How to chose rear end gearing?

LS

MYBAD79 said:

Garys 68 said:

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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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 an LS 6.0 bored .030 over engine with FI , 317 heads, 2800 stall converter, 2004R with shift kit, Holley ECU. I have the torque converter lock up wired using the brake pedal to unlock.

what's your rpm at 70 with the 3.08 gears that you have now?

your wheel outer diameter is 27" with the 255/60 tires, going to be just under 26" with 275/40 or 245/45 on 17"

63360" is a mile , one revolution of your wheel is (27x3.14) 84.78" therefore your wheel spins 747.35 revolutions in one mile, or 52314 revolutions for 70 miles, now devide that by 60 (60 min in one hour) and you get the rpm per minute:879.9

that rev per minute multiplied by the gear ratio is

879.9 x 3.08 = 2685 x 0.67 overdrive, 1798 rpm at 70 mph

879.9 x 3.73 = 3282 x 0.67 overdrive, 2198 rpm at 70 mph

yes, 4.11 gears would be a bit much

with the smaaler 17" wheels the rpm will be slightly higher so 3.55 gears might be perfect

MYBAD79 said:

what's your rpm at 70 with the 3.08 gears that you have now?

your wheel outer diameter is 27" with the 255/60 tires, going to be just under 26" with 275/40 or 245/45 on 17"

63360" is a mile , one revolution of your wheel is (27x3.14) 84.78" therefore your wheel spins 747.35 revolutions in one mile, or 52314 revolutions for 70 miles, now devide that by 60 (60 min in one hour) and you get the rpm per minute:879.9

that rev per minute multiplied by the gear ratio is

879.9 x 3.08 = 2685 x 0.67 overdrive, 1798 rpm at 70 mph

879.9 x 3.73 = 3282 x 0.67 overdrive, 2198 rpm at 70 mph

yes, 4.11 gears would be a bit much

with the smaaler 17" wheels the rpm will be slightly higher so 3.55 gears might be perfect

Click to expand...

Crap, all that math is too much, I went with a couple of calculators on the computer, one of which was tire rack....type in the variables....tire size, gear ratio, speed, they said what engine rpm was for O/D ratio or 1-1 speed....

AND then I got into it with Turtle Vette, who is MIA for some time now....

and so I got a GARMIN....and damn if even reviewing all the crap over again on the sites, they were RONG!!!!! must have to do with tire diameter or slippage at speed, which makes sense given the wind loading.....SO, I had to put in the other speedo driven gear, and it turns out to be totally accurate on my old '72 rebuilt mechanical speedo.....80 mph is really only 78 but at 100 i'm doing only 94....all speeds below 80 are dead nutz on.....

so I don't believe anyone, even a cop radar on speed bullshit, only GPS.....


:hunter::cussing:

AND SO the crooked Monkey Co. Faryland cops from decades ago, and their picking off motorists ticketing like mad.....makes me wonder....

:hunter::club:

donnie_19 said:

...I have an LS 6.0 bored .030 over engine with FI , 317 heads, 2800 stall converter, 2004R with shift kit, Holley ECU. I have the torque converter lock up wired using the brake pedal to unlock.

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Don,
With your cam (238/242 on 112 lsa) you will need the 2800 stall rpm, but I question your comment on hooking the lockup to the brake pedal. There must (should be) some way to prevent the TC from locking when you release the brake pedal to accelerate. Otherwise, the TC won't reach any stall rpm at all! Maybe there is a first gear lockout that you didn't mention? Or a time delay or minimum speed for lock-up operation?

As to rear end ratios, when I set the NCCC 2B drag record, I was using 4.11s with a 2.52 first gear ratio (10.35 total ratio) with sticky tires (MT ET Street Radials). These needed a very good track to hook as they would spin on a poorly prepped track or on the street. Conclusion, if your choices for a gear ratio bracket the golden 10:1, go with the 3.55. Provided your TC is working properly, your motor will annihilate the tires in first gear!

