

Thanks for the input guys. :)

I still have no idea what direction I'm going in with this car!
 
#21 ·
Jerry, did you just call my wagon a turd? Lol! ;)

...coming frim the guy with an LS7. Hahaha
 
#23 ·
Welcome to the board. Now go read the second post in this thread http://www.impalassforum.com/vBulletin/showthread.php?t=232899 and it should answer all your questions. If you have any further questions after doing some research and checking out all those links in that thread, feel free to start up your own topic and the guys here will be more than happy to help out, I'm sure. Odds are that there's someone local to you who's done it. ;)
 
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