How to chose rear end gearing?

donnie_19

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With my LS engine out for repair, now is the time to address my tranny.
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.
Transmission never seems to really engage in first. I have adjusted the TV cable to where the pressures look 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?
 
Look in tech info here in Tires & Wheels for BBShark's info about tire diameter for the size you want to run.
Multiply tire diameter times 3.14 for circumference.
Multiply 5280 feet per mile times 12 for inches per mile.
Divide your tire circumference into that figure for tire revolutions per mile.
Divide that by your choice of rear end ratios for driveshaft revs per mile.
Divide that by your final drive gear ratio for engine revs per mile.

I have heard that 10:1 is the optimal street ratio for launch, so multiply the rear end ratio by the first gear ratio to get that ratio. I came up with wanting a 3.73 rear.
 
I have a roller FI engine, 200 4r and 336 rear, 275-50-17 in rear tires, trick is, going to have to replace with 255/50/17 when time comes, reason is that 275 is NLA.....I used a Garmin GPS to prove speedo accuracy up to 80 mph.....
 
I have heard that 10:1 is the optimal street ratio for launch, so multiply the rear end ratio by the first gear ratio to get that ratio. I came up with wanting a 3.73 rear.

I've heard this as well and it seems to be commonly repeated on various websites but I've never found a source of technical info to back it up. I'm considering a swap to a 200-4r and so will be interested in this discussion.

If you have to choose between a final 1st gear ratio of slightly less than 10:1 or slightly more than 10:1, which do you choose?

For example, a 200-4r with 3.55 rear gear comes out to be 9.73:1. With a 3.73, it comes out to be 10.22:1.

There is also cruise rpm to consider. With 27.2" tall tires, the 3.55 will turn about 2,071 rpms at 70 MPH. The 3.73's will get you 2,175 rpms.

But, not everyone cruises at 70 or greater so in that case you'll be turning in the 1,500 to 2,000 rpm range when cruising between 50 and 70 MPH.

How does the camshaft specs play into all of this? For example, my camshaft comes with a recommend rpm range of 2,000 to 5,600. Should my cruise rpms be kept in this range? I could drop down to 3rd gear when needed but then my cruise rpms will be back where they were with the TH400.

Some say you shouldn't lug the engine below the camshaft's recommended operating range.

And then there is the torque converter. How does the selection of a torque converter influence the rest of this equation?

As a comparison, my Suburban puts out only about 40 HP less than my '73. It cruises at 70 around 1,800 rpms. It stays in overdrive between 50 and 70 MPH and at those speeds is turning less than 1,800 rpms. So I have no driveability issues with it at all. I'd like the Vette to be similar. Don't know if that is possible.

I'm looking for the best overall driveability. I plan to spend equal amounts of time at highway speeds as I will driving around town. Donnie's use of his Vette may be different and that may be the key factor - making sure everything is tailored to the way you intend to drive.

By the way, I have created an Excel spreadsheet which allows for comparison of two different tire/axle/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. It's nothing too fancy but it gets the job done.

DC
 
I'd use what you want for final drive ratio wheel revs per mile to determine the choice between a 3.73 & 3.55. In this case, I'd go with the 3.73---there's only 100 RWRPMile difference, and the 3.55 does seem a bit steep, potentially lugging a bit more.
If your cam is 'rated' 2000 to 5600 rpm, keep in mind it's only starting to come on at the lower rpm. It will have to be at higher rpm than that to be really kicking in.
I am bummed out because I used to have a Comp XE 262 cam with 114* lobe seperation angle, rated 1300 to 5000? rpm. (When I built the engine, I thought I was going to put TPI on it. Never came to pass.) The wide LSA gave it lots of low rpm torque. This time I put a CC 268 cam with 110* LSA, rated 1600 or 1800 to 5600 rpm. My stall converter is 2000 because I spend most of my time in town at lower speeds. (TH 400 with 2.75 first gear set & 3.36 rear end.) The new cam is not nearly as froggy off the line as the old one. It needs to ramp up to higher rpm before I can feel it. Maybe it's better at higher rpms, but I don't go there very often. But the off the line & traffic acceleration is definately weaker. (I'm thinking of advancing it 2* at some point to see if that helps the low end torque.)
 
Depending on tire choice the 17's could be close to stock diameter, like the 255-50 tire on 17x8 ... If you want to wider on the rear, like Gene says the 275-50 is not available anymore
If it was me.... I'd get the rims/tires first, 275-40 or 285-40 or 255-45 on 17" is all smaller diameter than stock, roughly 1".
Drive the car first, then decide on the gears, that's the best advice I can give you.... I have 3.55s but the 3.73 would be better, i dont like the big step from 4th to 5th gear with my tko .... The shorter gears would help ....
 
Cam spec sheet

Here is my cam sheet. CorvetteLSXcamspecs_zps616e671b.jpg
17" wheels and tires are going to be down the road after paint. Would like to get all my mechanical done before paint. What's a reasonable price for a rear end with correct gears? Should I swap or get mine re geared?
 
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With my LS engine out for repair, now is the time to address my tranny.
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.

A 3:08 is the perfect tranny for a Richmond 5 speed. It has a 1:1 fifth gear output, which is good for the 3:08, but has an extra low gear first gear. It's a manual transmission though, which is a big problem if you have an auto.

I think the auto tranny selections are all overdrive, so your 3:08 is not going to work.

