My 1970 LS6 has the original drivetrain and tow build sheets that show a 3.31 rear. It has F70x14 wide ovals on it. At 60mph it is taching just a hair over 3000rpm. This calculates to a 4.10 rear. I figured someone must have changed it out at some point. I have a TH400 automatic that shifts and operates normally as far as I can tell. I got the car up on the lift today and we checked it three times using the tried and true method of turning the rear wheels and counting driveshaft revolutions. It is a 3.31 rear! The original carrier is stamped CRV and dated. Anyone have any ideas? I should be taching around 2600 rpm's at 60mph with 3.31 gears. Any help would be greatly appreciated.:confused:

Rob:confused:

Your Tach is wrong!!!!

I'd start by verify the speedometer using mile markers before I did anything else.
Odometer and speedometer.

Tach and speedo appear normal but I will double check them again. The weird thing is I have been driving these cars for years and it feels like a 4.10 rear and not 3.31's It would be obvious if the torque converter was slipping and it is not. I roll out at 30 mph and get on it and the rear just breaks loose and will butn the tires off the rims.

Appearing normal doesn't count.
Check that 1 mile is 1 mile by the mile markers on the road and that it takes 60 seconds to cover 1 mile at 60 MPH.

Your Tach is wrong!!!!
Not uncommon for tach to be off by that much.

Have you factored in converter slippage?

I am not that familiar with automatics,but if the converter was slipping badly then wouldn't show up when you let off the gas or jumped onit. In other words wouldn't the tacj show a large drop or increase?

If it isn't a problem with the tach my guess is its the converter flashing past its stall point. You stated its still the original drivetrain so I assume you aren't making enough power to drive through the stall if its in good shape so it may be a bit worn out.

My 1970 LS6 has the original drivetrain and tow build sheets that show a 3.31 rear. It has F70x14 wide ovals on it. At 60mph it is taching just a hair over 3000rpm. This calculates to a 4.10 rear. I figured someone must have changed it out at some point. I have a TH400 automatic that shifts and operates normally as far as I can tell. I got the car up on the lift today and we checked it three times using the tried and true method of turning the rear wheels and counting driveshaft revolutions. It is a 3.31 rear! The original carrier is stamped CRV and dated. Anyone have any ideas? I should be taching around 2600 rpm's at 60mph with 3.31 gears. Any help would be greatly appreciated.:confused:

Rob:confused:

Well that tiny tire you have on there, could that be it??? I'm betting it's just over 24 inches in diameter, that means 2988 rpms at 65 mph with no converter slip. Factor in some slip, 200 to 300 rpm and you are in the ball park for you numbers.:thumbsup:

Check those tires and see how tall they are:beers::beers:

Did the same thing with my brothers car. We turned the rear tire 100 times and got something different every time. Didn't realize we needed to keep the other side wheel from turning to get a good count. But that rear wasnt a posi.

Well that tiny tire you have on there, could that be it??? I'm betting it's just over 24 inches in diameter, that means 2988 rpms at 65 mph with no converter slip. Factor in some slip, 200 to 300 rpm and you are in the ball park for you numbers.:thumbsup:

Check those tires and see how tall they are:beers::beers:

Wally

F70x14 tires measure 26.5 inches across.

Rob

Sounds as if the Tach my be out of calibration.....Possibly a converter issue. Have you checked how many mount bolts are in the converter? I wonder if someone replaced the original 6 bolt converter with a after market unit of unknown stall using 3 mount bolts.

Run a tape measure around the circumference of the tire, divide the number you get by pi (3.1416) and that will tell you the true tire height. No need to guess. Also, all non-lockup torque converters slip some, whether they are in good condition or not.

Wally

F70x14 tires measure 26.5 inches across.

Rob

OK, 2780 rpm at 65 with no converter slip. I didn't see where you said you have a lockup converter so a stock converter has some slip.

Need less rpm, put bigger tires on, way cheaper than changing gears, OD trans etc. Something in a 28 inch plus would work

Wally,let me get this straight. Are you saying that assuming I have the stock converter in the trans that with 3.31 gears and the factory original 14 inch tires this car would normally run at just over 3000rpm's at 60mph? It doesn't make any sense to me since in third gear it is still a 1.1 ratio. Is there that much difference between aTH400 and a 4spd in this car due to "slippage"?