BTW, the stock rear gear ratios are 3.08, 3.36, 3.55, 3.70, 4.11 and 4.56. There is NO 3.73 unless one goes aftermarket.
(I don't recall the really high ratio beyond 3.08 - really don't care!). Also be advised that 4.11s and lower require a 4 series posi case (added expense); 3.70s and higher use the 3 series case.

larrywalk said:

Conclusion, if your choices for a gear ratio bracket the golden 10:1, go with the 3.55

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Thanks for weighing in Larry. Your comments have helped me eliminate 3.70's as an option. That leaves 3.36 vs 3.55. Leaning toward the 3.55's from a cruise rpm standpoint but they give a total 1st gear ratio of 9.727 when coupled with the 200-4r. 3.36's end up at 9.206. How do these choices compare from a stop light to stop light driveability standpoint? I don't want a situation where I have to really baby the throttle at take off but I don't want to leave any performance on the table either. My only basis for comparison is my current situation which is a TH400 and 3.08's for a total 1st gear ratio of 7.638 which is definitely too low.

DC

mrvette said:

MYBAD79 said:

what's your rpm at 70 with the 3.08 gears that you have now?

your wheel outer diameter is 27" with the 255/60 tires, going to be just under 26" with 275/40 or 245/45 on 17"

63360" is a mile , one revolution of your wheel is (27x3.14) 84.78" therefore your wheel spins 747.35 revolutions in one mile, or 52314 revolutions for 70 miles, now devide that by 60 (60 min in one hour) and you get the rpm per minute:879.9

that rev per minute multiplied by the gear ratio is

879.9 x 3.08 = 2685 x 0.67 overdrive, 1798 rpm at 70 mph

879.9 x 3.73 = 3282 x 0.67 overdrive, 2198 rpm at 70 mph

yes, 4.11 gears would be a bit much

with the smaaler 17" wheels the rpm will be slightly higher so 3.55 gears might be perfect

Click to expand...

Crap, all that math is too much, I went with a couple of calculators on the computer, one of which was tire rack....type in the variables....tire size, gear ratio, speed, they said what engine rpm was for O/D ratio or 1-1 speed....

AND then I got into it with Turtle Vette, who is MIA for some time now....

and so I got a GARMIN....and damn if even reviewing all the crap over again on the sites, they were RONG!!!!! must have to do with tire diameter or slippage at speed, which makes sense given the wind loading.....SO, I had to put in the other speedo driven gear, and it turns out to be totally accurate on my old '72 rebuilt mechanical speedo.....80 mph is really only 78 but at 100 i'm doing only 94....all speeds below 80 are dead nutz on.....

so I don't believe anyone, even a cop radar on speed bullshit, only GPS.....


:hunter::cussing:

AND SO the crooked Monkey Co. Faryland cops from decades ago, and their picking off motorists ticketing like mad.....makes me wonder....

:hunter::club:

Click to expand...

LOLWUT???

Just curious - have you checked any of the various other sources?
GrumpyVette has a lot of discussion on shift points, gear ratios, etc.
And, there are a number of spreadsheets about that might be helpful too.

I found one on - of all places a DATSUN site_ that was nicely done and let me lay in all my data from my dyno sheet to help - mind you its all theory - until the hammer drops!
Here is a sample page:


Cheers - Jim

DC3 said:

larrywalk said:

Conclusion, if your choices for a gear ratio bracket the golden 10:1, go with the 3.55

Click to expand...

Thanks for weighing in Larry. Your comments have helped me eliminate 3.70's as an option. That leaves 3.36 vs 3.55. Leaning toward the 3.55's from a cruise rpm standpoint but they give a total 1st gear ratio of 9.727 when coupled with the 200-4r. 3.36's end up at 9.206. How do these choices compare from a stop light to stop light driveability standpoint? I don't want a situation where I have to really baby the throttle at take off but I don't want to leave any performance on the table either. My only basis for comparison is my current situation which is a TH400 and 3.08's for a total 1st gear ratio of 7.638 which is definitely too low.

DC

Click to expand...

If you already have 3.36s, they will do a respectable job, especially combined with a higher stall torque converter. To illustrate, with a 406 SBC in my '78 and a 10" diameter TC with ~2,800 rpm stall and 3.08 gears (and a Quadrajet!), it ran upper to mid 11s in the quarter! Either a 3.55 or a 3.36 will be fine unless you're using drag radials on the track.

Didn't read the previous posts, so my apologies if I missed something.

My opinion is that you want to be doing 2000 rpm at 70 mph. Give or take a little. This engine speed gives good mileage with reasonable engine wear and noise. My 68 does about 2000 rpm and the 08 does about 1800 rpm.

