donnie1956

I have a 2004 R Heavy Duty with stock 78 rear end 3:08. Currently have the stock 255/60/15 tires, but planning on 17's in the future. Powered by a LS engine.
Transmission never seems to really engage in first. I have adjusted the TV cable to where the pressures are correct in each gear. The shifts seemed to be solid both up and down. Took the tranny to a reputable Tranny Shop that is familiar with these older units. He said he our check it out but suggested that I really should change the rear gearing to either 3:73 or 4:11. Or change to a different transmission but that involves changing crossmember and drive shaft.
Here is the gearing for the 2004 R. 2.74 1.57 1.00 0.67.
I am not planning on any strip, but would like to spin the tires occasionally and have the engine run around 2000 RPM at 70 mph. With that info how do you chose a rear end gear?

gcusmano74

My 2 cents. Keep your trans.
I wouldn't go any shorter than 3.73 gears. If you want to put 4.11s in it, you will have to change out the carrier, as well. More $$$.

scottyp99

10:1 final drive in first gear is the old rule of thumb for performance applications, and a 3.73 diff gear gives you 10.22:1 in first gear, so that sounds about right.

Scott

Blue79

I'm running a 2004r in my 79 with stock size tires, 3:55 rear gears and at 75MPH with the converter locked, it's at 2400RPMs.

scottyp99

Blue79, according to an online rpm/speed calculator, your car should be pulling 2220 rpm at 75 mph. The tach and speedo are never 100% accurate, 2400 is actually pretty close to actual for these old cars.

OP, if you want to tach at 2000 rpm at 75, you're going to have to stick with the 3.08 diff gear. Can't have it both ways, sorry,

Scott

redwingvette

I to am running a 3.55 gear and think that it is a good cross between good starts and good highway speeds. Just my 2 cents worth.
My setup

DC3

He actually said he wanted to tach at 2,000 at 70. 3.36's would give about 1,969 and 3.55's would end up about 2,080.

DC

scottyp99

Oops! That's right, it was Blue79 who used the 75 mph figure. I got about the same numbers you have there using an online calculator, using 70 mph. Thanks for the head's up, DC3. I'm getting 2177 for the 3.73 gear, and that's my vote, if anybody is counting votes.

Donnie1956, if you are planning on going with 17" wheels, you can get 255/50/17 tires that are within 1/10" of the diameter of 155/60/15 tires, so no need to re-do calculations, or alter speedo gears. A gear change will need new speedo gears, obviously.

Scott

DC3

I wanted to go with 255/50/17 but there wasn't much of a choice in rubber in that size when I needed tires. I ended up with 235/55/17's which are also the same height.


I have created an Excel spreadsheet which allows for comparison of two different tire size/rear axle gear ratio/transmission scenarios. I've loaded it with data for various transmissions so you can select a particular transmission from a drop down box or enter specific parameters for any transmission that is not in the list. I'd be happy to email the spreadsheet to anyone who wants it, just send me your email address in a PM.

DC

scottyp99

I just sent you a PM, DC3, thanks for the offer.

Scott

johnt365

http://tri-5.chevyrides.com/rpm.php add a couple hundred rpm for your torque converter.

I have a 200-4R with 3.55, stock tires and a small block. The overdrive is pretty deep it seems to me. The trans was originally in a Monte Carlo SS with a 3.73 rear.

My car originally had 3.08's and I changed to 3.55's when I had a TH400 trans as a compromise .

I think if I had a 200-4R and was starting from 3.08, I would choose a 3.73. I think it would be easier to just put it in drive. As it is, I start out in third and manually put it in OD once I am on the highway.

donnie1956

I am leaning towards the 3.73. My torque convertor is 2800 rpm. My cam sheet. Whats a reasonalbe price for a rear end with correct gears. Should I get rebuild or have mine rebuilt?

jackwabbit703

Over drive tranny with 3.73 gears..... That's lotta FUN in all gears!!!

jackwabbit703

If I were you, I would get a rebuilt rear end. Zip corvette has 3.70 for around 900 I think.

http://www.zip-corvette.com/ProductD...GR-SR&CTitle=&


Just the ring and pinion set will cost you around 300 - 500 or more depending which makers (Richmond is okay but Yukon is very good but expensive).

If you go with the R&P swap, you or the shop will really need to know how to do it right. Nearest shops in my area will do the swap for around 400 - 700 in labor with no warranty.

So, you're in 700 - 1200 total cost for the R&P swap. Local shop I know will do the differential swap for 200. I'd paid little more and get the rebuilt rear end and it's virtually all plug and play with no tinkering with preload, gauging etc.

7T1vette

With a 3.73 rear gear, you will just burn tires and make smoke with 200-4R first gear...unless you have super sticky tires. And, if you do, then you need to beef up every part of the drive train to handle the torque.

If you will be having 'street' tires on the car, install a 3.55 gear MAX. A3.36 gear is a decent compromise for good take-offs and decent highway speed manners.

donnie1956

Currently leaning towards the 3.55 rear end.

gcusmano74

Good idea. Long term, gas can only get more expensive.

MIKE80

I'd go with the 370 gears if you're looking for more performance.

I run that gear with my T56 6 speed. At 70 mph I'm at about 3400 rpms in 4th (1.00), 5th (.74) is at about 2500 rpms, and 6th (.50) is at about 1700 rpms.

gkull

The first problem is: does it take off in first geat when you have the selector in "D"? If not you have a tranny problem that has nothing to do with the TV cable.

That cam is for higher rpm. So you don't want 2000 rpm at 70 mph. RPM does not determine MPG. Engine operating efficiently does.

I used a 4.11 with a 700R4 for 18 years. I always thought that a 3.90 rear was about the best choice

tracdogg2

Gkull is correct. That cam will not work with the OD. It will constantly be jumping in and out of lockup and OD. When it's out of lockup you will be running on the converter. 3.73 will work but 3.90-4.11 is a better choice for economy.
Zip differentials don't come with stub axles. Add a couple of hundred to the price. Yukon gears used to be very good but something has changed in the last year. I will no longer install Yukon gears in any diff.
You're trying to run the engine much slower than the minimum cruise rpm. You could get away with it with a standard trans and a 3.55 gear but not with and auto. You need to either run more gear or less cam.
Mike