What would the stock converter be in a 1970 with a 454 and TH400?(stall speed)
Anyone that has an LS5 or LS6 with a 3.31 rear and TH400 with stock tires feel free to chime in. What rpm's are you showing at 60 mph? Thanks

you can't say what the exact stall of a conv is, the same conv behind a smallblock with less torque won't stall as high as bigblock with way more torque

if you have access to a GPS like a tomtom or garmin, hook it up and go for a ride to check your speedo

same if you have access to a tach, hook it up, and compare the two

stall speeds of torque converters are torque sensitive, just because you bought a 2400 (example) stall, doesn't mean it's going to be spot on 2400, matters how much torque is applied

With a 1:1 final, a 3.31 rear, and 26.5" tall tires engine RPM will be 2519 @ 60. Factor in, say, 200 RPM of converter slip and you are at 2719. It isn't inconceivable at all for your factory tach to be off by 300.

If you had 4:10's in the rear, all else the same, RPM @ 60 would be 3120. Add 200 of slip, 3320.

You need to verify your speedo and/or your tach.

Dan

My factory tach reads 4500 rpm at 65 mph. Needless to say the tach is wrong, but it's original so I live with it.

My factory tach reads 4500 rpm at 65 mph. Needless to say the tach is wrong, but it's original so I live with it.

Doesn't the factory tach have a calibration screw? I know the knee-knockers in my '66s have one... Even if I had it appeared to be reading correct near idle (using another tach that maxed at 2000RPM), it still turned out to be off quite a bit at higher RPMs... I ended up calculating the RPMs based on tire size/gears/etc, and then finding the "true" 60 MPH on the speedo (by following my friend in a newer car at a steady 60 MPH)... Then adjusted the tach to read correctly...

I even got a great deal on a factory tach that didn't work (beautiful condition otherwise). Turned out, it just was adjusted all the way down so the seller thought it was broken...

Check/calibrate the tach first...

Doesn't the factory tach have a calibration screw? I know the knee-knockers in my '66s have one... Even if I had it appeared to be reading correct near idle (using another tach that maxed at 2000RPM), it still turned out to be off quite a bit at higher RPMs... I ended up calculating the RPMs based on tire size/gears/etc, and then finding the "true" 60 MPH on the speedo (by following my friend in a newer car at a steady 60 MPH)... Then adjusted the tach to read correctly...

I even got a great deal on a factory tach that didn't work (beautiful condition otherwise). Turned out, it just was adjusted all the way down so the seller thought it was broken...

Check/calibrate the tach first...

From what I've been told here they can't be calibrated with a screw like the separate housing ones. It's about 300 rpms high at idle and then the higher the revs the greater the distance from true reading.

My 1970 LS6 has the original drivetrain and tow build sheets that show a 3.31 rear. It has F70x14 wide ovals on it. At 60mph it is taching just a hair over 3000rpm. This calculates to a 4.10 rear. I figured someone must have changed it out at some point. I have a TH400 automatic that shifts and operates normally as far as I can tell. I got the car up on the lift today and we checked it three times using the tried and true method of turning the rear wheels and counting driveshaft revolutions. It is a 3.31 rear! The original carrier is stamped CRV and dated. Anyone have any ideas? I should be taching around 2600 rpm's at 60mph with 3.31 gears. Any help would be greatly appreciated.:confused:

Rob:confused:

Why not just pull the diff cover and count the teeth on the gears and you will know for sure whats in there?

I have the same size tires on my #'s matching 69 with 331/m20 with the correct speedo gears in trans and it tach's approx 2,600-2,625rpm @ 60mph.

3k rpm @ 60mph with your setup would normally be 373 gear but since you did the spin the d/shaft vs tire rotation calc and get 331 then your tach has to be off base unless you did the cal wrong.

Find a buddy with a small engine analyzer and hook it to see if your tack jives with the analyzer.

If you tach ends up ok then your speedo is incorrect ,have a buddy ride ahead of you on the road in a new car @ exactly 60mph to see what your speedo reads to see if that the issue.

Scott