I have a TKO 600 in the 68 and it has a 3.73 (?) rear end gear ratio. The TKO has an overdrive 5th gear. The 08 is an automatic and I think it has a 2.74 (!!) rear end gear ratio.

I think the 68 had a 3.73 rear end ratio. Anyhow with the stock Muncie 1:1 fourth gear, it was running about 3000 rpm at 60 mph. Just drove me nuts. The TKO 600 dropped me down to about 2000 rpm. About $3500. Actually, the TKO is a lot more stronger tranny than I need with my small block ZZ4. I think it's a 600 ft-kb tranny which I don't need. But anyhow, it's in the car and it's really nice to drive. Shifts nice.

68/70Vette said:

Didn't read the previous posts, so my apologies if I missed something.

My opinion is that you want to be doing 2000 rpm at 70 mph. Give or take a little. This engine speed gives good mileage with reasonable engine wear and noise. My 68 does about 2000 rpm and the 08 does about 1800 rpm.

I have a TKO 600 in the 68 and it has a 3.73 (?) rear end gear ratio. The TKO has an overdrive 5th gear. The 08 is an automatic and I think it has a 2.74 (!!) rear end gear ratio.

I think the 68 had a 3.73 rear end ratio. Anyhow with the stock Muncie 1:1 fourth gear, it was running about 3000 rpm at 60 mph. Just drove me nuts. The TKO 600 dropped me down to about 2000 rpm. About $3500. Actually, the TKO is a lot more stronger tranny than I need with my small block ZZ4. I think it's a 600 ft-kb tranny which I don't need. But anyhow, it's in the car and it's really nice to drive. Shifts nice.

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I have almost the same setup (and cars to compare) lol
my 06 is a 6spd and it can run in 6th gear anywhere from 50mph to 75mph which is what I consider highway speed - in traffic.

Now my '79 has 3.55 gears and the TKO with the lower OD .67 (well, higher OD, numerically lower compared to the .82 option).
Here's what I don't like and why I posted above leaning towards higher gears: 65mph and faster is fine in 5th gear. The speed limit here varies between 65 and 55 so most people drive between 55 and 75....when traffic slows down a little to 60 or slower The engine does not seem to be very happy at around 1800 rpm so I shift in 4th, accelerate and shift back in 5th. This gets really annoying after a while.
I am running a carb so that doesn't help my situation, with your FI you might not have this issue......

Will the 3:08 case work for 3:55 gears or is a different case required? I am currently adding a shift kit to the tranny while waiting on engine.

donnie_19 said:

Will the 3:08 case work for 3:55 gears or is a different case required? I am currently adding a shift kit to the tranny while waiting on engine.

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The 3.08 case (3-series) will also work with 3.36, 3.55, and 3.70 gears!

About the 10:1 ratio being golden...I'm an old guy, and back in the day it was considered that 12:1 was the preferred ratio to leave the line with.

With an automatic and a higher stall converter, this may not necessarily be the case. But with the higher gearing - say 3.08 vs 3.55 - it's all too easy to fry the tires with the 3.08's. While that may sound contradictory, from experience it's true. Once your tire let's loose with the 3.08's it is difficult to get it to grab again.

You could actually go as high as 4.11's with your first gear being as high as it is, and that multiplies out to the low 11's off the line. With your O.D. ratio you are back down into the low RPM's for the highway.

You can always play with rear tire sizes after that for your finer tuning for the RPM's you want. I don't know if you can fit a 29" tire in your wheel well, but compared to a 27" tire that will drop your RPM by about 7%.

Lots of things to play with.

84rzv500r said:

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The ROD 6 final gear is almost identical to the TKO II's, .80 and .85 respectively. What is the advantage to the extra spread in the lower gears?

Im not gonna get this exactly right but here goes...

2,3,4, and 5 are essentially the same as a Close ratio 4 speed, which is what I had originally (Super T 10)... now I have a very streetable low gear and a OD for Highway.

The thing I liked about the ROD is that it is hell for strong and the ratio selection is very broad for each gear also the 0.8 OD is rated for continuous use at max torque.

Really a BBC with such a broad Flat torque curve as the ramjet does not need so many gears... I almost went with a wide ratio muncie with an OD 4th gear set.