I have 17 inch wheels. They're Wheel Vintiques, which I believe are now sold as Billet Vintiques. They look like the stock rally wheels from, say 10 feet away.
 
What's a reasonable price for a rear end with correct gears? Should I swap or get mine re geared?

You can buy crap anywhere - cheap on Ebay, or more expensive from those so called rebuilders .... All these companies want and have to make money, that's the bottom line , many cut corners ...

Look at Gary's posts and check out how much time goes into the setup, I stopped by his shop last year and he explained in great detail what he does to these diffs. I am convinced nobody who's in the market for a profit does half the work that Gary does.... That said it's the time spent on the setup and the lithle details. If you can do this kind of work yourself and if you're willing to spend the time then defininetly do it yourself, buy the parts and do all the work that's described in Gary's threads (Gtr1999) , that's what I plan to do one day but for now my diff is fine and I have too much other stuff going on anyways ....

The gears are maybe $200-250 for quality stuff, then clutches, bearings , yokes, etc .... Maybe send the case to Gary and have him do the machining work that you cannot do in the garage .... Email or Pm him , he's always been very helpful....

I'm guessing if you buy quality parts you spend $700 .... Now the rebuild diffs are $1000 plus core .... For $300 i labor and profit you're not getting much if you buy a rebuild diff .....
 
Also, that is quiet a bit of cam duration... Mine is 230/236 so a little milder and the engine is not very happy at 1900-2100 in 5th gear which is exactly where it is when highway driving .... You might not be happy cruising at 65-70 mph ... My 5th gear is .62 overdrive, yours is .67 ??

This is why i said earlier I'll be happier with 3.73 or even 4.11 gears ....
 
Also, that is quiet a bit of cam duration... Mine is 230/236 so a little milder and the engine is not very happy at 1900-2100 in 5th gear which is exactly where it is when highway driving .... You might not be happy cruising at 65-70 mph ... My 5th gear is .62 overdrive, yours is .67 ??

This is why i said earlier I'll be happier with 3.73 or even 4.11 gears ....

My engine is a LS so it doesn't mind low RPM.
 
Judging by the comments here, a stock f body or C5 LS6 with a 4L60 trans and stock 2.73 rear shouldn't be able to get out of its own way or maintain OD at hiway speeds.
 
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Judging by the comments here, a stock f body or C5 LS6 with a 4L60 trans and stock 2.73 rear shouldn't be able to get out of its own way or maintain OD at hiway speeds.

I tell you what the biggest bitch is....crusing some commercial highway 3 lanes wide...and the light throttle sez to the trans to upshift, and i'm in 4th gear lockup at say 40 mph....and the engine lugs/pings.....it's gotta go on that, computer needs another chip setup....

:shocking:

Thing that gets ME is the lockup releases if I lift on the gas.....then re-locks when I hit the gas again.....but not with cruise control.....and this symptom is with the latest chip iteration....

maybe it saves gas, really....I dunno, but the old chip stayed locked when I lifted.....
 
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Judging by the comments here, a stock f body or C5 LS6 with a 4L60 trans and stock 2.73 rear shouldn't be able to get out of its own way or maintain OD at hiway speeds.

Stock cam is somewhere around 210 duration, stall is what ? 1800??? My LS2 runs fine at 1800rpm but that's with stock cam and a 6spd .... With a cam in the 240's duration it will not run great at 2000rpm ....
Not trying to sound like an ass but doing things over is expensive as I learned the hard way .... A few more details from the OP would help ... LS1 with FI or carb? Heads??
 
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I have a roller FI engine, 200 4r and 336 rear, 275-50-17 in rear tires, trick is, going to have to replace with 255/50/17 when time comes, reason is that 275 is NLA.....I used a Garmin GPS to prove speedo accuracy up to 80 mph.....
What brand of tire are you using? I have Nitto NT450, 275/50 17 on the rear of my 65 and they appear to still be available.
http://www.nittotire.com/Tire/nt450#size
http://www.discounttiredirect.com/direct/searchTiresBySize.do?ar=50&rd=17&cs=275

Tom
 
Damn Gene .... Run to your local DiscountTire and have them order a set ... They match their online price but then add the install and balance but you don't pay shipping ...

Good find !!!!!!
 
I agree to some extent but 4.11s seem a bit extreme.
His motor is a 6.0 efi with 317 heads. Most efi cams have enough LSA to still perform well in the lower rpm. Anyone want to calculate that for his cam?

Judging by the comments here, a stock f body or C5 LS6 with a 4L60 trans and stock 2.73 rear shouldn't be able to get out of its own way or maintain OD at hiway speeds.

Stock cam is somewhere around 210 duration, stall is what ? 1800??? My LS2 runs fine at 1800rpm but that's with stock cam and a 6spd .... With a cam in the 240's duration it will not run great at 2000rpm ....
Not trying to sound like an ass but doing things over is expensive as I learned the hard way .... A few more details from the OP would help ... LS1 with FI or carb? Heads??
 
Damn Gene .... Run to your local DiscountTire and have them order a set ... They match their online price but then add the install and balance but you don't pay shipping ...

Good find !!!!!!

HUMMM....some time ago I looked at Nitto/Tire Rack/Discount/whatever, and they listed it as NLA....guess they tooled up and made some....

but then again, I don't drive in the rain, and so seldom that it's not all that critical....putting those two tires on the front, but going to wait till the belts show on the worn tires.....:censored:
 
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