The ratio selection and spread should be made to keep the engine in the fat part of the torque curve so that when you shift the engine rpm dont fall below the power...

as you increase the speed the drag goes up by the cube of the speed and the power requirements to over come the drag are greater hence the gear spacing narrows to keep the rpms closer to the peak torque for max acceleration.

and don't think just because it has 200 mph gearing it will pull it... a C3 needs something like 700 flywheel HP to tag 200... by comparison my E55 AMG needs only 550 or so... can you say coefficient of drag


with all that said... the fun part is never running out of gears... or bullets...:mobeer:

84rzv500r said:

The thing I liked about the ROD is that it is hell for strong and the ratio selection is very broad for each gear also the 0.8 OD is rated for continuous use at max torque.

The ratio selection and spread should be made to keep the engine in the fat part of the torque curve so that when you shift the engine rpm dont fall below the power...

and don't think just because it has 200 mph gearing it will pull it... a C3 needs something like 700 flywheel HP to tag 200... by comparison my E55 AMG needs only 550 or so... can you say coefficient of drag

with all that said... the fun part is never running out of gears... or bullets...:mobeer:

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Enjoyed reading everyone's comments. The ROD(Richmond OverDrive) 5 speed is rated at 450 foot-lbs torque...is the 6 speed rated at more than 450?

a C3 needs something like 700 flywheel HP to tag 200. I wonder about this. In the early 1980's Glen Banks (now specializing in turbo setups for trucks) set the worlds land speed record for a stock bodied car. The stock bodied car was a 68 Corvette Convertible with a hardtop option. I saw the car and talked to Glen about it. He (his driver) drove the stock bodied 68 Corvette hardtop vert at 230 mph at Bonneville. IT WAS A TRiCK CAR. It was stock bodied..yes.. but.. the front end of the car air dam was about 1 1/2 inch above the road surface......the front engine coolant radiator had been removed and in it's place was a 50 gallon water tank, 400 lbs,..the 400 pound weight was to prevent the nose of the car from becoming airborne. The car was powered by a twin turbo 427 developing 1200 hp. Glenn told me he could get more hp out of the engine but he said he only wanted to run it at the boost level to set the worlds record and wanted to minimize the possibility of blowing up the engine/loosing the car, etc. If you want to run 230 mph with a stock bodied 68, loaded down with 400 pounds in the nose, you need 1200 hp. I'd guess that 700 flywheel hp will not get a C3 to 200 mph, and in any event if it did start to approach 200, the nose would lift off the ground a lot lower speeds.

I vaguely remember that the stock C2's will start to have nose liftup at about 120-130 mph. Can't cite this absolutely.

the Richmond 5 speed is 1:1 in 5th... the ROD has a bunch of OD ratios available for 6th from .91 to .52 and .when ordered my transmission from Richmond... talking to their tech support they wouldn't warranty the ROD behind the 502 Ramjet with out using the special 9 DP road race ratios K=.80 or L=.84.

never seen torque ratings on ROD6 I'm sure they exist. I know the Street 5 speed is rated at 450 in the lower gears but it drops off in the higher gears... I haven't been able to break it... my ROD6 and 3.08 Rear has held up well with annual fluid replacements...

I did snap my front differential mount off... built a new stronger one, more pinion down angle, urethane mounts instead of the stock rubber and added traction bars to take some of the load off.... didn't break at the drag strip... didn't break auto crossing... broke on an interstate in OD rolling on the throttle up a hill... BANG...

dunno about Gale Banks' vette... Id bet while the body was "stock" the body itself was tilted nose down about 2-3 degrees on the body mounts...
Chassis was lowered and there was prolly 2 rolls of duct tape sealing the thing up....

lotsa 700 HP c3's in the 200 zone... agreed not stock body'd.. If you look at the math the Coefficient of Drag is really important... IIRC a stock C3 is around 0.44, a C4 is around 0.33 and my ancient E55 AMG SEDAN (takes less than 30 seconds to bang into the Speed limiter at 168MPH from a dead stop) is around 0.28...

a good article...


http://www.hotrod.com/techarticles/body/hdrp_0609_aero_tricks_tips/

Anywho... depends on what you wanna do with the car.... 10 or 11 to 1 overall makes a great street-able 1st... space the rest of the gears to match the power charterisitcs of the motor.. my advice is do the math... simulation is much less costly than experimentation In this case...


:thumbs::thumbs: